
==== Front
Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)12487-5
10.1016/j.heliyon.2024.e36456
e36456
Research Article
Compost derived from olive mill cake: Effects on isohumic soil quality based on humic acids characterization
Masmoudi Saoussan a
Abid Wadii a
Medhioub Khaled a
Ammar Emna ammarenis@yahoo.fr
ab⁎
a Preparatory Institute of Engineering Studies in Sfax, Laboratory of Environmental Sciences and Sustainable Development (LASED), University of Sfax, B.P. 805 - 3018, Sfax, Tunisia
b National Engineering School of Sfax, University of Sfax, B.P. 1173 - 3038, Sfax, Tunisia
⁎ Corresponding author. National Engineering School of Sfax, University of Sfax, Sfax, Tunisia. ammarenis@yahoo.fr
17 8 2024
30 8 2024
17 8 2024
10 16 e364566 2 2024
14 8 2024
15 8 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc/4.0/ This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).
The compost effects on soil organic matter (SOM) stability were evaluated. Manure at 10 % ratio and compost at 10 %, 20 % and 40 % ratios (v/v) were added to the soil and their effects were compared to unamended control soil after 90-days of greenhouse-experiment. Humic acids (HA) and fulvic acids (FA) were extracted from two different soil-sample layers at 0–15 and 15–30 cm depth. The CHA/CFA ratio and the humification parameters were determined, and the soil-HA were characterized by spectroscopic methods (E4/E6 and FTIR). The humification parameters progress with time were affected by the amendment concentration. After 90 days, the treated soils HA’ FTIR spectra showed an increase in aromatic carbon polycondensation and O-containing groups reflecting the high degrees of molecular associations and humification of soil HA. Compared to 10 % manure application and 40 % compost ratio use, the applications of 10 % and 20 % compost ratios induced higher humification level and highly oxidized HA structure. Moreover, changes in the HA compositional and functional groups were noticed at the upper layer which exhibited higher reactivity compared to the lower layer which displayed more humified SOM. Through the humification process, the HA fraction was improved to reach more stable and complex macromolecules, where aromatic structures were bio-converted into highly functionalized compounds.

Graphical abstract

Image 1

Highlights

• Compost and manure were rich in humic acids (HA) fractions affecting soil quality.

• 10 % compost amended soil had higher molecular weight HA than that of 10 % manure.

• The humification progress depended on soil layer environmental conditions.

• Organic amendments humification raised aromatic-C content in soil HA structure.

• The 10 % and 20 % compost rates induced highly oxidized HA structure.

Keywords

Compost from olive mill waste
Amendment ratio
FTIR spectroscopy
Humification parameters
Manure
Soil layers
==== Body
pmc1 Introduction

In the arid and semi-arid regions, growing concerns by land desertification have been expressed worldwide, particularly to face agricultural soils deterioration due to the indiscriminate use of agro-chemicals and the accelerated soil organic matter (SOM) depletion through land use change and accelerated by the increase of drought event as well as temperature rise in relation to global climatic change [1]. This affects the C cycle and SOM return. Therefore, slowing down the soil degradation process and the development of efficient agriculture practices has become more important than ever before [1,2].

More particularly, in Tunisia, the olive oil industrial sector generates annually a huge amount of organic solid by-product, known as olive mill cake or olive pomace, during a short period of time (from November to February). It may produce more than 30 million tons of wastes per year; and a liquid-phase residue commonly known as olive mill wastewater representing about 7 × 105 m3 yearly [3]. These by-products and residues are considered as the main material-contamination's contributors in the olive oil producer countries due its high ecotoxicity [4,5]. The recycling of such olive oil residues after their composting was proposed as the best solution since this procedure is considered as an effective environmental, economic and sustainable process to reuse the organic residues [[5], [6], [7]]. While considering the SOM depletion, the compost use would be a solution to restore organic matter (OM) through soil amendment and this practice has recently much attracted researchers' attention. Indeed, previous investigations reported the beneficial effects of such by-product management on soil properties, including the soil structure, its water retention capacity and the humic substances content [[8], [9], [10], [11], [12], [13]]. Recently, Malone et al. [14] reported that compost and biochar increased SOM% by 3.1 and 6.5 units of SOM%, respectively. Composting is a commonly biological process where the waste organic materials are bio converted into stable products, including high type-humic substances content, promoting therefore the waste reuse and a sustainable agriculture [[15], [16], [17]]. It is a biodegradation process where eco-toxic organic residues are decomposed and harmless stabilized organic end-products are formed; these are high-quality fertilizers with humic-type substances, crucial indicators of compost-maturity [18]. These humic substances (HS) have a high capacity of adsorption, ion exchange, redox and complexion, which improve specially soil fertility. Consequently, composting is considered as a valuable resourcing process by recycling materials, attributing to the soil excellent conditioner and improver properties [15,18]. These HS are highly resistant to further biodegradation, valorizing the organic residues and providing thus a long-term sink for the soil carbon [19]. Indeed, in the soil, the redox environment, the microbial activity and the mineral surface interactions under defined moisture and temperature conditions control the HS formation [16,19,20]. Over the years, humic substances from compost are mineralized through extended microbial oxidation and consequently, only the most recalcitrant components are partially accumulated and incorporated into the soil [16,21]. Actually, the humic acids (HA) constitute the largest organic carbon (OC) fraction, representing 12–34 % of the total SOM at the upper soil layer [22].

Olive mill wastes (OMW) produced by the different olive oil extraction processes are characterized by high acidity, high salt content, low alkalinity, and a very high organic compounds content, including phenolic compounds [23,24]. These are considered their main phytotoxic source. This fact is related to possible negative physical, chemical, and biological effects on soil; potential phytotoxicity to crops, and potential risk to groundwater. The inhibition of soil microbial activity may in turn reduce soil fertility by inhibiting key processes in nutrient cycling responsible for the formation of labile forms of macro- and micro-elements [25].

Co-composting olive mill residues with other industrial and agricultural organic residues to provide adequate C/N, C/P and N/P ratios, could be a suitable treatment for the biological process initiation, and the phytotoxic effects reduction of phenolic and fatty compounds [24,[26], [27], [28]]. As composting advanced, a gradual humification of the composting substrates occurs, revealed by increases in pH and nitrogen content, and decreases in OM content, C/N ratio and lipid content. The lignocellulosic nature of olive-mill by-product requires a long composting time, with low nitrogen loss, and mineralization rates and an end-product rich in humic-like compounds [[29], [30], [31]]. Substrate composition is a key-controlling factor for both of the degradation and the humification processes. The relatively high lignocellulosic rate of the olive-oil residues may affect both of the composting performance and the compost quality, as lignin limits degradation and is recognized to be an important HS precursor [29,[31], [32], [33]]. The olive pomace derived composts were analyzed for their main characteristics, and were spread in agricultural soils; their use was considered as a contribution to the development of sustainable agriculture systems [34,35].

Many investigations deal with suitable methods developed for the compost maturity evaluation [[36], [37], [38]]. Compost OM biodegradability is the first criterion assessing compost maturity. Tests based on composts physical, chemical and biological characteristics have been proposed as well as methods dealing with the humified OM analyses, where the produced HS’ amounts measurement and their quantification are considered as suitable method to assess compost maturity [39]. Elemental analyses (C, N, H, O) and spectroscopic analysis: UV, Fourier transform infrared (FTIR), fluorescence spectroscopy and electron spin resonance (ESR) were investigated for compost maturity and stability evaluation [15,40,41].

The second criterion is rather biologic evaluating compost state by testing its effects on plant growth based on germination indexes, and confirming lack of any phytotoxicity or inhibition activity, and hence validating its use for agriculture purposes [29,42].

For centuries, animal manure has been recognized by its ability to slow or reverse declining soil organic levels of cropland, as well as its contributions to improving soil quality [[43], [44], [45]]. However, at present traditional organic amendments, such as farmyard manure, are locally scarce [43]. Manure contains both organic and inorganic soluble nutrients that may be dissolved into the liquid phase. Moreover, it includes toxic heavy metals and pathogenic microorganisms. As a result, manure requires proper treatment before been applied into agricultural land. When improperly treated, stored or recycled, manure can pollute rivers, soil ecosystems, and underground drinking water, thereby affecting all living organisms nearby [46,47].

At present, a few studies had focused on the application effect of the compost derived from olive mill cake residue’ recycling on physical as well as chemical properties, and soil HA structures. Indeed, the investigations of the changes in the humification process, induced by different organic amendments for a short term, remain quite limited in arid soils [7,8].

Therefore, the objective of the present study was to investigate the physical, the chemical and the structure of the soil HA to evaluate the soil quality after conventional manure, rich in nitrogen and available and olive mill cake compost incorporation. The HA evolution in amended soils was studied considering the experiment duration, the culture system and the compost ratio application at both studied layers. The effectiveness of the olive mill cake derived compost used as organic amendment for a sustainable increase of soil organic carbon content was also evaluated.

2 Materials and methods

2.1 Site and soil sampling

In this study, five soil samples were randomly collected from a greenhouse culture-system area in Mach 2021. The site is located in the Tunisian Union for Aid for Mental Insufficient: El Amal “U.T.A.I.M.” in the area of El Matar region in Sfax-Tunisia (+34.43° N, +10.41° E). The greenhouse experiment conditions were as follows: temperature: 16–30 °C, relative humidity: 60–80 %, light intensity: 4000-46,000-lx, and a photoperiod of 11–12/11-12-h.

The soil type in the study area is Haplic Calcisol (Aric, Pantoloamic, Ochric, Endoraptic) (according IUSS Working Group WRB. 2022 [48]). The calcisols have a homogeneous sandy loam to sandy clay loam texture, a calcic horizon (≤100 cm from the mineral soil surface) with secondary calcium carbonate accumulation (equivalent of ≥15 %). The soil pH value was around 8.12, and an OC content and a nitrogen decreasing with depth (Table 1). The soil moisture contents were more important at the 15–30 cm layer than that at superficial one (Table 2). The study was conducted based on a randomized experimental design, with split plots of 1.5 m2 each (Photo 1). The farm manure was applied at a ratio of 10 % and the compost was at 3 ratios of 10 %, 20 % and 40 % (v/v), and a plot involving soil without compost or conventional farm manure was experimented as a control. The organic amendments were incorporated manually into the soil with a shovel to a 30 cm depth (Photo 1). Three replicates were performed for each treatment, randomly established in a complete random block design; and in total 15 plots were achieved.Table 1 Composition and characteristics of the studied soil.

Table 1Soil layer (cm)	Sand (%)	Loam (%)	Clay (%)	pH	EC (mS/cm)	OC (%)	C/N	CaCO3 (%)	Ca	Mg	K	Na	CEC (cmol/kg)	
(g/kg)	
0–15	70.0 ± 1.5	10.5 ± 0.2	21.5 ± 0.4	8.14 ± 0.07	1.11 ± 0.02	0.91 ± 0.03	10.58 ± 0.14	18.34 ± 0.11	23.91 ± 0.35	2.74 ± 0.08	2.33 ± 0.08	2.23 ± 0.05	7.91 ± 0.09	
15–30	71.5 ± 2.1	7.5 ± 0.1	21.0 ± 0.3	8.10 ± 0.12	1.05 ± 0.02	0.62 ± 0.01	10.27 ± 0.64	19.25 ± 0.10	23.82 ± 0.32	2.54 ± 0.07	1.71 ± 0.03	2.50 ± 0.07	7.16 ± 0.06	
OC, organic carbon; CaCO3, soil carbonate content; CEC, cation exchange capacity.

Table 2 Moisture content of control soil and manure and/or compost amended soils at different studied layers.

Table 2Soil	Control	Manure treatment	Compost treatment	
Rate	10 %	10 %	20 %	40 %	
Layer (cm)	0–15	15–30	0–15	15–30	0–15	15–30	0–15	15–30	0–15	15–30	
Moisture (%)	
Initially	9.25 ± 0.09	12.15 ± 0.34	13.40 ± 0.38	14.72 ± 0.27	14.94 ± 0.19	15.63 ± 0.14	15.16 ± 0.35	15.80 ± 0.19	15.94 ± 0.33	16.95 ± 0.26	
At 90 days	10.71 ± 0.13	13.35 ± 0.26	14.11 ± 0.15	15.38 ± 0.42	15.71 ± 0.20	16.07 ± 0.37	16.40 ± 0.18	17.57 ± 0.24	16.68 ± 0.59	17.43 ± 0.37	

Soil samples were carried out (two samples for each plot, i.e. six samples for each treatment), initially (at day 0) (just after the amendment application) and after three months (at 90 days of the experimentation), simulating a medicinal plant cycle growth such as the basil (Ocimum basilicum L.), a time assessment sufficient to allow carbon biogeochemical cycle to be held, as previously investigated [49,50]. Soil plots were irrigated every seven days. The soil was sampled at the upper (0–15 cm) and the lower (15–30 cm) layers using an auger. All the collected samples were air dried, sieved through a 2 mm sieve, then filled in plastic bags hermetically closed, and stored at room temperature until analyses.

2.2 Compost preparation and analysis

The compost was prepared by co-composting seven mixed types of organic wastes made up of dry olive cake (30 %), poultry manure (23 %), sewage sludge (14 %), algae (Posidonia) (11 %), fresh olive cake (10 %), dehydrated olive mill wastewater (in solid state) (6 %), and almond shell (6 %). The raw materials were disposed in trapezoidal piles of 8000 kg, with approximate dimensions of 2.5 m height and a 3 × 20 m base. The composting technique conducted was the aerobic composting in windrow. The first raw material: dry and fresh olive mill cake, was kindly provided by a soap factory (SIOS-ZITEX, Sfax-Tunisia). It had acidic pH, high C/N ratio and relatively low mineral elements content. The second waste, the poultry manure was collected from an industrialized farm in the same area, Sfax-Tunisia. It was characterized by high pH, OM content and nitrogen content of 3.01 % of dry weight. and a low C/N ratio. The sewage sludge was provided from the municipal wastewater treatment plants located at the region of Sfax-Tunisia. This waste had high nitrogen and total phosphorous contents with relatively high mineral elements (P and Mg) and heavy metals (Fe, Cu, Zn) contents. The algae (Posidonia) have been collected from the Sidi Mansour beach of the Sfax region, Tunisia. This material was valorized for its high biodegradable OM, this was essential for boosting the composting process.

The olive mill waste, collected from the storage lagoon in Agareb (Sfax commune), had high OM and nitrogen contents with high mineral elements concentration, and the almond shells were obtained from the confectionary ‘‘Triki-Le Moulin’’ in Sfax (Tunisia). It included the highest total OM content and the lowest nitrogen content of all the experimented raw materials. These substrates with their specific characteristics were well mixed and aerated by mechanical turning to keep the moisture range of 45 %–60 % (Table 3).Table 3 Physico-chemical characteristics of the composted raw materials expressed on dry weight basis.

Table 3Parameters	Dry olive cake	Fresh olive cake	Poultry manure	Sewage sludge	Olive mill solid waste	Almond shell	
pH	4.90 ± 0.12	6.80 ± 0.07	8.40 ± 0.07	6.70 ± 0.08	5.80 ± 0.07	6.50 ± 0.16	
EC (mS/cm)	1.28 ± 0.01	2.34 ± 0.02	8.63 ± 0.16	8.64 ± 0.33	8.29 ± 0.04	1.25 ± 0.03	
OM (%/)	90.72 ± 3.08	91.61 ± 2.45	61.98 ± 0.83	59.49 ± 0.72	73.67 ± 1.74	98.03 ± 1.87	
TNK (%)	1.00 ± 0.01	1.07 ± 0.01	3.09 ± 0.04	4.01 ± 0.03	2.07 ± 0.06	0.210 ± 0.007	
C/N	52.80 ± 1.84	49.80 ± 1.32	11.70 ± 0.15	8.60 ± 0.10	20.70 ± 0.65	269.10 ± 7.21	
P2O5 (%)	0.18 ± 0.01	0.17 ± 0.01	3.77 ± 0.11	3.69 ± 0.08	0.58 ± 0.02	0.01 ± 0.01	
Ca (%)	1.27 ± 0.03	1.53 ± 0.04	15.78 ± 0.37	9.55 ± 0.09	2.31 ± 0.03	0.61 ± 0.01	
K (%)	0.35 ± 0.01	1.33 ± 0.04	3.24 ± 0.08	0.88 ± 0.01	3.11 ± 0.05	0.50 ± 0.01	
Mg (%)	0.07 ± 0.01	0.25 ± 0.01	0.76 ± 0.02	1.48 ± 0.01	0.49 ± 0.01	0.07 ± 0.01	
Fe (mg/kg)	734.00 ± 16.88	412.00 ± 6.34	718.00 ± 14.14	7802.00 ± 25.74	3665.00 ± 44.34	164.00 ± 2.19	
Mn (mg/kg)	10.00 ± 0.18	18.00 ± 0.47	344.00 ± 10.90	118.00 ± 3.15	71.00 ± 0.95	6.00 ± 0.06	
Cu (mg/kg)	11.00 ± 0.16	11.00 ± 1.77	61.00 ± 1.00	289.00 ± 3.95	42.00 ± 1.33	4.00 ± 0.10	
Zn (mg/kg)	10.00 ± 0.26	9.00 ± 0.16	385.00 ± 10.20	1049.00 ± 9.86	42.00 ± 1.31	6.00 ± 0.15	

During the thermophilic phase, the compost pile was turned mechanically twice each week, and once a week throughout the maturation period. To monitor the process, temperature was measured at different levels of the pile using an automatic thermometer. The end of the biological process was reached after 6 months, when the temperature of the pile was close to that of the ambient (approximately 30 °C) and re-heating did not occur.

The manure, the compost and the amended soils physicochemical characterizations were determined according to AFNOR standards [51]. The electrical conductivity (EC) and the pH were analyzed in a 1:10 (w/v) water-soluble extract. The total nitrogen content was determined by the Kjeldahl method, and the TOC content was estimated by oxidation with excess of dichromate in a sulfuric acid medium, and its determination by back titration of residual dichromate with ferrous sulphate [52]. The cationic exchange capacity (CEC) was determined by the percolation method using ammonium acetate buffer at pH 7, as described by Pansu and Gautheyrou [53].

2.3 Humic and fulvic acids extraction

The HA and FA were extracted from the manure, compost and soil samples according to the extraction method described by Stevenson [54]. The solutions of 0.1 M Na4P2O7 and 0.1 M NaOH were added to air-dried 2 mm-sieved samples at a ratio of 5:1. The mixture was mechanically shaken, and the supernatant was then separated from the residue after centrifugation at 6000g for 20 min and according to Amir et al. (2005) [55] the NaOH extraction would be repeated several times until colorless supernatants were obtained. The combined alkaline supernatants were acidified with concentrated HCl to pH ≈ 1. The solution was kept for 24 h at + 4 °C and then the acido-soluble fulvic acids were separated from the non acido-soluble HA by centrifugation at 6000g for 20 min.

The HA fraction was purified by washing the medium several times with distilled water. The HA and FA fractions were passed through a silver-membrane filter pore size of 53 μm to separate particulate OM and then dialyzed against distilled water with a Spectra-por membrane (1000 Da) to eliminate the salts excess. The resulting HA suspension was freeze-dried and kept for chemical and spectroscopic analysis.

2.3.1 Humic substances analyses

The OC in the HA and the FA extracts from manure, compost and soil were determined in triplicate using a multi N/C 2100 analyzer (Analytik Jena, Germany), and ash content (%) in the freeze-dried HA fraction was obtained after ignition at 550 °C.

The humification rate and index as well as the carbon content ratio of HA and FA were calculated as follows [56]:(1) Humification rate: HR (%) = C(HA+FA) / TOC × 100

(2) Humification index: HI (%) = CHA / TOC × 100

(3) Carbon HA / Carbon FA ratio = CHA / CFA

The E4/E6 ratio was calculated as the absorbance ratio measured at 465 nm and 665 nm of the solution prepared by dissolving 3.0 mg of each HA fraction in 10 mL of NaHCO3 0.05 M [57].

The HA Fourier transform infrared spectra were recorded over the range of 4000-400 cm−1 on prepared pellets using 2 mg of HA and 100 mg of dried KBr, which were placed under vacuum and gradually pressed to reach 6 tons within 10 min. The FTIR spectra were obtained using a PerkinElmer 1720 FTIR spectrophotometer equipped with an MIR TGS detector (Spectrum 100 Instrument, PerkinElmer Inc).

2.4 Statistical analyses

The statistical data analysis and the correlation coefficients were determined using the SPSS 19.0 program for Windows. The experimental data were expressed as means and standard deviations and were compared statistically using one-way ANOVA. Significant differences between the treatments were analyzed using Fisher test at a level of p < 0.05 to determine the effect of compost and manure application on soil parameters at the upper and the lower soil layers, and the differences between soil parameters during the experimentation progress.

A two-way ANOVA and partial eta squared were used to perform and assess respectively, the statistical significance of the tested factors and their relative contribution to statistically explain the obtained results.

Pearson correlation procedure (p < 0.05) was performed to establish the soil parameters correlation. A principal component analysis (PCA) determined potential correlations between soil humic substances properties, humification parameters and soil depth.

3 Results and discussion

3.1 Manure and compost characteristics

The final compost was characterized by its relatively high OM and total nitrogen contents when compared to manure, and a C/N ratio around 16, reflecting its stability (Table 4). Both of the manure and compost had high macronutrient contents playing an important role in improving soil fertility. The CHA/CFA ratio, the humification rate and index of the compost were higher than those of the manure. These data revealed that compost included high polymerization degree and high molecular weight HA, confirming an advanced humification level, and thus the humic substances stability (Table 4).Table 4 Physico-chemical characterization of the manure and compost.

Table 4Parameters	Manure	Initial mixture	Final compost	
pH	8.17 ± 0.03a	7.56 ± 0.05	7.86 ± 0.09b	
EC (mS/cm)	5.63 ± 0.12a	6.37 ± 0.14	9.07 ± 0.27b	
OM (%)	28.23 ± 1.12a	64.92 ± 1.07	57.62 ± 1.48b	
OC (%)	20.23 ± 0.57a	43.57 ± 0.23	28.30 ± 0.40b	
TNK (%)	1.09 ± 0.02a	1.87 ± 0.01	1.92 ± 0.07b	
C/N	19.83 ± 0.15a	23.19 ± 0.39	15.89 ± 0.55b	
CEC (cmol/kg)	45.71 ± 1.21a	20.34 ± 0.54	22.53 ± 0.77b	
PT (g/kg)	4.79 ± 0.12a	5.86 ± 0.09	10.04 ± 0.06b	
Ca (g/kg)	8.39 ± 0.08a	29.49 ± 0.27	34.65 ± 0.46b	
K (g/kg)	8.66 ± 0.17a	10.35 ± 0.35	11.92 ± 0.42b	
Mg (g/kg)	3.21 ± 0.08a	4.13 ± 0.06	5.76 ± 0.03b	
HA (%)	0.35 ± 0.01a	0.38 ± 0.02	0.69 ± 0.01b	
CHA/CFA	1.35 ± 0.02a	1.60 ± 0.01	2.38 ± 0.01b	
HR (%)	3.02 ± 0.02a	1.40 ± 0.02	3.46 ± 0.03b	
HI (%)	1.73 ± 0.02a	0.87 ± 0.01	2.44 ± 0.03b	
E4/E6	4.87 ± 0.03a	5.98 ± 0.01	4.24 ± 0.02b	
EC, electrical conductivity; OM, organic matter; OC, organic carbon; TNK, total nitrogen Kjeldahl; CEC, cation exchange capacity; PT, total phosphorous; HA, humic acids; CHA, humic acids carbon; CFA, fulvic acids carbon; HR, humification rate; HI, humification index; E4/E6, absorbance ratio at 465 nm and 665 nm.

Different letters indicate significant differences (p < 0.05).

3.2 Compost rate effects on CHA/CFA ratio

3.2.1 Initial amendment effects

At the 10 % compost amended soil upper layer, the CHA/CFA ratio increased significantly by 1.26 folds while compared to that of the control soil (Fig. 1). This increase was significantly higher than that issued from the manure applied at the same ratio (1.12 folds) (p = 0.008). This could be explained by the initially compost HA content (0.69 %) which was around 2 folds that of the manure (Table 4). Furthermore, the compost ratio increase contributed to a significant CHA/CFA ratio rise, thus confirming its maturity and stability, which enriched the soil in HA fraction [58]. Moreover, for all the amendments (compost and manure), the CHA/CFA ratio increased from 1.01 folds (at 40 % compost treatment) to 1.25 folds (at 10 % compost treatment), with soil depth, considering 0–15 to 15–30 cm layers. This result could be related to the HA interaction with the mineral particles surface modifying the wettability, the surface charge and the particles aggregation. In this perspective, Kloster and Avena [59] linked this accumulation of the humic material to the HA association to the soil mineral fraction, which would develop the clay-humic complexes. This process has a significant effect on SOM conservation and carbon sequestration; it exhibited the positive impact of using compost as a product of waste management.Fig. 1 CHA/CFA of control soil, manure amended soil (10 %) and compost amended soils at the three experimented rates (10 %, 20 % and 40 %) in soil layers. Different uppercase letter (for a given treatment), and lowercase letter (for the same soil layer) indicate significant difference at p > 0.05.

Fig. 1

3.2.2 Polymerization degree progress at the amended soils

At day 90 of the experiment, both of the amended soil layers showed a significant increase in CHA/CFA ratio values compared to the control soil and to the initial experimented soil state (p < 0.05), except for the high compost ratio treatment (40 %) at the upper layer (Fig. 1). The CHA/CFA ratio increase, also known as the ‘‘degree of polymerization”, reflected that the HA were synthesized with time from simple molecules (FA) in all the soil layers and at the different experimented amendments and ratios, as recently pointed out by Shan et al. [58].

Moreover, Grishina [60] considered the ratio of CHA/CFA to determine the humus typology: humic (CHA/CFA > 1.5), fulvic-humic (CHA/CFA = 1.0–1.5), and humic-fulvic (CHA/CFA = 0.5–1.0). In the present study, the lowest CHA/CFA ratio values were observed in the control soil (CHA/CFA = 1.26). The CHA/CFA > 1 exhibited that an important part of carbon was present in the HA fraction compared to FA. Consequently, compost and manure amendments significantly increased the CHA/CFA ratio. This indicated an intensification in the carbon associated to the HA fraction, which underwent a humus type change from fulvic-humic in the control to humic in manure and compost amended soils at the end of the experiment. However, the noticed HA fraction increase was relatively low while compared to the large OM amount applied. Such improvement was in correlation with the experimented amendment quality (compost or manure) (Table 4). Indeed, the compost was characterized by a high total carbon content of 28.30 %, but only 3.46 % (<10 %) was associated to NaOH extractable humus-like substances (CHA + CFA) [61]. The manure had a total organic carbon (TOC) of 20.23 %, but only 3.01 % (<10 %) was associated with humus like substances.

At 40 % compost treatment, CHA/CFA ratio value decreased with the time progress. This could be the result of a significant increase in the CFA content simultaneously with only slight change in the CHA concentration, as previously found by Debska et al. [62]. The relatively low value of this ratio (1.68) could be attributed to the soil intensively fertilized with 40 % compost. According to Zhang et al. [63], the FA formation is affected by the compost amendment, which provides the primary material for humification, while microbial activity influences the FA formation. In the present study, this low CHA/CFA value when 40 % compost was applied would be the consequence of the lignocellulosic fraction characterizing the olive-pomace compost and the initial humification stages where microbial activity was enhanced by the compost rate providing carbon source for its growth [64].

At 15–30 cm layer, the CHA/CFA ratio increased significantly in amended soil compared to the upper layer. This fact could be caused by the physical retention of particulate OM, expected from the substrate particularly rich in carbonates, as described by Balesdent et al. [65]. In the soil, calcium originating from CaCO3, may improve the stability of organic C retention at the mineral’ surfaces, this could affect the SOM and the aggregates stability, as indicated by Dou et al., 2023 [66]. In the present study, the particulate OM included in compost fine-fraction would be protected by the soil carbonates, which exceeded 18.34 %.

Additionally, when considering the CHA/CFA ratio values evolution, these decreased with time when increasing the compost ratio. This fact could be attributed to the important hard biodegradable fraction present in high compost ratios, declining the humification process. Conversely, Daouk et al. [67] attributed the low CHA/CFA ratio of compost issued from agricultural production amended soils to the important labile OM and its reactivity fraction. Indeed, Rivero et al. [68] indicated that the decrease of the CHA/CFA ratio in the compost-amended soil (obtained from water extraction) was due to the OM decomposition, which affected this ratio, and its ionization degree. Moreover, the crop could also contribute to this humic substances progress through the plant debris input, affecting the carbon cycle. Otherwise, plant residue can increase soil carbon pools [69,70]. As mentioned by Menichetti et al. [71], plant roots has a longer residence time in soils than manure-derived C inputs, and therefore it interacts in effectiveness at maintaining and accumulating the SOM. As a consequence, this ratio depended on the soil texture and its properties, as well as the climate, influencing the soil C-cycle.

3.3 Humification indices evaluation

3.3.1 Humification index (HI) and humification ratio (HR) at the initial time

The HR and the HI may assess the soil OC humification. At the experiment beginning and at the 0–15 cm layer depth, the humification parameters (HR and HI) did not exhibit any significant difference (p > 0.05) at compost or manure amended soils when applied at the same ratio (10 %), and compared to the control soil (Fig. 2a, b). Moreover, when compared to the initial control soil value, the HR decreased by 54.6 % and 67.8 %, while the HI decreased by 44.6 % and 59.4 % for the 20 % and 40 % compost ratios, respectively. These findings could be ascribed to the soil enrichment by the hard biodegradable fraction (lignin) with the applied compost ratio increase. This result was unexpected since while considering the TN content, the C/N ratio (Table 4) and the important fraction of lignin-type carbon of the olive cake compost [72], an OM humification is predictable to occur. Indeed, generally the OM addition causes SOM humification (HI and HR increase), and soil C and N can only be assimilated, and recycled through the microbial biomass. Moreover, as mentioned by Gomez-Muenoz et al. [73], when recalcitrant compounds are available, the telluric microorganisms start decomposing the most recalcitrant compound (compost lignin-C with a higher C/N ratio) and consequently, nitrogen immobilization occurs hindering the humification process. Furthermore, compost included different particle size fractions, which when incorporated to soil, induced differences on the distribution of the carbon content related to the HA and the FA. Therefore, these effects may contribute to interpret the results found while considering the 20 % and 40 % compost application. According to Rossi and Beni [74], the HR and HI decreases is due to the amounts of not-humic extracted carbon provided by composted sludge amendment.Fig. 2 (a) Humification ratio (HR) and (b) humification index (HI) of control soil, manure amended soil (10 %) and compost amended soils at different rates (10 %, 20 % and 40 %) in soil layers. Different uppercase letter (for a given treatment), and lowercase letter (for the same soil layer) indicate significant difference at p > 0.05.

Fig. 2

Furthermore, at 15–30 cm layer, the humification parameters were significantly higher at all the amended soils than at the 0–15 cm layer, where the highest values were recorded at 20 % compost amended soil and the lowest values appeared in the manure amended soil could be associated with the input of stable organic substances from the manure that are less susceptible to microbial biotransformation. Actually, at this deep layer, different trends were expressed by the HR and HI values in the amended soil with different compost ratios (10 %, 20 % and 40 %), this result suggested that SOM had high humification degree in these three compost-ratios of the investigated amended-soils.

3.3.2 Compost rate effects on the humification indices at the end of the experiment

The HR and HI values were relatively lower at 10 % manure than at 10 % compost treatment, which was probably due to low lignin fraction included in manure [73] and it's low CHA/CFA (Fig. 2(a and b)). The increase of the humification parameters after 10 % compost application could be related the olive cake compost beneficial effect on soil biological activity [75].

At 0–15 cm layer, the compost ratio increase contributed to HR and HI decrease (p = 0.001) at the end of the experiment, suggesting that the soils amended with high compost ratios (20 % and 40 %) presented a relatively hard biodegradable fraction (the lignin fraction) that required longer time to be humified and bioconverted into HA [58]. The same effect was noticed by Gigliotti et al. [76] who reported that in the 0–30 cm the HI and HR values decreased respectively by 3.89 % and 32.03 % after 6 years of urban waste compost application (540 t ha−1), compared to the control clay-loam soil. This compost ratio was about 4.7 folds higher than that of the 40 % compost ratio (113 t ha−1) applied in the present study, explaining the relatively low decrease noticed in this sandy-clay-loam experimental soil (isohumic soil) comparing to that recorded for the clay-loam soil. The discrepancies between the findings in the study and those reported by Gigliotti et al. [76] may be attributed to the compost specific characteristics. Indeed, olive pomace composts have a high proportion of C in the form of lignin, and lignocellulosic that is more difficult to degrade than urban composts. Hence, the organic amendments (the compost or the manure) would affect the microbial and the chemical processes in the soil. Consequently, it may be concluded that the humic quality of recycling the agro-waste by composting field would be better than that of the municipal solid waste [76,77]. In addition, the compost derived from olive mill cake would include coarse particles (2–6 mm) with an important ligniocellulosic fraction [33]. This fraction a recalcitrant form of carbon could be protected from decomposition by physical mechanism (the preservation of this fraction), which leads to the accumulation of humic materials as a way to maintain the C-stocks in amended soils [21]. In this context, Wickings et al. [78] found selective preservation of lignins in corn and grass litters during three growing seasons, and the selective preservation became more pronounced over that time. In the present study, the continual presence of this fraction might be considered characteristic of the humification processes in Mediterranean ecosystems [79,80], playing a role in the accumulation of stable C forms in soil.

Compared to the initial time, the amended soils showed similar HR values, except for 10 % manure, and all treatments increased their HI significantly. This result could be attributed to the high soil carbonate content (19.25 %) which may protect the native SOM against degradation by physical retention at deep soil [81]. The humic acids could interact with Ca and form calcium-humate. Therefore, these recalcitrant substrates persisted within a profile and might accumulate through time in a process of selective selection of OM [82].

Furthermore, at 15–30 cm layer, the humification parameters values (CHA/CFA ratio, HR and HI) were significantly higher than those at 0–15 cm layer in all the experimented soils. In the studied soil where carbonate content is exceeding 15 %, the Ca may contribute to a strong association between HA and mineral surfaces. This could be a significant factor to Ca-enhanced OM accumulation in amended soil, as described by Barreto et al. [83]. Moreover, the different microenvironments within the soil layers could be related to the soil humification deference between the soil layers. According to Purtova et al. [10], the anaerobic conditions hamper the biological activity and the OM mineralization. This effect enhanced HA accumulation in the deep soil layers, improving the HA polymerization degree and as a result, SOM stability was higher at this deep layer. Daouk et al. [67] reported that at the 15–30 cm layer, the humification parameters increase was attributed to rapid downwards migration of the humic substances. These authors explained this phenomenon by the important irrigation and the sandy soil quality as enhancing factors.

3.3.3 The compost and manure effects on the humification progress

At day 90 of the experiment, the HI and HR were significantly increased at the upper-layer compost amended soils comparing to the values recorded initially (Fig. 2(a and b)). Indeed, the HR values progress were of 1.41, 1.75 and 2.10 folds at respectively 10 %, 20 % and 40 % compost-ratios amended soils; and the HI values progress were of 1.63, 1.94 and 2.00 folds at respectively 10 %, 20 % and 40 % compost-ratios amended soils. However, in the 10 % manure amended soil, the HR decreased by 10.2 % and the HI increased by only 6.4 %. Indeed, the compost HR and HI were higher than those of the conventional manure. Actually, these data revealed that the compost HA fraction contains highly humified organic compounds that are resistant to degradation, confirming an advanced humification level and the humic substances stability. These results highlighted that the compost amended soil had an improved humification level compared to the manure amended soil. Considering the raw materials composted, the HI and HR could evaluate the humification status of OM from soil and compost, the efficiency of the process, suggesting the compost beneficial effect on soil quality compared to manure.

3.3.4 Humification indicators progress trends versus compost rate application

The HI progress with time were respectively by 63.43 %, 93.65 %, 99.57 % at 10 %, 20 % and 40 % compost amended upper soil layers. At this upper layer, the HR and the HI increases versus compost ratio displayed linear mathematical models, respectively:(4) Percentage increase HR = 38.19 HR - 40.73; R2 = 0.994

(5) Percentage increase HI = 32.60 HI - 17.1; R2 = 0.868

This improvement correlation indicated that the SOM humification process increased with compost ratio during the experiment in a greenhouse culture-system. In fact, the HA sorption effect would be promoted by the greater specific surface area of the fine compost fraction. In the present study, the investigated compost was rich in fine particles, which presented a surface area more important than that of the coarse fraction, and would contribute to increase the amount of the HA sorption to soil mineral phase. Indeed, it was reported that at the end of the composting process, the compost fine particles (<2 mm) represented more than 60 % of the amendment mass in a comparable amendment condition [84]. As a result, interactions between the native soil and the compost HA would be improved, contributing to the accumulation of the HA fraction in the soil. In this context, Kleber and Johnson [21] underlined that sorption interactions between SOM and mineral surfaces hindered the OM decomposition. In addition, the high compost rate application probably increased the recalcitrant fraction in soil (lignin fraction). The soil conditions may affect the lignin decomposition, which seemed to be selectively preserved in the studied soil. In this context, Gul et al. [85] studies’ exhibited the relationship between the decomposition of lignin and the soil conditions, and the soil management practices, which can favor lignin preservation in soil.

During the experiment time, the HR and HI increased at the 15–30 cm layer for all the compost treated soils (Fig. 2(a and b)). Consequently, the humification process progress with time was more intense at the 0–15 cm than at the 15–30 cm layer, probably attributed to the important aerobic microbial activity after the organic amendment addition [3,86].

3.4 E4/E6 ratio variation

3.4.1 Initial E4/E6 ratio

At the beginning of the experiment, the same compost and manure ratio of 10 % treated soils showed a significant lower E4/E6 ratio values than in the control superficial layer (p < 0.05) (Fig. 3). This indicated that compost and manure contributed to a soil HA rising rate, characterized by a high degree of aromatic-C polycondensation and molecular weight fraction. This indicated the positive effect of manure and compost amendment in improving the humification level. According to Kumada [87], low E4/E6 values are correlated to high aromatic carbon molecular weight and condensation. Furthermore, the E4/E6 ratio was increased significantly with compost ratio applied. Indeed, this effect was multiplied by 1.64 folds and 1.06 folds, respectively when doubling the amendment rate from 10 % to 20 %, and from 20 % to 40 %. This result would be probably due to the low molecular weight and the less polymerized fraction dominance. According to Kleber and Jhonson [21], E4/E6 ratio provides an index for the extent to which decomposition has progressed. High compost rate application (20 % and 40 %) could affect the soil microbial activity. The decrease of the decomposition progress could be responsible for the decrease in the humification degree. This was specifically intensified in greenhouse culture-system conditions. This finding is in line with Santos et al. [88] results who reported an E4/E6 ratio increase in sewage sludge amended soil after 7 years in a field-experiment. They attributed this effect to a low humification degree, due to the presence of a newly formed humic substances (HS) and HA fractions, since this sewage sludge was characterized by low humification degree. More recently, Song et al. [89] reported that the E4/E6 ratio increase reflected that HA became more aliphatic and that such ratio was related to the increased soil OC and HS fractions in the soils receiving manure. These findings showed that when comparing control soil to 20 % and 40 % compost treated soils; the E4/E6 ratios were statistically equal. Consequently, 20 % and 40 % compost ratios application had positive effects on the soil HS quality, considering the molecular weight and the polycondensation degree of the aromatic carbon.Fig. 3 E4/E6 ratio of control soil, manure amended soil (10 %) and compost amended soils at different rates (10 %, 20 % and 40 %) in soil layers. Different uppercase letter (for a given treatment), and lowercase letter (for the same soil layer) indicate significant difference at p > 0.05.

Fig. 3

3.4.2 Three months after the amendment application

After 90 days, a E4/E6 ratio decrease was noticed at all the experimented soils, varying from 11 % (control) to around 52 % (20 % compost amended soils), at 0–15 cm layer, suggesting a progressive HA molecular-size enhancement with the added amount of manure or compost, characterizing humification. Furthermore, the E4/E6 ratio was lower at 15–30 cm than at 0–15 cm layer, informing about HA accumulation over time, characterized by high aromatic carbon condensation and high molecular-weight compounds establishment, especially at the deep soil layer, where important HA aromatic structures were observed (20 % compost ratio) [7].

At the 20 % and 40 % compost treated soils, the E4/E6 ratio drop with time was by 52.4 % and 49.4 % respectively at 0–15 cm layer. This effect confirmed that the high molecular weight components were enhanced with progressively increasing ratios of added compost (20 % and 40 %), indicating that compost incorporation in soils increased the soil humus molecular weight. This fact is consistent with the data obtained by the HR and the HI values in the present study.

These results are in agreement with previous works which reported that the E4/E6 ratio is related to the polycondensation degree, the molecular weight and the HA humification degree [7]. However, at 15–30 cm layer, the E4/E6 ratio values increased with time particularly at 20 % and 40 % compost ratios amended soils in relation with the partial incorporation of rich N and low molecular weight compounds as well as HA rich functional groups content [54,68]. Indeed, as mentioned by Campitelli et al. [90], the E4/E6 value suggested the aggregation level, with high values indicating a more aggregated humic macromolecules, a greater aliphatic character, less molecular condensation and therefore an incomplete humification degree. It is also a mean to control the OM biodegradation and HS synthesized. In the present study, these different evolution patterns would depend on the soil biomass distribution in the different soil layers, and its activity, affected by the amendment type and ratio, as well as by the environment oxygenation and moisture.

At the 15–30 cm layer, the E4/E6 ratio values increased by 2.05 and 1.08 folds at 10 % manure and compost amended soils, respectively when compared to the control soil, and were statistically equal at these same amended soils ratio (manure and compost). Furthermore, for both of the amendments (compost and manure), the E4/E6 ratio decreased significantly with depth (p < 0.001) for more than 1.1 folds and 1.6 folds respectively from 0 to 15 to 15–30 cm, revealing an intense humified OM and condensed aromatic structures at the deep layer.

3.4.3 E4/E6 ratio progress with time evaluation

Considering the E4/E6 ratio values progress with time, the decrease was more important and significant (p = 0.01) in 10 % compost-amended soil (36.3 %) than in 10 % manure-amended one (33.9 %) at 0–15 cm layer. Consequently, the difference of compost and manure maturity degree could have an effect on the SOM humification and the HA polymerization degree. Besides, the E4/E6 ratio drop with time was more important at 0–15 cm than at 15–30 cm layers at 10 % compost treatment, probably linked to the dominance of the aromatic constituents in the HA structure, at this upper layer. This reflected the compost quality and the effectiveness of the waste materials biotransformed into compost. Indeed, the compost produced derived from olive mill cake with relatively high OC content in the fine fraction, that improved the OC sorption to soil mineral matrix as well as the HA polycondensation and the degree of aromaticity through humification. In this perspective, Santos et al. [88] indicated that a difference in clay content between the 0–20 cm and 20–60 cm depths of around 7 % interfered significantly with the detection of the humic properties, such as the humification degree. According to Angst et al. [91], the SOM stabilization is directly related to the soil texture and mineralogy. However, conversely to what was recorded at the same compost ratio (10 %), manure induced E4/E6 ratio drop with time more important at 15–30 cm than at 0–15 cm layers. Thus, soil amended with manure has more stable humic compounds at 15–30 cm and therefore a better impact on the soil structure. Despite the differences between compost and manure, both of them induced an interesting humification but with different degrees. A two-way ANOVA (Depth × Time) was performed, coupled with a partial eta squared analysis to extract the main effects of each factor on the OM humification level, the results of the tested soil variables are presented in Table 5. The highest significant p-value was depth. This analysis confirmed that the soil depth have the most significant relevance affecting SOM humification.Table 5 Two-way ANOVA results of soil variables. PES: Partial eta squared.

Table 5Factor	p-value	PES	
Depth	<0.001	0,465	
Time	0.001	0,178	

3.5 FTIR spectroscopy characterization

3.5.1 Initial HA FTIR spectra characteristics

The FTIR spectra of the HA extracted from both of the studied layers of the control, the manure and the compost treated soils, exhibited similar absorption bands, but with different intensities (Fig. 4). The spectra assignment, in Table 6, and interpretation were principally based on the previous studies [[92], [93], [94]].Fig. 4 FTIR spectra of humic acids extracted from control soil, 10 % manure amended soil and 10 %, 20 % and 40 % compost amended soils at 0–15 cm (a) Initially (at day 0 of the experimentation), (b) After 90 days (at day 90 of the experimentation); and at 15–30 cm soil layer (c) Initially (at day 0 of the experimentation), (d) After 90 days (at day 90 of the experimentation).

Fig. 4

Table 6 Main absorbance bands in FTIR spectra and their assignments in HA amended soils.

Table 6Wavenumber (cm−1)	Assignment	
3200–3600	O–H vibration of the hydroxyl group of various functional groups and N–H stretching of amides and amines	
3100	Stretching C–H of aromatic bands	
1730	C=O stretching of carboxylic and ketonic groups
Unconjugated C=O in xylans (hemicellulose)	
1670–1640	Aromatic C=C vibrations and C=O stretching of the amide groups (amide I band)	
1420–1450	C–H deformation of lignin and carbohydrate structures and/or phenolic and COO− groups asymmetric stretching	
1040	C–O stretching of polysaccharides groups	
950–1020	Aromatic C–H deformation and stretching vibration of Si–O groups	

Initially, five main bands of the HA extracted FTIR spectra were evidenced in the 0–15 cm layer. First, an intense broad band at 3460 cm−1 attributed to O–H vibration of the hydroxyl group of various functional groups and N–H stretching of amides and amines (Fig. 4a). Among all treatments, no absorption band near 2920 cm−1 (due to C–H stretching vibrations of fatty acids, waxes and various aliphatic components: CH2 and CH3 groups, and characteristic for olive pomace composts) was recorded at soil HA FTIR spectra.

The second band at approximately 1645 cm−1 characterized the aromatic C=C vibrations and C=O stretching of the amide groups (amide I band). The third bands, recorded at 1395-1444 cm−1, were ascribed to C–H deformation of lignin and carbohydrate structures and/or phenolic and COO− groups asymmetric stretching.

The fourth band observed at about 1111 cm−1 was attributed to polysaccharides C–O stretching. The last shoulders observed at about 600–900 cm−1 could be ascribed to aromatic C–H deformation [7]. Compared to the control soil, the FTIR spectra showed that after the organic amendment addition, the phenolic O–H groups and N-including groups, the aromatic C=C functions, the C–H lignin and COO− groups and the C–O polysaccharides groups increased, especially at compost treated soils (10 %, 20 % and 40 %). This was due to compost HA richness in these functional moieties (Fig. 5b), compared to the manure (Fig. 5a). Indeed, the HA isolated from pomaces are characterized by high contents of phenolic OH groups, N-containing components and polysaccharides groups [7,84]. The absence of the aliphatic structure could be due to a preferential biodegradation of fatty acids component and aliphatic structures (CH2 and CH3 groups); which could induce that in the present study, the soil HA included a high aromatic compound in their structure [95]. Furthermore, the increase at 1420 and 1044 cm−1 indicated the possible contribution of the ash content to this band. Indeed, the ash content in the HA fractions was of 1.23–2.68 %.Fig. 5 FTIR spectra of humic acids extracted from (a) Manure and (b) Compost.

Fig. 5

At the 15–30 cm layer, the HA FTIR spectra exhibited high intensity attributed to phenolic O–H groups and aromatic carbon, compared to those recorded at the upper layer (Fig. 4c). However, the band intensity of polysaccharides groups was low, revealing a relatively low microbial activity under low oxygenation, at this deep layer.

3.5.2 HA structural specifications after 90 days

At 90 days of the experiment, the O–H of various groups bands (3364 cm−1), the aromatic C=C (1649 cm−1), the oxygenated groups (1430 cm−1) and the polysaccharides structures (1031 cm−1) (Fig. 4b) were higher at the 10 % compost than at the manure-treated soils, at the upper layer, which could be correlated with the various organic substrates biodegradability's. Indeed, the compost treated soil showed the highest OM reactivity, probably because of the typology of its composted materials, lignin-rich materials (olive mill cake) which is resistant but a proportion is degraded mostly under aerobic conditions yielding large amounts of aromatic, phenolic and carboxylic acid (COOH) [96]. Moreover, the contribution of ash contents to the FTIR spectrum at 1031 cm−1 band is probable.

In addition, the highest HA bands intensity was exhibited in the 10 % and 20 % compost amended soils. However, at the 40 % compost ratio, the carbohydrates and oxygenated groups, as well as polysaccharides substances relative absorption intensities were the lowest, suggesting a less OM decomposition degree [97].

In all humic acid spectra, there are wide bands between 3200 and 3100 cm−1 indicating the presence of the aromatic C–H band (at 3100 cm−1) [98,99] and carboxylic and ketonic (C=O) groups appears also in the band centered at 1775 cm−1; which suggested their increase in the HA structure through the humification process. According to Medina et al. [100], the high carboxylic acidic groups in humic substance increased the surface charge and the amount of adsorbed minerals and/or metals, thus promoting the aggregate formation of the humic substances-minerals and/or the humic substances-metals complexes.

At the compost amended soils, the high band intensities recorded at 880 - 905 cm−1 confirmed the high aromatic character, reflecting the HA stability [21]. Indeed, aromatic C may be adsorbed to minerals in the coarse silt/sand fractions and remained there [101].

Considering the experiment time-progress, the compost and manure applications changed the HA chemical and structural characteristics in the soil upper layer. The main characteristic is the increase in the band's intensities, particularly at 10 % and 20 % compost-amended soils. Furthermore, at a 10 % ratio, the FTIR spectra indicated that various functional groups were present at higher relative concentrations in compost HA fraction than that in manure. This reflected that the SOM humification and its transformation processes might differ depending on the organic amendment type (compost or manure). The O–H and N–H stretching of the various functional group bands (3364 cm−1) increased and shifted towards low frequencies due to the N–H peptide low contribution to these bands [102]. Two new bands at 3100 cm−1 (aromatic structures) and at 1750 cm−1 (carboxylic groups) were present, and an increase in C=C aromatic and C=O ketonic structures (at 1650 cm−1) and in C–H aromatic groups (at 878 cm−1) were noticed. An increase in polysaccharides was also observed. As reported by Gigliotti et al. [76], the polysaccharide that increased in the six-year amended soil, could be ascribed to the humification process. As a result, the humification induced aromatic carboxylic groups' formation and hence, aromatic structures accumulation.

At the 15–30 cm soil layers, the extracted HA were characterized by:- O–H and N–H stretching bands of various functional groups (3370 cm−1), which were more intense at the control than in the manure and the compost treated soils (Fig. 4d);

- A new band at 3100 cm−1 evidenced in all the treated soils, which would be attributed to aromatic C–H, revealing the progressive aromatic structures formation;

- A shoulder appeared at 1750 cm−1 having the same intensity in all the treated soils, indicating peptide and ester bonds breaking, resulting in the release of alkyl material and the persistence of free carboxyl groups, at the end of the experiment;

- Aromatic C=C and N–H amide groups (1650 cm−1) with the highest intensity was observed in the 20 % compost amended soil, reflecting the aromatic ring polycondensation;

- Intense C–H deformation of lignin and carbohydrate structure and/or antisymmetric stretching of COO− groups (1430 cm−1) revealed in all the amended soils, due to the OM oxidation and/or the concentration of undegraded lignin at 90 days of the experiment;

- The C–O polysaccharide bands (1007 cm−1) relatively higher in the amended soils than in the control soil, particularly at the 10 % manure and the 20 % compost treated soils, reflecting the intense HA reactivity at the 15–30 cm depth; and a probable contribution of the ash content to this band;

- The C–H aromatic bands (900 cm−1) presence, which were more intense at the 10 % and the 20 % compost treatments, indicating the aromatic compounds concentration.

Moreover, a decrease in the O–H of various group bands intensity (3460 cm−1) was observed at 10 % and 40 % compost treatments, at 15–30 cm layer. This effect was different from that observed in the soil upper layer, reflecting a less oxidized OM at the deep layer. The C–H deformation of lignin and carbohydrates structures and/or antisymmetric stretching of COO− groups (1434 cm−1) and polysaccharides structures (1015 cm−1) increased indicating that during the humification process, the transformation of alcohol function (3460 cm−1) in O-containing groups, particularly carboxylic and ester groups, takes place. Furthermore, 10 % compost amended soil showed lower O–H of various groups bands (3460 cm−1), aromatic C=C (1648 cm−1), oxygenated groups (1433 cm−1) and polysaccharides structures (1015 cm−1) than manure at the same ratio application. This result indicated the low contribution of the N-including groups to these bands, which would reflect an advanced level of HA stabilization and the more condensed molecules presence in the 10 % compost treatment. Furthermore, the 10 % and 20 % compost ratios application improved the soil HA characteristics, especially their aromatic character and oxygenated groups, at the upper layer.

The HA IR spectroscopic study presented amended soil ray spectra having more intense signals at the 10 % and 20 % compost amended soils in comparison with unamended soil and 40 % compost amended soils signals including aromatic structures and carboxylic groups. Consequently, the 10 % and 20 % compost ratios represent a suitable treatment for promoting OM humification, improving the amended soil OC status under greenhouse system condition.

3.6 Soil properties correlation using principal component analysis (PCA)

A principal component analysis (PCA) was performed for the soil physico-chemical properties and the humification parameters of the manure and the experimented compost ratios. The first two PCA axes of the soil parameters accounted for 71.71 % of the observed variation. The first axis (PC1) accounted for 54.52 % and the second axis (PC2) accounted for 17.19 % of the total variability. The correlation matrix of the amended soils main physico-chemical properties and humification parameters (Table 7) showed a strong relationship between soil parameters, compost ratio and soil depth. Indeed, a high correlation coefficient between OC content and soil humic matter (HA and FA) content was noticed. Three groups were found by a plan representation of correlated parameters (Fig. 6).Table 7 Correlation matrix between the main parameters of manure compost amended soils, and physico-chemical as well as humification indicators.

Table 7	Depth	Rate	pH	EC	OC	TN	C/N	HA	FA	CHA/CFA	HR	HI	E4/E6	
Depth	1.000													
Rate	0.000	1.000												
pH	0.236	0.292	1.000											
EC	−0.349	0.098	−0.126	1.000										
OC	−0.546a	0.450	−0.234	0.619b	1.000									
TN	−0.583a	0.462	−0.255	0.592a	0.995b	1.000								
C/N	−0.346	0.470	0.150	0.481	0.602b	0.566a	1.000							
HA	−0.549a	0.472	−0.207	0.530a	0.835b	0.846b	0.718b	1.000						
FA	−0.727b	0.262	−0.304	0.700b	0.784b	0.805b	0.571a	0.840b	1.000					
CHA/CFA	0.473	0.187	0.212	−0.269	−0.175	−0.192	0.137	0.101	−0.412	1.000				
HR	0.851b	−0.187	0.275	−0.493	−0.780b	−0.807b	−0.398	−0.618b	−0.743b	0.493	1.000			
HI	0.843b	−0.060	0.317	−0.491	−0.684b	−0.711b	0.251	−0.454	−0.729b	0.680b	0.955b	1.000		
E4/E6	−0.616b	−0.017	−0.109	0.643b	0.620b	0.607b	0.330	0.325	0.492	−0.307	−0.768b	−0.737b	1.000	
Depth, soil depth; EC, electrical conductivity; OC, organic carbon; TN, total nitrogen; CEC, cation exchange capacity; HA, humic acids; FA, fulvic acids; CHA, humic acids carbon; CFA, fulvic acids carbon; HI, humification index; HR, humification rate.

a Significant at the p < 0.05 level.

b significant at the p < 0.01 level.

Fig. 6 Principal component analysis of unamended, 10 % manure amended soil, and 10 %, 20 % and 40 % compost amended soils from the two soil layers (0–15 and 15–30 cm). EC, electrical conductivity; FA, fulvic acids; HA, humic acids; HI, humification index; HR, humification rate; OC, organic carbon; TN, total nitrogen.

Fig. 6

Photo 1 Compost incorporation into the soil and the studied layers.

Photo 1

The first group presented by the soil properties: OC, total nitrogen, C/N, HA, FA, E4/E6 and electrical conductivity were positively correlated. The OC had positive impact on soil HA and FA contents, indicating that the organic amendments application significantly improved the HS fraction in the treated soils. This result is in line with that of Song et al. [89] after manure long-term fertilization.

The second group included the soil depth, the HI and the HR, with high correlation coefficients between the soil depth and the humification parameters. Therefore, lower layer displayed the highest humification degrees and SOM stability.

Moreover, these parameters were negatively correlated to the E4/E6 ratio. This confirmed that at the deep soil layer, the environment conditions such as soil moisture and oxygen availability required for telluric flora activity, enhanced the humification process and the HA formation including high molecular-weight fractions. Furthermore, these conditions, differing at the upper and the deep soil layers, promoted the HA recalcitrance and their adsorption to the soil mineral phase [7,84]. This finding confirmed that the SOM humification would be more affected by the soil environment conditions (at different soil depth) than the compost ratio incorporated.

The third group is made up of the CHA/CFA and the HI. The intermediate position of the HI could be due to the fact that both of these parameters indicated the soil HA fraction proportion. The amendment ratio as well as the soil pH didn't exhibit any correlation with the three previous mentioned groups.

It should be noticed that the present study evidenced the importance of the different investigated soil chemical parameters correlations, explaining their progress in the different soil layers, while Santos et al. [88] results' didn't exhibit any difference in the investigated parameters by PCA.

4 Conclusions

In the present study, the results showed that the application of the compost produced may generate both qualitative and quantitative changes in humic substances, according to the amendment type and the investigated ratio in a concentration dependent way.

The compost application at a low ratio (10 %) induced a marked increase in CHA/CFA ratio and humification parameters as well as a decrease in E4/E6 ratio. The FTIR spectra indicated that the compost application improved the soil HA aromatic character and polycondensation trough humification and lignin degradation, among other processes. Furthermore, the highest aromatic structure concentrations were found at the upper layer with the 10 % and 20 % compost ratio treatments. The humification process exhibited variations within the soil profile, and the deep soil layer (15–30 cm) displayed the highest humification degrees, organic matter stability, with a high positive correlation between CHA/CFA, HR, HI, and soil depth. Considering the soil carbonated state, the biosynthesized HA may interact with Ca and form humates, a recalcitrant complex, in a process of selective selection of organic matter.

Overall, with the experiment progress, the main soil HA compositional changes and structural properties modification were observed in the upper soil layer (0–15 cm) due to its subsequently intense organic matter reactivity, related to the different local environmental conditions. Therefore, the results demonstrated that the compost spread at a low concentration represents the best substrate for soil amendment, contributing to its restoration by highly humified OM, through the humification process, improving hence its quality.

Funding

The authors declare no specific funding for this work.

Data availability statement

The data that support the findings of this study will be made available on request from the corresponding author.

Data associated with this study had not been deposited into a publicly available repository.

CRediT authorship contribution statement

Saoussan Masmoudi: Writing – original draft, Investigation, Formal analysis, Data curation, Conceptualization. Wadii Abid: Writing – original draft, Supervision, Methodology. Khaled Medhioub: Writing – review & editing, Resources, Methodology, Conceptualization. Emna Ammar: Writing – review & editing, Visualization, Validation, Supervision, Resources, Methodology, Conceptualization.

Declaration of competing interest

The authors declare no conflict of interest. The authors declare that they have no competing financial assistance provided by a third party for the reported work. They have no financial interest or relationship - within the last 3 years - related to the subject matter.

Acknowledgements

This work was carried out in the Tunisian Union for Aid to Mentally Disabled "El Amal" (U.T.A.I.M), an agricultural educational centre in Sfax-Tunisia, in join with the research Laboratory of “Environmental Sciences and Sustainable Development” in the framework of its Research and Development activities through an integrated aromatic and medicinal plants multiplication and culture project. The facilities and services of the Agricultural Center "El Amal" are gratefully acknowledged. The authors are also grateful to El Kindi composting company (Tunisia) for providing the compost.
==== Refs
References

1 Siles J.A. De la Rosa J.M. González-Pérez J.A. Fernández-Pérez V. García-Díaz C. Moreno J.L. García C. Bastida F. Long-term restoration with organic amendments is clearer evidenced by soil organic matter composition than by changes in microbial taxonomy and functionality Appl. Soil Ecol. 198 2024 105383
2 Badewa E.A. Yeung C.C. Whalen J.K. Oelbermann M. Compost and biosolids increase long-term soil organic carbon stocks Can. J. Soil Sci. 103 2023 510 521
3 Abid W. Ammar E. Date palm (Phoenix dactylifera L.) wastes valorization: a circular economy approach Mediterranean Fruits Bio-Wastes: Chemistry, Functionality, and Technological Applications 2021 403 430
4 Delgado A. Chammem N. Issaoui M. Ammar E. Bioactive phytochemicals from olive oil and table olives processing by-products Bioactive Phytochemicals from Vegetable Oil and Oilseed Processing By-Products 2022 1 37
5 Khalil J. Jaafar A.A.K. Habib H. Bouguerra S. Nogueira V. Rodríguez-Seijo A. The impact of olive mill wastewater on soil properties, nutrient and heavy metal availability – a study case from Syrian vertisols J. Environ. Manag. 351 2024 119861
6 Baron V.S. Dick A.C. Lemke R. Greer K. Mapfumo E. Grazing and fertilizer, compost or manure application effects on a meadow bromegrass pasture on a thick black chernozem I. Productivity and sustainability Can. J. Soil Sci. 103 2023 510 521
7 Sellami S. Hachicha S. Chtourou M. Medhioub K. Ammar E. Bioconversion of wastes from the olive oil and confectionery industries: spectroscopy study of HA Bioresour. Technol. 28 2007 285 1298
8 Masmoudi S. Magdich S. Rigane H. Medhioub K. Rebai A. Ammar E. Effects of compost and manure application rate on the soil physico-chemical layers properties and plant productivity, Waste Biomass Valori. 11 2020 1883 1894
9 Pergola M. Persiani A. Palese A.M. Di Meo V. Pastore V. D'Adamo C. Celano G. Composting: the way for a sustainable agriculture Appl. Soil Ecol. 123 2018 744 750
10 Purtova L.N. Kostenkov N.M. Shchapova L.N. Assessing the humus status and CO2 production in soils of anthropogenic and agrogenic landscapes in Southern regions of the Russian far East Eurasian Soil Sci. 50 2017 42 48
11 Royer A.C. de Figueiredo T. Fonseca F. Lado M. Hernánde Z. Short-term effects of olive-pomace-based conditioners on soil aggregation stability Agronomy 14 5 2024 2 18
12 Rueda M.P. Domínguez-Vidal A. Llorent-Martínez E.J. Aranda V. Ayora-Canãda M.J. Monitoring organic matter transformation of olive oil production residues in a full-scale composting plant by fluorescence spectroscopy, Env Technol. Innov. 35 2024 103695
13 Vignozzi N. Andrenelli M.C. Agnelli A.E. Fiore A. Pellegrini S. Short-term effect of different inputs of organic amendments from olive oil industry by-products on soil organic carbon and physical properties Land 12 2023 1628
14 Malone Z. Berhe A.A. Ryals R. Impacts of organic matter amendments on urban soil carbon and soil quality: a meta-analysis J. Clean. Prod. 419 2023 138148
15 Abid W. Ben Mahmoud I. Masmoudi S. Triki M.A. Mounier S. Ammar E. Physico-chemical and spectroscopic quality assessment of compost from date palm (Phoenix dactylifera L.) waste valorization J. Environ. Manage. 264 2020 110492
16 Li F. Ghanizadeh H. Cui G. Liu J. Miao S. Liu C. Song W. Chen X. Cheng M. Wang P. Zhang Y. Wang A. Microbiome - based agents can optimize composting of agricultural wastes by modifying microbial communities Bioresour. Technol. 374 2023 128765
17 Ofei-Quartey M.N.L. Appiah-Efah E. Akodwaa-Boadi K. Ampaw B. Taylor T.S. Millogo Z.E.N. Enhancing the economic potential of organic waste by co-composting using ratio modelling toward a circular economy J. Mater. Cycles Waste Manag. 25 2023 1560 1580
18 Shi M. Zhao Y. Zhu L. Song X. Tang Y. Qi H. Cao H. Wei Z. Denitrification during composting: biochemistry, implication and perspective Int. Biodeterior. Biodegrad. 153 2020 105043
19 Guo X.-X. Liu H. Wu S. Humic substances developed during organic waste composting: formation mechanisms, structural properties, and agronomic functions Sci. Total Environ. 662 2019 501 551 30695750
20 Magdich S. Ammar E. Olive mill wastewater agronomic valorisation through the spreading and co composting: impacts on soil properties and olive tree performance Mediterranean Fruits Bio-wastes: Chemistry, Functionality, and Technological applications 17 2022 91 114 Chapter
21 Kleber M. Johnson M.G. Advances in understanding the molecular structure of soil organic matter: implications for interactions in the environment Adv. Agron. 106 2010 77 142
22 Banach-Szott M. Debska B. Tobiasova E. Properties of humic acids depending on the land use in different parts of Slovakia Environ. Sci. Pollut. Res. 28 2021 58068 58080
23 Attard K. Lia F. The antioxidant and bioactive potential of olive mill waste The Power of Antioxidants – Unleashing Nature's Defense against Oxidative Stress Chapter 1, 2023 1 37
24 Enaime G. Dababat S. Wichern M. Lübken M. Olive mill wastes: from wastes to resources Environ. Sci. Pollut. Res. 31 2024 20853 20880
25 Dantas Palmeira J. Araújo D. Mota C.C. Alves R.C. Oliveira M.B.P.P. Ferreira H.M.N. Fermentation as a strategy to valorize olive pomace, a by-product of the olive oil industry Fermentation 9 2023 442
26 Bouhia Y. Lyamlouli K. Fels L.E. Youssef Z. Ouhdouch Y. Hafidi M. Effect of microbial inoculation on lipid and phenols removal during the co-composting of olive mill solid sludge with green waste in bioreactor Waste Biomass. Valor. 12 2021 1417 1429
27 Hachicha S. Cegarra J. Sellami F. Hachicha R. Drira N. Medhioub K. Ammar E. Elimination of polyphenols toxicity from olive mill wastewater sludge by its co-composting with sesame bark J. Hazard Mater. 161 2009 1131 1139 18513861
28 Vaz T. Quina M.M.J. Martins R.C. Gomes J. Olive mill wastewater treatment strategies to obtain quality water for irrigation: a review Sci. Total Environ. 931 2024 172676
29 Alburquerque J.A. Gonzálvez J. Tortosa G. Ait Baddi G. Cegarra J. Evaluation of ‘‘alperujo’’ composting based on organic matter degradation, humification and compost quality Biodegradation 20 2009 257 270 18814039
30 Haddad G. El-Ali F. Mouneimne A.H. Humic matter of compost: determination of humic spectroscopic ratio (E4/E6) Curr. Sci. Int. 4 1 2015 56 72
31 Tomati U. Madejon E. Galli E. Capitani D. Segre A.L. Structural changes of humic acids during olive mill pomace composting Compost Sci. Util. 9 2 2001 134 142
32 Hu Z.-T. Huo W. Chen Y. Zhang Q. Hu M. Zheng W. Shao Y. Pan Z. Li X. Zhao J. Humic substances derived from biomass waste during aerobic composting and hydrothermal treatment: a Review Front. Bioeng. Biotechnol. 10 2022 878686
33 Serramiá N. Sánchez-Monedero M.A. Fernández-Hernández A. García-Ortiz Civantos C. Roig A. Contribution of the lignocellulosic fraction of two-phase olive-mill wastes to the degradation and humification of the organic matter during composting Waste Manage. 30 10 2010 1939 1947
34 Bueno L.A. Bosch-Rubia G. Sanchez-Montilla R. Monetta P. Use of compost from olive oil by-products as soil amendment: effect on physical and chemical properties of loamy-and soils Acta Hortic. 1057 2014 417 422
35 García-Randez A. Marks E.A.N. Pérez-Murcia M.D. Orden L. Andreu-Rodriguez J. Martínez Sabater E. Cháfer M.T. Moral R. Is the direct soil application of two-phase olive mill waste (Alperujo) compatible with soil quality protection? Agronomy 13 2023 2585
36 Adani F. Ubbiali C. Generini P. The determination of biological stability of composts using the dynamic respiration index: the results of experience after two years Waste Manage. (Tucson, Ariz.) 26 2006 41 48
37 Baffi C. Dell'Abate M.T. Nassisi A. Silva S. Benedetti A. Genevini P.L. Adani F. Determination of biological stability in compost: a comparison of methodologies Soil Biol. Biochem. 39 2007 1284 1293
38 Gomez-Brandon M. Lazcano C. Dominguez J. The evaluation of stability and maturity during the composting of cattle manure Chemosphere 70 2008 436 444 17689588
39 Inbar Y. Hadar Y. Chen Y. Characterization of humic substances formed during the composting of solid wastes from wineries Sci. Total Environ. 113 1992 35 48
40 Ait Baddi G. Alburquerque J.A. Gonzálvez J. Cegarra J. Hafidi M. Chemical and spectroscopic analyses of organic matter transformations during composting of olive mill wastes, International Biodeter Biodegrad. 54 2004 39 44
41 Spaccini R. Piccolo A. Molecular characteristics of humic acids extracted from compost at increasing maturity stages Soil Biol. Biochem. 41 2009 1164 1172
42 Diacono M. Ferri D. Ciaccia C. Tittarelli F. Ceglie F. Verrastro V. Ventrella D. Vitti C. Montemurro F. Bioassays and application of olive pomace compost on emmer: effects on yield and soil properties in organic farming Acta Agric. Scand. -Sect B 62 6 2012 510 518
43 Fang L. Lakshmanan P. Su X. Shi Y. Chen Z. Zhang Y. Sun W. Wu J. Xiao R. Chen X. Impact of residual antibiotics on microbial decomposition of livestock manures in Eutric Regosol: implications for sustainable nutrient recycling and soil carbon sequestration J. Environ. Sci. 147 2025 498 511
44 Gao S.-J. Gao J.-S. Cao W.-D. Zou C.-Q. Huang J. Bai J. Dou F.-G. Effects of long-term green manure application on the content and structure of dissolved organic matter in red paddy soil J. Integ. Agri. 17 8 2018 1852 1860
45 Li J. Xu Y. Zhang Y. Liu Z. Gong H. Fang W. Ouyang Z. Li W. Xu L. Quantifying the mitigating effect of organic matter on heavy metal availability in soils with different manure applications: a geochemical modelling study Ecotoxicol. Environ. Saf. 276 2024 116321
46 Mostafa M.A.M. Hegazy A.S.S.I. El-Sedfy O.M.F. Abd El-Rhaman Z.M. Characterization and metal loading capacity of humic acids derived from composted rice straw and olive pomace affected by the humification degree ST-J. Soil Sci. Agroclimatology 18 1 2021 1 6
47 Yang X. Zhang S. Wu D. Huang Y. Zhang L. Liu K. Wu H. Guo S. Zhang W. Recalcitrant components accumulation in dissolved organic matter decreases microbial metabolic quotient of red soil under long-term manuring Sci. Total Environ. 934 2024 173287
48 IUSS Working Group WRB World Reference Base for Soil Resources, International Soil Classification System for Naming Soils and Creating Legends for Soil Maps fourth ed. 2022 International Union of Soil Sciences (IUSS) Vienna, Austria 234
49 Usman A.R.A. Kuzyakov Y. Stahr K. Dynamics of organic C mineralization and the mobile fraction of heavy metals in a calcareous soil incubated with organic wastes Water Air Soil Pollut. 158 2004 401 418
50 Pérez-Lomas A.L. Delgado G. Párraga J. Delgado R. Almendros G. Aranda V. Evolution of organic matter fractions after application of co-compost of sewage sludge with pruning waste to four Mediterranean agricultural soils. A soil microcosm experiment Waste Manage. (Tucson, Ariz.) 30 2010 1957 1965
51 AFNOR (Association Française de Normalisation) Recueil des normes afnor, Qualité de l'eau, méthodes d'analyses 1997 Afnor, Paris
52 Anne P. Sur le dosage rapide du carbone organique des sols Ann. Agron. 15 1945 161 172
53 Pansu M. Gautheyrou J. L'analyse du sol minéralogique, organique et minérale 2003 Springer-Verlag Paris 992
54 Stevenson F.J. Humus Chemistry: Genesis, Composition, Reactions 1994 John Wiley and Sons Canada 512
55 Amir S. Hafidi M. Lemée L. Merlina G. Guiresse M. Pinelli E. Structural characterization of humic acids, extracted from sewage sludge during composting, by thermochemolysis–gas chromatography–mass spectrometry Process Biochem. 41 2 2006 410 422
56 Senesi N. Composted materials as organic fertilizers Sci. Total Environ. 81/82 1989 521 542
57 Campitelli P.A. Velasco M.I. Ceppi S.B. Chemical and physicochemical characteristics of humic acids extracted from compost, soil and amended soil Talanta 69 2006 1234 1239 18970708
58 Shan G. Lu H. Li Q. The properties and dynamic changes of DOM subfractions during food waste and sugarcane leaves co-composting Environ. Sci. Pollut. Res. 25 2018 7433 7442
59 Kloster N. Avena M. Interaction of humic acids with soil minerals: adsorption and surface aggregation induced by Ca2+ Environ. Chem. 12 6 2015 731 738
60 Grishina L.A. Humus Formation and Humus Status of Soils 1986 Moscow State University Moscow, Russia 243
61 Wu S. Stability and Maturity Evaluation of Biosolids Compost 2001 PhD Dissertation University of Florida, Gainesville, United State of America 97
62 Dębska B. Długosz J. Piotrowska-Długosz A. Banach-Szott M. The impact of a bio-fertilizer on the soil organic matter status and carbon sequestration—results from a field-scale study J. Soils Sediments 16 2016 2335 2343
63 Zhang X. Zhao Y. Meng H. Li L. Cui H. Wei Z. Yang T. Dang Q. Revealing the inner dynamics of fulvic acid from different compost-amended soils through microbial and chemical analyses J. Agric. Food Chem. 68 2020 3722 3728 32129998
64 Becher M. Pakuła K. Czapliński K. Soil organic matter quality in soils with different levels of manure fertilization Environ. Prot. Nat. Resour. 31 2 84 2020 17 23
65 Balesdent J. Chenu C. Balabane M. Relationship of soil organic matter dynamics to physical protection and tillage Soil Till. Res. 53 2000 215 230
66 Dou X. Zhang J. Zhang C. Ma D. Chen L. Zhou G. Li J. Duan Y. Calcium carbonate regulates soil organic carbon accumulation by mediating microbial communities in northern China Catena 231 2023 107327
67 Daouk S. Hassouna M. Gueye-Girardet A. Niang S. Pfeifer H.R. UV/Vis characterization and fate of organic amendment fractions in a dune soil in Dakar, Senegal Pedosphere 25 2015 372 385
68 Rivero C. Chirenje T. Ma L.Q. Martinez G. Influence of compost on soil organic matter quality under 17 tropical conditions Geoderma 123 2004 355 361
69 da Silva Santana M. Andrade E.M. de Sá Barreto Sampaio E.V. Ferreira T.O. Salviano A.M. da Silva D.J. Cunha T.J.F. Giongo V. Do agrosystems change soil carbon and nutrient stocks in a semiarid environment J. Arid Environ. 201 2022 104747
70 Heinemann H. Hirte J. Seidel F. l Don A. Increasing root biomass derived carbon input to agricultural soils by genotype selection – a review Plant Soil 490 2023 19 30
71 Menichetti L. Ekblad A. Kätterer T. Contribution of roots and amendments to soil carbon accumulation within the soil profile in a long-term field experiment in Sweden Agric. Ecosyst. Environ. 200 2015 79 87
72 Miranda I. Simões R. Medeiros B. Nampoothiri K.M. Sukumaran R.K. Rajan D. Pereira H. Ferreira-Dias S. Valorization of lignocellulosic residues from the olive oil industry by production of lignin, glucose and functional sugars Bioresour. Technol. 292 2019 121936
73 Gómez-Muñoz B. Hatch D.J. Bol R. García-Ruiz R. The compost of olive mill pomace: from a waste to a resource - environmental benefits of its application in olive oil groves Sustainable Development - Authoritative and Leading Edge Content for Environmental Management 2012 INTECH 459 484 (Chapter 20)
74 Rossi G. Beni C. Effects of medium-term amendment with diversely processed sewage sludge on soil humification-mineralization processes and on Cu, Pb, Ni, and Zn bioavailability Plants 16 2018 1 10
75 García-Ruiz R. Victoria Ochoa M. Belén Hinojosa M. Gómez-Muñoz B. Improved soil quality after 16 years of olive mill pomace application in olive oil groves Agron. Sustain. Dev. 32 2012 803 810
76 Gigliotti G. Businelli D. Giusquiani P.L. Composition changes of soil humus after massive application of urban waste compost: a comparison between FT-IR spectroscopy and humification parameters Nutr. Cycl. Agroecosyst. 55 1999 23 28
77 Cao X. Williams P.N. Zhan Y. Coughlin S.A. McGrath J.W. Chin J.P. Xu Y. Municipal solid waste compost: global trends and biogeochemical cycling Soil Environ. Health 1 2023 100038
78 Wickings K. Stuart Grandy A. Reed S.C. Cleveland C.C. The Origin of Litter Chemical Complexity during Decomposition, Ecology Letters 2012 Blackwell Publishing Ltd/CNRS 1 9
79 Miralles I. Ortega R. Sánchez-Marañón M. Soriano M. Almendros G. Assessment of biogeochemical trends in soil organic matter sequestration in Mediterranean calcimorphic mountain soils Soil Biol. Biochem. 39 2007 2459 2470
80 Semenov V.M. Tulina A.S. Semenova N.A. Ivannikova L.A. Humification and nonhumification pathways of the organic matter stabilization in soil: a review Eurasian J. Soil Sci. 46 2013 355 368
81 Duchaufour P. Pédogenèse et Classification 1977 Masson Paris 477
82 Rowley M. I Nvestigating Calcium Mediated Accumulation of Soil Organic Carbon at the Nant Valley Alpage, Vaud Alps, Switzerland 2020 Thèse de doctorat UNIL, Lausanne 305
83 Barreto M.S.C. Elzinga E.J. Ramlogan M. Rouff A.A. Alleoni L.R.F. Calcium enhances adsorption and thermal stability of organic compounds on soil minerals Chem. Geol. 559 2021 119804
84 Masmoudi S. Jarboui R. El Feki H. Gea T. Medhioub K. Ammar E. Characterization of olive mill wastes composts and their humic acids: stability assessment within different particle size fractions Environ. Technol. 34 2013 787 797 23837330
85 Gul S. Yanni S.F. Whalen J.K. L Ignin Controls on Soil Ecosystem Services: Implications for Biotechnological Advances in Biofuel Crops, in: Lignin vol. 14 2014 Nova Science Publishers, Inc. 375 416
86 Senesi G.S. Martin-Neto L. Villas-Boas P.R. Nicolodelli G. Milori D.M.B.P. Laser-based spectroscopic methods to evaluate the humification degree of soil organic matter in whole soils: a review Sci. Total Environ. 18 2016 1292 1302
87 Kumada K. Chemistry of Soil Organic Matter first ed. 1987 241 Tokyo, Japan
88 Santos L.M. Simões M.L. Melo W.J. Martin-Neto L. Pereira-Filho E.R. Application of chemometric methods in the evaluation of chemical and spectroscopic data on organic matter from Oxisols in sewage sludge applications Geoderma 155 2010 121 127
89 Song X. Liu S. Liu Q. Zhang W. Hu C. Carbon sequestration in soil humic substances under long-term fertilization in a wheat-maize system from North China J. Integr. Agr. 13 3 2014 562 569
90 Campitelli P. Velasco M. Ceppi S. Characterization of humic acids derived from rabbit manure treated by composting-vermicomposting process J. Soil Sci. Plant Nutr. 12 2012 875 891
91 Angst G. Pokorný J. Mueller C.W. Prater I. Preusser S. Kandeler E. Meador T. Straková P. Hájek T. Buiten G. Angst S. Soil texture affects the coupling of litter decomposition and soil organic matter formation Soil Biol. Biochem. 159 2021 108302
92 Miikki V. Senesi N. Hänninen K. Characterization of humic material formed by composting of domestic and industrial biowastes Chemosphere 34 1997 1639 1651
93 Castaldi P. Alberti G. Merella R. Melis P. Study of the organic matter evolution during municipal solid waste composting aimed at identifying suitable parameters for the evaluation of compost maturity Waste Manage. (Tucson, Ariz.) 25 2005 209 213
94 Ait Baddi G. Alburquerque J.A. Gonzálvez J. Cegarra J. Hafidi M. Chemical and spectroscopic analyses of organic matter transformations during composting of olive mill wastes Int. Biodeter. Biodegr. 54 2004 39 44
95 Kobayashi T. Yanagi Y. Sumida H. Chemical properties of purified commercial humic acids and their comparison with those of soil humic acids certified by Japan humic substances society Humic Sub. Res. 14 2018 33 42
96 Flores R.M. Origin of coal as gas source and reservoir rocks Coal and Coalbed Gas 2014 Elsevier 97 165
97 Inbar Y. Chen Y. Hadar Y. Humic substance formed during the composting of organic matter Soil Sci. Soc. Am. J. 54 1990 1316 1323
98 Akimbekov N. Qiao X. Digel I. Abdieva G. Ualieva P. Zhubanova A. The effect of leonardite-derived amendments on soil microbiome structure and potato yield Agronomy 10 2020 147
99 El Hassani F.Z. Fadile A. Faouzi M. Zinedine A. Merzouki M. Benlemlih M. The long term effect of olive mill wastewater (OMW) on organic matter humification in a semi-arid soil Heliyon 6 2020 e03181
100 Medina J. Monreal C. Chabot D. Meier S. González M.E. Morales E. Parillo R. Borie F. Cornejo P. Microscopic and spectroscopic characterization of humic substances from a compost amended copper 6 contaminated soil: main features and their potential effects on Cu immobilization Environ. Sci. Pollut. Res. 7 24 2017 14104 14116
101 Han L. Sun K. Jin J. Xing B. Some concepts of soil organic carbon characteristics and mineral interaction from a review of literature Soil Biol. Biochem. 94 2016 7 121
102 Mansuy L. Bourezgui Y. Garnier-Zarli E. Jardé E. Réveillé V. Characterization of humic substances in highly polluted river sediments by pyrolysis methylation-gas chromatography-mass spectrometry Org. Geochem. 32 2001 223 231
