Soil Humic Substances Research Articles

Stocks of carbon, total nitrogen and humic substances in soil under different cropping systems

This study aimed to evaluate total carbon and nitrogen and stocks of the humic fractions of soil organic matter under different cropping systems at the experimental farm at the Federal University at Grande Dourados – UFGD. Soil samples were collected from two layers (0-10 and 10-20 cm) from an oxisol with a clay texture. The systems studied were as follows: non-tillage (NTS), tillage (TS), eucalyptus and pasture. Natural vegetation from Dourados, Mato Grosso do Sul, Brazil was used for comparison. For statistical analysis of the C and N stocks, the model: Y = ? + Ai + rep (A) ik + Eijk was used. The replacement of TN one for CT decreased the total organic carbon and C in the stocks of humic substances (fulvic acid, humic acid and humin) in the soil just three years after adoption, especially in the 0-10 cm layer. However, soils under eucalyptus trees acquired increased carbon stock in the most active fractions, such as the fractions of fulvic and humic acids (0-20 cm layer). Regardless of the cropping system, the largest C and N stocks were measured for the humin fraction, followed by humic acid and fulvic acid. The total N and humic and fulvic acid levels under the conditions of maintenance of TN for 15 years increased when compared with CT, but not in soils under eucalyptus trees.

Reconstructing past climates using carbon isotopes from fulvic acids in cave sediments

Little is known about the potential of organic substances found within cave sediments to provide useful reconstructions of past climatic changes. Here, we present a study of the carbon isotopes of fulvic acids extracted from sediments from a cave in west-central Florida. Fulvic acids are the most contemporaneous representation of soil humic substances derived from vegetation growing above the cave. Radiocarbon dating of these sediments constrains their depositional period to be late-Holocene. The range of the fulvic acid δ13C record shows the dominance of C3 vegetation during this period, but with changes in abundance and the shifting presence of C4 plants. The δ13C values record both the Little Ice Age and Medieval Climate Anomaly which produce drier and wetter conditions for Florida, respectively. Our conclusion that the stable carbon isotopic values of the cave sediment record as a proxy for vegetation change as driven by variability in precipitation is strongly supported by a comparison with the δ13C and δ18O values of two uranium-series dated speleothems from this part of Florida, with both being proxies for precipitation change.

Compositional and Structural Difference of Fulvic Acid from Black Soil Applied with Different Organic Materials: Assessment After Three Years

Knowledge of different effects of various organic materials on soil humic substance is important for both environmental safety and sustainable agriculture. A pot experiment was conducted at Jilin Agricultural University, Jilin Province in northeast China to discover the influence of herb residue, animal excrement, woody residue, animal remnant on fulvic acid (FA) composition and structure using differential thermal analysis-thermogravimetric (DTA-TG), fourier transform infrared spectroscopy (FTIR) and elemental analysis. DTA-TG showed the range of peak temperature in the first exothermic reaction increased following the trend: CK>herb residue>animal excrement>woody residue=animal remnant, and the most weight loss was observed in animal excrement. Moreover, the second exothermic reaction of CK- and animal excrement-FA was presented as double peaks, the order of weight loss in that area was animal remnant>CK>woody residue>animal excrement>herb residue. According to FTIR, herb residue displayed higher adsorption intensity at 2 950, 1 420, 1 240 and 1 030 cm−1, animal excrement was in reverse. At the same time, herb residue- and animal excrement-FA had an absorption peak at 1 720 cm−1, while other organic materials didn't have this peak. As elemental analysis showed, FA isolated from various treatments was significantly distinct. It was clearly shown from our results that FA composition and structure in amended soils may be affected in different ways and at various extents on dependence of the nature and origin of amendment.

Complexes of the antimicrobial ciprofloxacin with soil, peat, and aquatic humic substances

Natural organic matter (NOM) is implicated in the binding of antibiotics by particles in soils and waters. The authors' previous computational study revealed structural rearrangement of both hydrophilic and hydrophobic moieties of NOM to favor H-bonding and other intermolecular interactions, as well as both competition with ion-exchange reactions and bridging interactions by NOM-bound divalent cations. The importance of these interactions was investigated using fluorescence-quenching spectroscopy to study the adsorption of ciprofloxacin (Cipro), a fluoroquinolone antibiotic, on 4 reference humic substances (HSs): Elliott soil humic acid (HA), Pahokee peat HA, and Suwannee river HA and fulvic acid. A simple affinity spectrum HS model was developed to characterize the cation-exchange capacity and the amount of H-bond donor moieties as a function of pH. The adsorption results stress the influence of both pH conditions and the type of HS: both soil HA and peat HA exhibited up to 3 times higher sorption capacity than the aquatic HS at pH ≥ 6, normalizing to the aromatic C content accounted for the differences among the terrestrial HS, and increasing the concentration of divalent cations led to a decrease in adsorption on aquatic HA but not on soil HA. In addition, the pH-dependent speciation models of the Cipro-HS complexes illustrate an increase in complexation due to an increase in deprotonation of HS ligands with increasing pH and, at circumneutral and alkaline pH, enhanced complexation of zwitterionic Cipro only in the presence of soil HA and peat HA. The findings of the present study imply that, in addition to electrostatic interactions, van der Waals interactions as facilitated by aromatic structures and H-bond donating moieties in terrestrial HS may facilitate a favorable binding environment. Environ Toxicol Chem 2013;32:1467-1478. © 2013 SETAC.

Influence of Soil Humic and Fulvic Acid on the Activity and Stability of Lysozyme and Urease

Humic substances (HS), including humic acids (HA) and fulvic acids (FA), are important components of soil systems. HS form strong complexes with oppositely charged proteins, which will lead to changes in the enzyme activity. The effect of soil HS on the activity and stability of two enzymes was investigated as a function of pH, ionic strength, and mass ratio HS/enzyme. Humic acid (JGHA) and fulvic acid (JGFA) are negatively charged, lysozyme is net positive at pH values below 10.4, and urease is net positive below pH 5.2 or net negative above pH 5.2. The enzyme activities in the HS-enzyme complexes were suppressed when the enzymes were oppositely charged to the HS. The largest activity suppression was observed around the mass ratio HS/enzyme where the HS-protein complex was at its isoelectric point (IEP). At the IEP strong aggregation of the complexes led to encapsulation of the enzyme. The ionic strength was important; an increase decreased complex formation, but increased aggregation. Due to the larger hydrophobicity of JGHA than JGFA, the reduction in enzyme activity was stronger for JGHA. The enzyme stability also decreased maximally at mass ratio around the IEP of the complex when HS and protein were oppositely charged. When urease and HS were both negatively charged no complexes were formed, but the presence of JGHA or JGFA improved the activity and stability of the enzyme.

Retention Effects of Soil Humic Substances on the Diffusive Transportation of Metal Ions During Sediment Porewater Membrane Dialysis Sampling

In situ sampling techniques such as membrane dialysis are widely applied to the monitoring of heavy metal contamination in water-saturated environments. In this study, batch experiments were conducted to investigate the influence of humic substances (HS) on the diffusive transportation of metal ions across sampling membranes. The presence of HS substantially slowed the transportation of metal ions by 22.2, 20.3, 17.4, 16.6, and 14.9 % for Zn2+, Cu2+, Ni2+, Co2+, and Mn2+, respectively, in the 100-mg/L HS-amended treatments relative to the treatments without HS under acidic conditions (pH = 5.0). The retention effect of HS on the metal ions appeared to be relieved at a higher pH of 8.5. However, HS also slowed the transportation of metal ions (Cu2+, Zn2+, Ni2+ and Co2+) into the sampling cells when membranes soaked previously in porewater solution. The retention effects of HS can be attributed to the enhanced adsorption of metal ions on the surfaces of the membranes. Further study in saturated sediments verified that the membrane dialysis technique could underestimate the concentrations of metal ions by 13.3–40.2 % at the presences of HS. These findings suggest that collected porewater data using the membrane dialysis technique should be interpreted with caution with the consideration of in situ geochemical conditions such as HS or pH in water-saturated environments.

Labile and Recalcitrant Carbon in Crop Residue and Soil under No-Till Practices in Central Region of Argentina

We evaluated the effects of the amount and type of C compounds (labile/recalcitrant) of crop residues on the soil organic matter (SOM) components under no-tillage practices in semiarid region of Argentina, with the aim of establishing criteria for sustainable agriculture. Five agricultural situations were evaluated: soybean/wheat rotation (wheat crop and at bare fallow); soybean monoculture; soybean/maize rotation and maize/wheat/potato rotations with biannual ploughing. In each situation crop residues and soil samples were collected. In residue samples, biomass (total and compo- nents), organic carbon, insoluble fibers, and soluble C compounds were evaluated. In soil samples total SOM, non-humic substances (NHS), humic substances (HS), humic (HA), and fulvic acids (FA) were analyzed. The total biomass and its components differed among situations and were lower in the situation with plough, while the fiber contents were higher in situations that includes wheat crop. Total SOM, NHS, HA content were higher in the plowed situation (57.20 g kg -1 , 47.30 g kg -1 , 7.80 g kg -1 respectively). We concluded that the total SOM quantity in continuous no-tillage is not affected by the different crop rotations and sequences, but the HA quantity (stable SOM) seems to be favored by the high recalcitrant compounds amount of the crop residues (particularly wheat). For these reasons, the following criteria for sustainable agri- culture management in semiarid zones can be suggested: a) continuous no-tillage utilization; b) summer crop annual rota- tions; and c) winter wheat crop instead of bare fallow.

Realistic soil C sink estimate in dry forests of western Argentina based on humic substance content

Due to high temporal variation of soil organic matter in arid regions, estimates of annual sequestered C might be overestimate. We assessed the soil stable organic matter (humic substances) in the transitional area between Dry Chaco and Monte eco-regions in western Argentina, as an approach to estimate realistic soil C sink. Soil samples were taken during wet and dry seasons in four sites along precipitation gradient. In each site three soil cover situations (under tree, under shrubs and on bare soils) were sampled (n = 5) and the quantity and type of residues (tree and shrub leaves, woody material, grasses and forbs) were recorded. Soil organic matter and humic substances (humic and fulvic acids) content were analyzed and non-humic substances were calculated by the differences between organic matter and humic substances. Soil humic substance proportion respect to SOM was low (20%) in all sites and it did not correspond with the precipitation gradient. Non-humic substances were lower in wet season indicating high C lability. The most important factors that affected soil humic substance content were the type and quantity of organic residues and soil cover type. Our results suggest that previous C sink estimations in Argentina dry forest probably are overestimated.

The Impact of Humic Substances as Remediation Agents to the Speciation Forms of Metals in Soil

Humic substances (HS) are the most widespread group of organic substances in natural environment and have high stability. The main terrestrial reserves are found in the form of naturally occurring ore, peat or lignite. The aim of this paper is to study possibilities to use HS as agents for remediation of contaminated with heavy metals soil and impacts of HS of metal speciation forms in it. It has been proved that HS are able to bind to metal ions and change their speciation forms in soils. The ability to form complexes with metal ions depends on the type of soil, type of metal, as well as concentrations of HS in soil. The study was carried out in experimental conditions and analytical method to determine availability of metals to living organisms was used for spiked soils. Remediation by using HS has shown good result for diminishing the content of biologically available copper and lead.

Caracterização potenciométrica de substâncias húmicas extraídas de solos da Antártica Marítima

Monitoring of soil carbon storage may indicate possible effects of climate change on the terrestrial environment and it is therefore necessary to understand the influence of redox potential and chemical characteristics of humic substances (HS) of Antarctic soil. Five soils from King George Island were used. HS were extracted, quantified and characterized by potentiometry and the content of total carbon and nitrogen determined. HS of these soils had greater aliphatic character, low content of phenolic groups, lower acidity and lower formal standard electrode potential, compared to HS of soils from other regions, suggesting they are more likely to be oxidized.

Study on C-Functional Groups of Soil Humus Fractions Affected by Phosphate Using C 1s Near-edge X-ray Absorption Fine Structure Spectroscopy

Synchrotron based near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was applied to study the C-functional group chemistry of various soil humus fractions (including fulvic acid, humic acid and humin) and to explore the reaction mechanisms between phosphate and humus. The results indicated that the humus fractions contained similar functional groups but at different proportions. Humic acid and humin, with the increasing acid. insolubility, exhibited higher content of hydrophobic functional groups and higher aromaticity while lower carboxylic C when compared with fulvic acid. The presence of phosphate could favor the release of the hydrophilic carboxylic functional groups into soil solutions through substitution reactions as well as electrostatic effects, and thus exert significant effects on the functional group chemistry of soil humic substances. The results will be great helpful to study the speciation and composition of C-functional groups with NEXAFS spectroscopy and to understand the reaction mechanisms of phosphate in soil micro-interfaces.

Characterization of organic matter content of Hungarian agricultural soils

Humic substances have proved to be very important fractions in soils, playing a key role especially in agricultural soil and influencing chemical and physical soil properties. Spectroscopic methods are widely used to identify the quality of soil humic substances. In this study, 16 soil samples were selected from the Soil Bank of the Soil Science Laboratory at Szent István University, Gödöllő. The samples were extracted using the Hot Water Percolation (HWP) method and the amount of dissolved organic carbon (DOC) in each fraction was measured. The kinetics of the DOC extraction process with the HWP method was estimated. The Ultraviolet Visible (UV-VIS) technique was used to characterize the properties of HWP-dissolved organic carbon (HWP-DOC), measuring absorbance at 200–700 nm. Among the humification parameters, the absorbance ratios at 254 and 365 nm (E2/E3) and 465 and 665 nm (E4/E6), the specific UV absorbance (SUVA) and the UV absorbance ratio index (URI) were estimated. The K factor (humus stability coefficient) and E2/E3 and E4/E6 in NaOH and NaF extracts were also measured. The properties of HWP-DOC were similar in most of the soil samples. There was a good correlation between the content of HWP-DOC and the absorbance at 254 nm. URI, SUVA, E2/E3 and E4/E6 indicated that most of the HWP-DOC in the samples consisted of fulvic acid components with greater activity, simpler structure and low molecular weight.

Comparison of soil organic matter composition after incubation with maize leaves, roots, and stems

As plants are major contributors to soil organic matter (OM), the chemical composition of plant tissues entering the soil should be related to the soil OM composition. We examined the OM composition of maize leaves, stems, and roots, to determine if differences in plant tissue composition altered the soil OM composition during a 36-week degradation experiment. Lignin phenols were measured by gas chromatography/mass spectrometry (GC/MS), and soil OM and humic substances were characterized using solid-state 13C and solution-state 1H nuclear magnetic resonance (NMR) spectroscopy respectively. Lignin phenol composition, oxidation, and depletion relative to cutin-derived OH-acids changed less in leaf- compared to stem- and root-amended soils over time, and may be due to greater vanillyl concentrations in leaves. Soil amended with stems had higher concentrations of carbohydrates in soil OM. Humic substances from leaf-amended soils had higher concentrations of aliphatic components, likely due to higher concentrations of aliphatic compounds in leaf tissues, which suggests that compounds derived from leaves are potential contributors to the stable pool of soil OM. After 36-weeks of incubation, the contribution of microbial-derived OM was greatest in humic extracts from root-amended soils, and increased contribution from these compounds was detected earlier in these soils than stem- and leaf-amended soils. This indicates that root amendment may enhance contributions from microbial-derived OM. Our study suggests that changes in soil OM composition over time was related to the chemical composition of the plant tissue, and demonstrates the important link between plant chemistry and soil OM turnover.

The effect of organic matter on aggregation under different soil management practices in a vineyard in an extremely humid year

Soil organic matter is an important agent responsible for binding soil mineral particles together and creating soil structure stability. Soil management can modify the content of soil organic matter and soil structure stability. Therefore, the aim of this study was to evaluate the effect of soil organic matter on soil structure stability under different soil management practices in Rendzic Leptosol, in an extremely humid year. In 2006, an experiment of the different management practices in a productive vineyard was established in the locality of Nitra-Dražovce (Slovakia). The following treatments were established: 1. control (grass without fertilization), 2. T (tillage), 3. T + FM (tillage + farmyard manure), 4. G + NPK3 (grass + NPK 120–55–195 kg·ha− 1), and 5. G + NPK1 (grass + NPK 80–35–135 kg·ha− 1). Soil samples were collected every month, during the year 2010. The results showed that the aggregation process and soil structure stability were mainly dependent on the stable form of soil organic matter or the physically protected soil organic matter inside of aggregates in all soil management practices in a vineyard. In the treatment with plowed farmyard manure, the soil structure stability decreased, during 2010. In this case, humic substances stability had a statistically significant influence on soil structure stability. The lower the values of color quotient of humic substances in soil were, the lesser was the decrease in soil structure stability.

Birnessite-Induced Binding of Phenolic Monomers to Soil Humic Substances and Nature of the Bound Residues

The nature of the abiotic birnessite (δ-MnO(2))-catalyzed transformation products of phenolic compounds in the presence of soil organic matter is crucial for understanding the fate and stability of ubiquitous phenolic carbon in the environment. (14)C-radioactive and (13)C-stable-isotope tracers were used to study the mineralization and transformation by δ-MnO(2) of two typical humus and lignin phenolic monomers--catechol and p-coumaric acid--in the presence and absence of agricultural and forest soil humic acids (HAs) at pH 5-8. Mineralization decreased with increasing solution pH, and catechol was markedly more mineralized than p-coumaric acid. In the presence of HAs, the mineralization was strongly reduced, and considerable amounts of phenolic residues were bound to the HAs, independent of the solution pH. The HA-bound residues were homogeneously distributed within the humic molecules, and most still contained the unchanged aromatic ring as revealed by (13)C NMR analysis, indicating that the residues were probably bound via ester or ether bonds. The study provides important information on δ-MnO(2) stimulation of phenolic carbon binding to humic substances and the molecular distribution and chemical structure of the bound residues, which is essential for understanding the environmental fates of both naturally occurring and anthropogenic phenolic compounds.

Dissolution of Humic Substances from Highly Humic Volcanic Ash Soil as Affected by Anionic Surfactant, Electrolyte Concentration and pH

Dissolved humic substances separated from soils play an important role in the material cycle because they adsorb nutrients and contaminants and move with water. This study was conducted to investigate the influence of anionic surfactant, pH and electrolyte concentration on the dissolution of humic substances from a highly humic volcanic ash soil. The soil used in the experiment has a negative charge and the anionic surfactant, sodium dodecylbenzene sulfonate, has also the negative charge. The absorbance of supernatant of soil solution at different surfactant concentration and different electrolyte concentration (0.001 M, 0.01 M, 0.1 M & 0.5 M) of NaCl at pH 4.5 and 6.5 was measured at the wave-length of 400 nm; this corresponds to the relative concentration of dissolved humic substances. The surfactant adsorption and its equilibrium concentration under the same solution condition of the absorbance measurement were also measured in order to get their effect on dissolved humic substances. The zeta potential of soil particles was measured in order to evaluate the influence of electrostatic potential on dissolution of humic substances. The concentration of dissolved humic substances increased at higher surfactant concentration and adsorption, at higher pH and at lower electrolyte concentration, because the electrostatic repulsive force between the soil particles and the dissolving humic substances became larger. Therefore, surfactant concentration and adsorption, pH and electrolyte concentration are important when considering the fate of humic substances in soils.

Comparative Use of Soil Organic and Inorganic Amendments in Heavy Metals Stabilization

Remediation strategies are capable to mitigate negative effects of heavy metals (HMs) on soils. The distribution of cooper (Cu), zinc (Zn), and chromium (Cr) was evaluated in a contaminated soil after adding biosolid compost (BC) and phosphate fertilizer (PF). A greenhouse assay and sequential extraction procedure were performed to determine the fractionation of HM in contaminated and remediated soil. In BC treatment, among 4 to 6% of Cu was associated with soil humic substances. Without amendments and with fertilizer application, Zn solubility increased by 15.4 and 8.4%, respectively, with experiment time. Although Cr was significantly adsorbed to the inorganic fraction, with compost application there was a transfer to organic fraction. A single amendment application is not suitable for immobilizing all metals of concern, because there are diverse union’s behaviors between HM and soil matrix. As the organic matter and phosphate fertilizer were effective in reducing mobility of Cu, the organic matter was more effective in the immobilization of Cr, and inorganic amendment induced the Zn precipitation, results from this pilot study suggest a combined use of these two amendments for soil remediation strategies. However, liming may be further needed to prevent soil acidification on longer time scales. Also, we propose the use of chemical and biological remediation strategies for potential improvement of effectiveness.

Characterization and analysis of soil humic acids by off-line combination of wide-pore octadecylsilica column reverse phase high performance liquid chromatography with narrow bore column size-exclusion chromatography and fluorescence detection

Liquid chromatography method arranged around RP-HPLC using step-wise gradients of dimethylformamide (DMF) in pH 3.0 aqueous phosphate buffered mobile phase and a wide-pore (30 nm) 4mm inner diameter (I.D.) octadecylsilica column was applied to the fractionation separation of soil and peat humic substances (HS), respectively. Combination of acceptable DMF solvating properties for HSs together with wide pore RP sorbent improved surface interactions of the analytes and suppressed influence of size-exclusion effects in RP-HPLC. Individual fractions collected from the RP-HPLC were off-line analyzed by size-exclusion chromatographic method (SEC) using 99/1 DMF/aqueous phosphate buffer pH 3.0 with Spheron HEMA 100 stationary phase filled in a 2.2 mm I.D. column. Both methods provided reproducibility of characterization profiles and robustness resulting from excellent reproducibility of HSs fraction retention times (±0.5% RSD) of peaks enforced by the step gradient shape. Obtained results indicate that the methods can be combined in a compatible, compact, automatic, 2D orthogonal separation system for fluorimetric characterization of such complicated natural substances as are examined humic acids and obtain so more information about their character.

Effects of heat treatment on fluorescence properties of humic substances from sandy soil in arid land and their Hg(II) binding behaviors

The topsoil temperature in arid areas of Xinjiang, China can be up to about 80°C in summer. This may significantly affect the chemical properties of soil humic substances. However, the effects of high temperature on characteristics of soil humic substances and their complexation with toxic metals are still poorly known. In the present study, binding of Hg(II) to unheated soil humic substances and heated soil humic substances from sandy soils was comparatively investigated using three-dimensional excitation–emission matrix (EEM) fluorescence spectroscopy. Two fluorescent peaks (peak I at Ex/Em = 365–370/470–474 nm; peak II at Ex/Em = 270–275/468–472 nm) identified as humic-like fluorescence were observed in the EEM spectra of humic substances. Both peaks were clearly quenched by Hg(II), indicating the strong interaction of humic-like substances with Hg(II), and showed blue shifts after heat treatment. Heat treatment caused an increase of the fraction of accessible fluorophore (fa), binding sites number (n) and effective quenching constants (logKa), indicating that more binding sites in humic substances could bind Hg(II) and form more stable humic substances–Hg(II) complexes after heat treatment. However, a decrease of binding constants (logKb) suggested that heat treatment would reduce the binding capacity of each binding site of humic substances to Hg(II). This study implies the transport of Hg(II) may be affected by high temperature in the arid zone due to the modification of the physicochemical properties of humic substances in soil.

Chemical and structural characterization of soil humic substances under agroforestry and conventional systems

Studies have proven that the agroforestry systems in the semi-arid region of the State of Ceará, Brazil, induce an increase in soil organic C levels. Notwithstanding, there is no information if this increase also results in qualitative changes in different pools of soil organic matter. The objective of this study was to verify the possible chemical and structural alterations in fulvic and humic acids of a Luvisol in areas adopting agroforestry, traditional intensive cultivation and native forest in a long-term experiment conducted in the semi-arid region of Ceará State, Brazil. The study was conducted in an experimental area of the National Goat Research Center (Embrapa) in Sobral, CE. The following treatments were evaluated: agrosilvopasture (AGP), silvopasture (SILV), intensive cultivation under fallow (ICF), and areas with native forest (NF). Soil fulvic and humic acids fractions were extracted from the 0-6 and 6-12 cm layers and characterized by elemental composition, thermogravimetry and infrared spectroscopy analyses. The elemental composition analysis of humic acids confirmed the data found for fulvic acids, showing reduction in the C, H and N levels, followed by an increase in O contents in the AGP and ICF treatments over SILV and NF. In all treatments, except to SILV in the 0-6 cm layer, the percentage of mass loss was highest (300-600 °C) for humic acids in the thermally most stable region. Despite the similarity between infrared spectra, soil fulvic acids in the SILV treatment extracted from 6-12 cm depth decrease the absorption bands at 1708 and 1408 cm-1 followed by an increase in the absorption band at 1608 cm-1 attributed to aromatic C=C groups. This behavior suggests an increase in the aromatic character of the structure. The AGP and ICF treatments, which increase the soil tilling, favored the maintenance of humic substances with a more aromatic character in the soil than SILV and NF. The less aromatic humic substances in the SILV treatment resulted in an increase of exchange sites of soil organic matter, indicating improved nutrient cycling and maintenance of productivity in the system.