Fluorescent properties of water-extractable organic matter in low-gradient, clay soils offer insight for management and restoration

In weathered tropical soils, the inherent pedoclimatic characteristics strengthen the role of soil organic matter (SOM) pools within the carbon cycle for both forest and agricultural ecosystems. The fast SOM turnover and the humid climate hasten the impact of land use and emphasize the importance of water extractable organic matter (WEOM) on soil organic carbon (SOC) dynamics. The goal of this study was to determine the short‐term dynamics of bulk SOM and WEOM pool in weathered Brazilian soil, as conditioned by both land use and type of plant inputs. To this aim we used the appraisal of the natural δ13C‐OC signature to follow the variation of SOC pools and to overcome the analytical issues related to low OC concentration of WEOM fractions. In a one‐year laboratory incubation, soil samples from a natural forest and an adjacent site under sugarcane monoculture were amended with either forest litter or sugarcane straw to evaluate SOM and WEOM dynamics. The bulk SOM was mainly affected by inherited conditions rather than on the organic amendments. The incorporation of sugarcane residues in the forest site produced a partial replacement of the original SOC, while the amendments to the cultivated soil promoted a stable increase in SOC. The analysis of WEOM highlighted its influence as a reactive pool in the short‐term SOM dynamics. The δ13C variation of WEOM from cultivated soil corroborated the significant ponderal rate of OC input from forest litter that accounted for 22% of total dissolved carbon. Unexpected evidence on the shift of isotopic dilution between SOC pools indicates that the evaluation of δ13C to determine the origin of the SOC fraction should be conservatively adopted. The results outline that the steady state of SOM in forest systems may undergo a rapid decline with limited counteracting effects of exogenous OM inputs. Conversely, OC depleted agricultural soils may benefit from organic amendments, thus acting as an effective sink for SOC accumulation.

The dynamics of water extractable organic matter (WEOM) in common arable topsoils: II. Influence of mineral and combined mineral and manure fertilization in a Haplic Chernozem

The subalpine ecosystems of the Bieszczady Mountains are characterized by a mosaic of blueberry shrubs (Vaccinium myrtillus) and tall-grass vegetation, with significant implications for soil organic matter (SOM) dynamics. This study explores how vegetation type influences the content and spectroscopic properties of water-extractable organic matter (WEOM) in the topsoil horizons (O and A) in this region. WEOM is a crucial, bioavailable component of SOM that plays a significant role in nutrient cycling and carbon sequestration, particularly in sensitive mountain ecosystems.Samples of topsoil horizons (O and A) were collected from 20 sites dominated by blueberry shrubs or tall-grass vegetation. Water extracts were analyzed to determine WEOC and WETN concentrations using TOC analyzers. The chemical properties of WEOM were characterized via FTIR-ATR spectroscopy and UV-Vis spectrophotometry. Specific ultraviolet absorbance (SUVA254) and absorbance ratios (E2/E4, E2/E6, and E4/E6) were calculated to assess the aromaticity and molecular composition of WEOM.The O horizons of soils under blueberry shrubs exhibited significantly higher WEOC concentrations compared to those under tall-grass vegetation. However, WETN concentrations were not significantly different between vegetation types. The WEOC/WETN ratio was higher in soils under blueberry shrubs, indicating more carbon-rich WEOM in these areas.Spectroscopic analyses revealed notable differences in WEOM composition. FTIR spectra showed more pronounced bands associated with aliphatic compounds and carboxylic groups in WEOM from shrub-dominated soils, suggesting a higher proportion of less decomposed organic matter. UV-Vis spectroscopy indicated higher SUVA254 values for WEOM in grass-dominated soils, reflecting greater aromaticity and advanced decomposition. In the A horizons, differences in WEOC and WETN concentrations and WEOM properties were minimal, likely due to microbial homogenization and reduced vegetation influence.The results highlight how vegetation significantly affects WEOM quantity and quality, especially in the organic-rich O horizons. Soils under blueberry plants exhibit higher WEOC concentrations and carbon-dominated WEOM, which may improve carbon retention and slow decomposition rates. In contrast, tall-grass vegetation helps to produce more aromatic WEOM, indicative of advanced microbial processing. These findings suggest that shrubification, driven by climate change, can influence WEOM composition and stocks, with implications for carbon cycling and nutrient dynamics in subalpine ecosystems.This study emphasizes the importance of vegetation type as a key determinant of WEOM properties, shaping both the storage and bioavailability of nutrients in mountain soils. These insights are essential for effective vegetation management and the preservation of ecological functions in fragile subalpine zones.

Conversion of Brazilian savannah to agricultural land affects quantity and quality of labile soil organic matter

Soil organic matter pools provide insight into the potential stabilization or degradation of the soil resource by various long-term land management systems. Potentially mineralizable carbon (PMC) in soil is a measure of easily decomposable carbon and considered an important pool of soil organic matter. PMC was measured in soil samples taken from six different land management types viz. grass, legume, cinnamon, coconut, vegetable and a planted forest located in the low country wet zone of Sri Lanka. Soil samples taken from the different fields were incubated and carbon mineralization during 0-3, 3-10 and 10-24 day periods were determined as total CO2 evolved using alkali (NaOH) traps. Land management significantly influenced the total carbon mineralized during the incubation. Soils from the grass and coconut fields recorded the significantly high PMC compared to other land management types. The PMC in soils ranged from 298 to 428 mg/kg of soil in different land management treat ments. PMC was also calculated based on the carbon mineralized as a fraction of total organic carbon (TOC) in soil. Planted forest had the lowest PMC as a fraction of TOC (1.6%) compared to the highest recorded in the coconut field (5.2%). Results of this study suggest that PMC is related to the vegetation type and the nature of plant litter that supply organic matter to soil. It also appeared that labile carbon is more prevalent under grass vegetation compared to other land management types. PMC in soil could be used as an indicator to assess the amount of labile and easily decomposable carbon in soil.

Abstract. Soil organic matter (SOM) is redox-active, can be microbially reduced, and transfers electrons in an abiotic reaction to Fe(III) minerals, thus serving as an electron shuttle. The standard procedure to isolate organic matter (OM) from soil involves the use of alkaline and acidic solutions and the separation of humic acids (HAs) and fulvic acids (FAs). This process potentially leads to unwanted changes in SOM chemical and redox properties. To determine the effects of extraction conditions on the redox and electron-shuttling properties of SOM extracts, we prepared HA, FA, and water-extractable organic matter (OM) extracts, applying either a combination of 0.1 M NaOH and 6 M HCl or ultrapure water (pH 7), from soil samples collected from the subsoil (0–15 cm, A horizon, pH 6.5–6.8) in Schönbuch forest, Baden-Württemberg, Germany. Both chemical extractions (NaOH∕HCl) and water extractions were done in separate experiments under either oxic or anoxic conditions. Furthermore, we applied the NaOH∕HCl treatment to a subsample of the water-extractable OM to separate HA and FA from the water-extractable OM. When comparing the amount of carbon extracted from soil by different extraction methods, we found that FA and HA chemically extracted from the soil can make up to 34 %–40 % of the soil organic carbon pool while the water-extractable OM only represents 0.41 %–2.74 % of the total soil organic carbon. The higher extraction efficiency of the chemical extraction is probably due to the deprotonation of carboxyl and phenol functional groups under high pH. Anoxic extraction conditions also led to more extracted carbon. For water-extractable OM, 7 times more C was extracted under anoxic conditions compared to oxic conditions. This difference was probably due to the occurrence of microbial reduction and dissolution of Fe(III) minerals in the soil during the anoxic water extraction and thus the concomitant release of Fe(III) mineral-bound organic matter. To compare the redox activity of different SOM extracts, the electron-exchange capacity (EEC) of all extracted HA, FA, and water-extractable OM was analyzed and our results showed that, under anoxic extraction conditions, the HA chemically isolated from the water-extractable OM had 2 times higher EEC values compare to the water-extractable OM itself, suggesting the potential formation of redox-active aromatic functional groups during the extraction with NaOH under anoxic conditions by condensation reactions between amino acids, aldehydes, and hydroxyl- and catechol-containing molecules. We also performed a microbial Fe(III) reduction experiment with all extracts and found that higher EEC of extracts in turn resulted in a higher stimulation of microbial Fe(III) mineral reduction by electron shuttling, i.e., faster initial Fe(III) reduction rates, and in most cases also in higher reduction extents. Our findings suggest that OM extracted with water at neutral pH should be used to better reflect environmental SOM redox processes in lab experiments and that potential artefacts of the chemical extraction method and anoxic extraction condition need to be considered when evaluating and comparing abiotic and microbial SOM redox processes.

Dissolved organic matter is the most mobile part of soil organic matter. At the same time, its change and transformation processes occuring during soil erosion have not been sufficiently studied. The goal of the work was to assess the optical properties of water-extractable organic matter (WEOM) in arable soils of different degree of degradation from erosion and sedimentation in a plowed small arable catchment in the Kursk region. We studied WEOM of arable Protocalcic Chernozems (noneroded and moderately eroded) and their analogue with soil matter sedimentation – Novic Protocalcic Chernozems. WEOM was isolated from aggregates 2–1 and 10 mm. Aqueous extracts were characterized by their organic carbon and nitrogen content. Optical properties were assessed based on absorption spectra and three-dimensional fluorescence spectra. It was shown that in terms of the main quantitative indicators of soil organic matter – the content of organic carbon and nitrogen, as well as the pH value – washed away and reclaimed soils were close to each other and differed significantly from Protocalcic Chernozems. At the same time, both the quantitative and qualitative indicators of WEOM showed a different trend: the WEOM of Novic Protocalcic Chernozems differed significantly from noneroded and moderately eroded Protocalcic Chernozems. Besides, some indicators of WEOM (nitrogen content, SUVA254, S350–400 и SR) depended on the size of the aggregates from which WEOM was obtained (2–1 or 10 mm). In addition, the fluorescent properties of WEOM depend on the size of the aggregates. The obtained data allow us to conclude that the properties of WEOM in a small arable catchment in the central forest-steppe zone are largely determined by the processes of destruction of non-water-stable aggregates and the consolidation of their particles, as well as the leaching of water-soluble organic matter. When aggregates are destroyed by water, their particles migrate with flows along the slope, and organic matter undergoes decomposition; in depressions, particles accumulate, consolidate into blocky structural units, while the properties of their WEOM change significantly, both due to the degradation of organic matter and as a result of its leaching.

BackgroundMinimum tillage (MT) and organic farming (OF) are increasingly conducted in agricultural managements from the interest of optimizing soil conditions and developing sustainable agriculture. Our understanding of their effects on water-extractable organic matter (WEOM) is still insufficient.MethodsTo study the effects of MT and OF on WEOM, we analyzed soil materials sampled at two depths (0–8-cm-upper soil and 12–25-cm-deeper soil) from long-term field experiments using different farming and tillage methods. The content, composition, and quality of WEOM were examined.ResultsThe results showed organic farming significantly decreased water-extractable organic carbon and nitrogen, but had positive effect on WEOM humic-like components revealed by parallel factor analysis with excitation–emission matrix, soil organic carbon (SOC), total nitrogen (TN), as well as SOC/TN. In addition, organic farming increased the aromaticity and condensation of WEOM as indicated by specific UV absorption and humification index. MT had no effect on WEOM both quantitatively and qualitatively but significantly decreased SOC and TN of the whole investigated soil profile. The depth effect was significant with strong stratification of WEOM, WEOM components as well as SOC and total N in upper soil. Moreover, the WEOM spectroscopic quality showed sharp differences between the upper and deeper soils.ConclusionsThe results indicated that in the combined presence both tillage management and farming management, farming management imposed more influence on WEOM than tillage, and organic farming may facilitate the transformation of WEOM and lead to formation of WEOM with high stability. MT significantly changed the distribution of SOC and WEOM in soil, profile but did not increase the contents of SOC and WEOM in the site of the present study. However, the presence of larger pool of WEOM in MT + OF treatment at upper soil is likely to fuel possibly greater microbial activity and more rapid nutrient cycling in soil which can be favorable practice with potential in improving soil conditions in view of developing a sustainable ecosystem in the studied siteGraphical abstractThe impacts of agricultural practices on soil water extractable organic matter

Response of water extractable organic matter and its fluorescence fractions to organic farming and tree species in poplar and robinia-based alley cropping agroforestry systems

Soil Organic Carbon Trends Over 100 Years in the Morrow Plots

Dissolved and water-extractable organic matter in soils: a review on the influence of land use and management practices

Yan, Y., He, H., Zhang, X., Chen, Y., Xie, H., Bai, Z., Zhu, P., Ren, J. and Wang, L. 2012. Long-term fertilization effects on carbon and nitrogen in particle-size fractions of a Chinese Mollisol. Can. J. Soil Sci. 92: 509-519. The response of soil organic matter (SOM) dynamics to long-term fertilization may be deduced from changes in the accumulation and distribution of different soil organic carbon (SOC) and nitrogen (N) pools. The SOC and N in particle-size fractions were therefore measured to assess the influences of pig manure and synthetic fertilizer application on the characteristics of these pools. A long-term fertilization experiment, established in 1979 in the Mollisol area (Gongzhuling, China) was used for this study. Composite soil samples (0-20cm) were collected in 2005 from 12 treatment plots that had received annual applications of pig manure, synthetic fertilizers or combinations of both. Soils were fractionated into fine clay (<0.2 µm), coarse clay (0.2-2 µm), silt (2-50 µm), fine sand (50-250 µm) and coarse sand (250-2000 µm) and then SOC and N contents in each particle-size fraction were measured. Although most of the SOC and N were associated with clay and silt fractions, the large proportion of silt in the soil mass played a key role in the retention of SOC and N. The application of pig manure alone increased accumulation of SOC and N in each particle-size fraction, but preferential enrichment was found in the coarse sand fraction. This indicates that pig manure is efficient in restoring SOM in the temperate Chinese Mollisol under a tilled maize (Zea mays L.) monocropping system and having a long frozen period in winter. The application of synthetic fertilizers had no clear effect on SOC and N accumulation or their distribution in particle-size fractions. However, the combined application of pig manure and synthetic fertilizers enhanced the accumulation of SOC and N in all particle-size fractions, and led to a shift of SOC and N from fine to coarse particles. We extended the hierarchy model for SOC protection to consider a shift in SOC accumulation from fine to coarse particles, depending on the initial SOC content of the specific soil. The findings reveal a clear positive interaction between pig manure and synthetic fertilizers that may improve the quantity of SOM in the temperate Chinese Mollisol.

The interaction of organic matter with mineral components of the solid phase of soils is the most important process that regulates the cycle and balance of carbon in the biosphere. The adsorption of humic acids on minerals is accompanied by their fractionation in size, composition, and amphiphilicity, thus decreasing their heterogeneity. Despite a strong interest in studying the regularities and mechanisms of the interaction between natural organic matter and layered aluminosilicates, it is necessary to take into account the natural diversity of soil organic matter, adsorption conditions, and mineral composition. This study was designed to investigate the adsorption regularities of fulvic acid (FA) and water-extractable organic matter (WEOM) isolated from horizon H of peaty-podzolic-gleyic soil on kaolinite and muscovite. Sorbates and sorbents were examined by the following methods: high-pressure size exclusion chromatography (HPSEC), high-performance liquid chromatography (HPLC), and potentiometric titration. The specific surface areas of the sorbents were determined by the sorption of N2 molecules. We found that hydrophobic components of FA and WEOM are mainly adsorbed on mineral surfaces. The adsorption of FA and WEOM on kaolinite and muscovite is followed by decreased hydrophobicity of organic matter and decreased heterogeneity of its amphiphilic properties in an equilibrium solution. At pH levels around 6, sorption of organic matter from FA solution containing 19% and 81% hydrophilic and hydrophobic components, respectively, onto kaolinite and muscovite occurs mainly due to hydrophobic components. Hydrophobic interactions on siloxane surfaces are the main mechanism to fix FA on both minerals. Kaolinite adsorbs slightly more organic carbon per unit area than muscovite. The adsorption of WEOM from a solution with 41% hydrophilic and 59% hydrophobic components results not only from hydrophobic and hydrophilic components but also from hydrophobic and electrostatic interactions and depends on pH. The most hydrophobic fractions of organic matter are adsorbed from the hydrophobic components on the surface of both minerals. Under conditions of the performed experiments at pH < 5, more WEOM is adsorbed on muscovite than on kaolinite.

Water-extractable organic matter linked to soil physico-chemistry and microbiology at the regional scale

Quantity and chemistry of water-extractable organic matter in surface horizons of Arctic soils under different types of tundra vegetation – A case study from the Fuglebergsletta coastal plain (SW Spitsbergen)