Water-extractable Soil Organic Matter Research Articles

Effects of long-term application of organic materials on soil water extractable organic matter, fulvic acid, humic acid structure and microbial driving mechanisms

Effects of long-term application of organic materials on soil water extractable organic matter, fulvic acid, humic acid structure and microbial driving mechanisms

Effects of burn severity on organic nitrogen and carbon chemistry in high-elevation forest soils

Fire frequency and severity have increased in recent decades in the western United States, with direct implications for the quantity and composition of soil organic matter (SOM). While the effects of wildfire on soil carbon (C) and inorganic nitrogen (N) have been well studied, little is known about its impacts on soil organic N. Since organic N is the most abundant form of soil N in conifer forests and dominant source of plant N facilitated by symbiotic mycorrhizae and mineralization, better understanding of post-fire organic N chemistry will help address a critical gap in our understanding of fire effects on SOM. Here, we characterized changes to organic N chemistry across fire severity gradients resulting from two wildfires that burned lodgepole pine (Pinus contorta) forest along the Colorado/Wyoming border, USA. One representative gradient was selected for high-resolution analysis based on results from bulk data (total C and N, and pH). Mineral soils were collected from two depths in low, moderate, and high severity burned areas and adjacent, unburned locations one year following the Ryan and Badger Creek fires. Nuclear magnetic resonance spectroscopy and 21 tesla ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry analysis showed that N content and aromaticity of water-extractable SOM (0–5 ​cm depth) increased with burn severity, while minimal changes to 5–10 ​cm depth were observed. Heterocyclic N species are generally higher in toxicity compared to their non-nitrogenated counterparts, which prompted soil toxicity measurements. Complementary Microtox® analysis revealed a positive relationship between increased fire severity and increased soil toxicity to Aliivibrio fischeri (microbial test species). These findings add to our molecular-level understanding of organic C and N responses to wildfire severity, with likely implications for nutrient cycling, forest recovery and water quality following severe wildfire.

Tree mycorrhizal type controls over soil water-extractable organic matter quantity and biodegradation in a subtropical forest of southern China

Tree mycorrhizal type is believed to be a key factor controlling soil organic matter dynamics in forests, yet its effects on soil dissolved organic matter (DOM) quantity and biodegradation remain unknown, especially in sub/tropical forests. Here, we used soil water-extractable organic matter (WEOM) as a proxy for soil DOM, and measured water-extractable organic carbon (WEOC), total nitrogen (WETN), and total phosphorus (WETP) concentrations, and WEOM chemical composition (i.e., aromaticity and molecular weight) in the 0–10 cm and 10–30 cm mineral soil depths along the gradient ranging from arbuscular mycorrhizal (AM)- to ectomycorrhizal (ECM)-associated tree dominance in a subtropical secondary forest of southern China. Subsequently, we assessed soil WEOM biodegradation using a 42-day aerobic incubation experiment. At each soil depth, WEOC concentration and WEOC:WETN ratio increased with increasing ECM tree dominance, whereas WEOM aromaticity and molecular weight exhibited an opposite changing trend. Moreover, WEOC concentration correlated positively with soil organic C, total N, total P, and sand content, but correlated negatively with soil pH and clay and silt contents. Soil WEOM biodegradation did not change with ECM tree dominance in 0–10 cm depth but decreased with increasing ECM tree dominance in 10–30 cm depth. Soil WEOM biodegradation was negatively related to WEOC:WETP and WETN:WETP ratios, but was positively related to WEOM aromaticity and molecular weight. These findings suggested that tree mycorrhizal type is a potential indicator of soil DOM concentration and biodegradation, and highlight that the shifts in dominance of AM- and ECM-associated trees will influence soil DOM dynamics by altering soil texture, C:N:P stoichiometry, and C quality in subtropical forests.

Variations in the concentration, composition, and sources of WEOM in erosion and deposition landscapes over an erosion intensity gradient on the Loess Plateau of China

Water-extractable organic matter (WEOM) is the most active and mobile fraction of soil organic matter and responds sensitively to soil erosion. However, previous studies on the chemical nature of WEOM in the erosion and deposition landscapes were controversial at site-level studies and mainly focused on surface soils, limiting our assessment of WEOM’s environmental fate at larger and deeper scales. Here, we collected eroded soils and deposited sediments from a depth of 0–200 cm in five eroding catchments with an erosion intensity gradient across the Loess Plateau. We examined the concentration, composition, and potential sources of WEOM using UV–visible absorption and fluorescence excitation-emission matrix-parallel factor analysis. Soil WEOM quantity and quality varied significantly in the erosion and deposition landscapes, with greater variations in soils below 20 cm, where WEOM in the deposition zone was characterized by higher concentration, aromaticity, hydrophobicity, and UVA and UVC humic-like components derived from terrestrial and soil organic matter. WEOM variations in deep soils increased with increasing erosion modulus among sites, and were mainly driven by changes in soil organic carbon concentration, aggregate stability, and carbon to nitrogen ratio. However, the pattern of WEOM parameters between erosion and deposition zones at 0–20 cm was opposite to that in soils below 20 cm. Vegetation inputs compensated the loss of organic matter in sites with suitable climatic status and weak erosion, and led to higher concentrations and exogenous features of WEOM in the surface erosion area. Concludingly, the WEOM concentration, components, and sources differed significantly between erosion and deposition zones, and these differences, as well as their response to erosion intensity, revealed an opposing pattern in surface and deep soils.

Soil water extractable organic matter under long‐term dryland cropping systems on the Texas High Plains

AbstractWater extractable organic matter (WEOM) is critical for soil fertility because of its role as a readily available C substrate for microbial metabolism. In the semi‐arid Texas High Plains, dryland cropping systems are common because of limited precipitation and irrigation water. Little is known about the long‐term effects of dryland cropping on soil WEOM content and composition, and how these differ from native grassland. The objective of this study was to use soils collected near Amarillo, TX, in 1977 and 2013 to examine the long‐term (72 yr total cropping length) effects of dryland cropping systems [continuous wheat and wheat–fallow rotations] and tillage methods (disk plow, stubble‐mulch tillage, and delayed stubble‐mulch tillage) on WEOM characteristics, compared with native grassland. In cropped plots, water‐extractable organic carbon (WEOC) concentrations, a measure of WEOM content, were depleted regardless of tillage or crop, with reductions up to 13% in 1977 and 60% in 2013, compared with native grassland in 2013. Other parameters of WEOM changed over time, where cropped soils from 2013 had proportionally more WEOM in aromatic, phenolic, and lignin‐like forms, attributed to recently incorporated crop residues. In contrast, WEOM in cropped soils from 1977 and native grassland (2013) contained more functional groups (e.g., aliphatic, aromatic, polysaccharide, phenol, etc.) and humic‐like substances than cropped soils in 2013. This suggests that long‐term dryland cropping led to depletion of stable, humic‐like WEOM. Enhancement of WEOM stocks and long‐term sustainability in the Texas High Plains may require more vigorous conservation systems than currently used.

Combination of3D Fluorescence/PARAFAC and UV-Vis Absorption for the Characterization of Agricultural Soils from Morocco.

The present study, combining UV-Visible absorption and 3D fluorescence supported by PARAFAC chemometric analysis, focused on the characterization of soil water extractable organic matter (WEOM) in the zone of Doukkala located near the Atlantic coast of Morocco. The extracts, in water, of a set of 30 samples covering the four main types of agricultural soils in the region (commonly labeled Tirs, Faid, Hamri and R'mel) were investigated. [Formula: see text] and [Formula: see text] absorbance ratios [Formula: see text] and [Formula: see text] spectral slopes, along with their ratios[Formula: see text], as well as the fluorescence [Formula: see text] and humification [Formula: see text] indices were calculated and interpreted. In the four soil types, these parameters revealed, on the one hand, organic materials of terrigenous origin with some biological component, and showed, on the other hand, that these materials are in similar stages of humification with an important humic character. In all the soils investigated, 3D fluorescence crossed with PARAFAC chemometrics highlighted the absence of any protein component and revealed the prevalence of the fulvic acids fraction in the organic matter humic material.

Organic matter in aqueous soil extracts: Prediction of compositional attributes from bulk soil mid-IR spectra using partial least square regressions

Water-extractable organic matter (WEOM) is labile and a key component of soil organic matter. Thus, the prediction of soil WEOM concentration and composition-related characteristics is of great interest. The main objective of this study was to model and predict dissolved organic C (DOC) concentration and WEOM composition-related attributes of aqueous soil extracts using mid-infrared (IR) spectra of bulk soils coupled with partial least square (PLS) regression. Absorbance of UV light at 254 nm (Abs254), considered proportional to the concentration of aromatic substances in soil extracts, and light emission intensities proportional to concentrations of some components controlling WEOM fluorescence, were used as composition-related attributes of the soil extracts. The DOC-normalized Abs254 and emission intensities were used as composition-related attributes of WEOM. Application of PLS regressions for predicting spectroscopy-based composition-related attributes of aqueous soil extracts, using bulk soil IR spectra, is novel. Mid-IR spectra were determined for 216 soil samples collected from different (i) Israeli climate regions (Mediterranean and Semi-arid), (ii) types of land use (field crops, orchard and non-cultivated land), (iii) two depths (0–10 and 30–60 cm), (iv) sampling seasons (Fall and Spring). Prediction of DOC concentrations in the soil extracts was of limited and variable success evaluated by the coefficient of determination and slope of the linear regression of predicted vs measured values, with some soil subsets yielding no satisfactory predictions. However, Abs254 and emission intensities of fluorescent humic-like components were predicted more successfully than DOC concentrations, suggesting that WEOM aromatic and fluorescent components are better presented in soil IR spectra compared to WEOM aliphatic substances. Yet, prediction of the specific UV absorbance (SUVA), using bulk soil IR spectra, was less successful than prediction of Abs254. Prediction of DOC-normalized emission intensities of fluorescent components (analogous to SUVA in fluorescence spectroscopy) was not successful. The differences between the success of predicting extract properties, i.e., Abs254 and fluorescence emission intensities, and the prediction of DOC-normalized derivatives, suggest that concentrations of aromatic (fluorescent) components in soil extracts are better predicted using PLS regression analysis of bulk soil mid-IR spectra than the WEOM composition.

Adsorption of Fulvic Acid and Water Extractable Soil Organic Matter on Kaolinite and Muscovite

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.

Quantitative and Qualitative Responses of Soil Water-Extractable Organic Matter to Carbon and Nitrogen Management Practices in Loess Soil

Soil-dissolved organic matter (DOM) drives the carbon (C) and nitrogen (N) cycles in agroecosystems. Despite many studies on DOM dynamics, hardly any attention has been directed toward DOM quality, particularly DOM composition. The aim of this study was to elucidate how C and N management practices alter soil water-extractable organic matter (WEOM) in a loess soil agroecosystem. Field experiments were conducted with a winter wheat monoculture. Three N fertilization rates (0, 120, and 240 kg ha−1 year−1) were applied for 17 years (2002–2019), combined with five C practices (zero, low, and high rates of sheep manure or wheat straw) for three years (2016–2019). The results reveal that soil organic carbon (SOC) and water-extractable organic carbon (WEOC) concentrations in the topsoil (0–20 cm) were increased by organic amendments considerably but were not affected by N fertilization. The fluorescence excitation–emission matrix spectra (EEM) of WEOM were resolved to two humic-like components (C1 and C2) and two soluble microbial byproduct-like components (C3 and C4). The proportions of C1 and C2 were increased, while the proportion of C3 was decreased by both C and N management practices. In conclusion, organic amendments increased both WEOM quality and its proportion of humic-like components, whereas N fertilization increased the proportion of humic-like components without variations of WEOM quality in the topsoil of loess soil.

Depletion of Soil Water-Extractable Organic Matter With Long-Term Coverage by Impervious Surfaces

Water-extractable organic matter (WEOM) in soil is the critical substrate that fuels microbial-driven biogeochemical cycles. However, questions remain regarding whether and how expanding impervious surface area under global urbanization may alter soil WEOM cycling. Based on absorbance and fluorescence spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), we compared the content and chemical signatures of soil WEOM under impervious surfaces with those in adjoining open areas and evaluated the impacts of types (complete sealing by concrete and partial sealing by house structures) and durations (1.5, 27, and 114 years) of impervious surface coverage. The content of soil WEOM and its chromophoric and fluorescent fractions were not significantly changed (less than 20%) after 1.5 years of coverage by concrete and house structures. However, these parameters decreased by more than 30% with 27 and 114 years of coverage by the residential home structures. The microbial-humic-like and protein-like fluorescent WEOM persisted preferentially over the terrestrial-humic-like and nonfluorescent WEOM. FT-ICR MS results suggest various degrees of depletion of biochemical groups in WEOM. While the water-extractable lipid-like compounds increased with 1.5 years of coverage, all studied biochemical groups were depleted with long-term coverage, which might reduce the microbial processing of suberin-derived compounds. This study highlights the remarkable impacts of soil sealing on reducing substrate availability for microbial carbon processing in urban environments.

Responses of soil WEOM quantity and quality to freeze–thaw and litter manipulation with contrasting soil water content: A laboratory experiment

Future climatic change is likely to increase the occurrence of soil freeze–thaw (FT) events in high latitude and/or high altitude zones, which can substantially influence the dynamics of dissolved organic matter (DOM) released into the soils. However, it is not clear how the quantity and quality of soil DOM respond to changing FT patterns under different soil moisture and litter manipulation conditions in northern temperate forest stands. In this study, litter-amended and non-amended forest soils were incubated for 360 days at three soil moisture levels (30%, 60%, and 90% water-filled pore space) and three intensities of FT disturbance (low, high, and none). We quantified heterotrophic respiration, enzyme activity, microbial biomass, and water-extractable organic matter (WEOM) as a proxy for DOM in soils during the incubation experiment. The quality of WEOM was characterized by biodegradability, UV absorbance and parallel factor analysis modelling of fluorescence excitation emission matrices. Concentrations of water-extractable organic carbon (WEOC) and biodegradable WEOC declined continuously in all treatments over the 360-day incubation period. The dominant component of fluorescent WEOM shifted from humic- and fulvic- like components during the first 108 days of incubation, to protein-like components of microbial origin, characterized by high aromaticity, at the end of the incubation period. Litter amendment, FT disturbance, and their interaction increased WEOC concentrations in soils during the early 108-day incubation period, particularly in soils with low moisture and high FT intensity, but these increases disappeared after 252 days incubation at 15 °C. Litter-derived fluorescent WEOM in soils without FT disturbance was dominated by protein-like components after 14 days of incubation, but these were replaced by humic- and fulvic-like components after 108 days. This replacement effect was weaker in soils with FT disturbance, which we attribute to changes in microbial properties, including enzyme activity, microbial biomass and activity.

Fluorescence spectroscopy: A sensitive tool for identifying land-use and climatic region effects on the characteristics of water-extractable soil organic matter

Water-extractable organic matter (WEOM) is an indicator of active and labile pools of soil organic matter (SOM). Land-use is expected to have a major effect on SOM, yet the effects of different land-uses on WEOM are not well understood. Reported trends in the changes in water-extractable organic C (WEOC) concentrations following changes in land-use are inconsistent or even contradictory. Fluorescent WEOM components, including humic-like and proteinaceous substances, may be useful for elucidating long-term effects of different land-uses on WEOM content and composition. Therefore, the objectives of this research were (i) to characterize SOM and WEOM in soils from different climatic regions that were subjected to different land-uses and (ii) to compare the sensitivity of fluorescent components in WEOM to that of other SOM/WEOM attributes for evaluating the impact of land-use and climatic region on SOM. Two soil types from different climatic regions (Mediterranean and semi-arid) that were collected from land subjected to different uses (field crops, orchards and non-cultivated plots) and characterized for total soil organic C and total N. The concentration of WEOC, UV absorbance at 254 nm and excitation-emission matrices (EEMs) of fluorescence were determined in diluted solution extracts. Parallel-factor (PARAFAC) analysis was applied to identify meaningful fluorescent components of WEOM and quantify their presence. Most of the quantitative measures used for SOM and WEOM characterization demonstrated insignificant and/or inconsistent relationships with the type of land-use and climatic region. However, EEM-PARAFAC-derived fluorescent components present in soil WEOM and, specifically, their relative fractions exhibited significant associations with the climatic region and type of land-use. These results may support the use of EEM-PARAFAC fluorescent components as indicators for the assessment of soil health.

Dissolved organic matter in surface runoff in the Loess Plateau of China: The role of rainfall events and land‐use

AbstractExploring the chemical characterization of dissolved organic matter (DOM) is important for understanding the fate of laterally transported organic matter in watersheds. We hypothesized that differences in water‐extractable organic matter (WEOM) in soils of varying land uses and rainfall events may significantly affect the quality and the quantity of stream DOM. To test our hypotheses, characteristics of rainfall‐runoff DOM and WEOM of source materials (topsoil from different land uses and gullies, as well as typical vegetation) were investigated at two adjacent catchments in the Loess Plateau of China, using ultraviolet–visible absorbance and excitation emission matrix fluorescence with parallel factor analysis (PARAFAC). Results indicated that land‐use types may significantly affect the chemical composition of soil WEOM, including its aromaticity, molecular weight, and degree of humification. The PARAFAC analysis demonstrated that the soils and stream water were dominated by terrestrial/allochthonous humic‐like substances and microbial transformable humic‐like fluorophores. Shifts in the fluorescence properties of stream DOM suggested a pronounced change in the relative proportion of allochthonous versus autochthonous material under different rainfall patterns and land uses. For example, high proportions of forestland could provide more allochthonous DOM input. This study highlights the relevance of soils and hydrological dynamics on the composition and fluxes of DOM issuing from watersheds. The composition of DOM in soils was influenced by land‐use type. Precipitation patterns influenced the proportion of terrestrial versus microbial origins of DOM in surface runoff. Contributions of allochthonous, terrestrially derived DOM inputs were highest from forested landscapes.

Decadal Changes in Soil Organic Matter Due to Microaggregate and Hot Water Extractable Pools

Core Ideas Hot water extractable and microaggregate C explained 60% of soil C change over time. Hot water extractable and microaggregate N explained 47% of soil N change over time. No significant soil order effects on changes in C and N in fractions. Storage of C and N within aggregates is important for long‐term stabilization of soil organic matter (SOM). We investigated whether changes in C and N associated with physical soil fractions and the hot water extractable pool were correlated to changes in topsoil C and N over three decades. Archived soil samples from three soil orders collected from 46 sites across New Zealand were physically fractionated and the aggregate abundance (and C and N contents) of fractions determined; hot water extractable C (HWC) and hot water extractable N (HWN) were also measured. Together the change of C and N in hot water extractable SOM, microaggregate within macroaggregate, and free microaggregate fractions explained 60 and 47% of the change in whole soil C and N, respectively. Soil order was not a significant factor in the model suggesting that similar processes were operating in all three soil types. In summary, the development of strategies that enhance the storage of labile SOM and microaggregates could reverse the trend of loss of SOM and its associated ecosystems services.

Short-term modifications of soil microbial community structure and soluble organic matter chemical composition following amendment with different solid olive mill waste and their derived composts

Production and disposal of olive-mill wastes represent a problem of environmental relevance particularly in the Mediterranean countries where they are generated in huge quantities.A field trial was carried out to evaluate the short-term effects of soil amendment with solid olive mill waste (OMW) and its compost in comparison to the olive mill waste derived from the new two-phase treatment (humid OMW) and its resulting compost. The impact of treatments on soil water extractable organic matter and soil microbiota was determined. During a 120days-long field trial, it was observed an initial increase of water extractable organic carbon (WEOC), total reducing sugars and total phenol compounds for all the amended soils. Even if humid OMW had the highest WEOC and total phenol compounds concentration, it increased to a lesser extent their content in the soil, probably due to the rapid utilisation of this particular organic matter by microorganisms. After three days from spreading, all amendments induced rapid modifications of both fungal and bacterial communities. However, while the bacterial community restored its initial state at the end of the trial, suggesting their high resilience capacity, the fungal populations changes remained until 120days. The overall results indicated that all types of the organic materials tested as soil amendments had an impact on the soil microbial communities, especially in the early days of the field trial, without negative effects on the soil chemical characteristics.

Water-extractable organic matter and its fluorescence fractions in response to minimum tillage and organic farming in a Cambisol

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

Impact of Management Practices on Water Extractable Organic Carbon and Nitrogen from 12-Year Poultry Litter Amended Soils

Water extractable organic carbon (WEOC) and nitrogen (WEON) are two key parameters of soil water extractable organic matter (WEOM). Proper management of manure application rate in combination with tillage and cropping management could maintain appropriate WEOC and WEON concentrations in soils while decreasing the risk of their runoff from cropland and pastures. The objective of this research was to determine the effect of poultry litter (PL) application on WEOC and WEON in soils under different crops, tillage regimes, and grazing strategies. From 2001 to 2012, PL was applied at multiple rates to cultivated fields in a corn-oat/wheat-hay rotation or to pastures grazed by cattle or ungrazed. Soil samples (0 - 15 cm) were analyzed for KCl-extractable mineral N, and WEOC, and WEON contents. In addition, Ultraviolet-visible (UV-vis) and fluorescence spectroscopies were used to characterize WEOC stability. Organic N levels were higher at the high PL application rates. The soil C:N ratio narrowed as the PL application rate increased. However, the soil from pastures which received PL tended to have a wider range of C:N ratios than soil from the cultivated fields, despite identical PL application rates. The spectral analyses indicated that WEOC properties were responsive to management and PL application rate; therefore, this parameter may be used as a guide to provide best management strategy for manure application.

Ultrahigh-resolution FT-ICR mass spectrometry for molecular characterisation of pressurised hot water-extractable organic matter in soils

Dissolved organic matter in soil is a highly reactive pool of organic matter and is of great importance for biogeochemical cycles in soil. A better understanding of its dynamics relies on its molecular characterisation. Here, the molecular composition of water-extracted organic matter at elevated pressure and temperature (PH-WEOM) obtained from 120 Burgundy soils was investigated using high-field Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR MS). Unsupervised multivariate statistical analysis (UMSA) was used to retrieve classes of samples with specific molecular characteristics. Accordingly, van Krevelen diagram, Kendrick mass defect (KMD), as well as aromaticity index (AI) and aromaticity equivalent (Xc), were applied to present a pool of ubiquitous molecular formulas and to evaluate the PH-WEOM molecular heterogeneity in the sample set. Based on UMSA, the PH-WEOM from forest soils revealed a clearly distinct molecular composition, with major contributions from lignin- and tannin-like compounds, and with its aromaticity related to soil characteristics, especially the soil pH. No clear evidence of land-cover influence on the PH-WEOM molecular composition was found for cropland and grassland soils, but the role of pH was also identified for these samples, and agrees with molecular patterns attributed to microbial activity, with the presence of compounds with high H/C ratio. A group of samples from cropland soils developed on residual formations is characterised by a very specific molecular composition, rich in aliphatic organosulfur-like compounds, highlighting the importance of specific soil processes in the molecular composition of PH-WEOM. This work demonstrates the potential of FT-ICR MS to resolve the high chemical complexity of PH-WEOM in soils and the intricate influences of both biotic and abiotic environmental factors on the molecular composition of PH-WEOM in soils.

Quantity and Nature of Water‐Extractable Organic Matter from Sandy Loam Soils with Potato Cropping Management

Core Ideas 2‐ and 3‐yr potato crop rotations did not increase the levels of water‐extractable organic matter (OM). Potato crop rotation increased the less humified portions. Irrigation stimulated decomposition of non‐potato crop residues. Irrigation increased levels of water‐extractable OM in potato crop rotation soils. Composted manure addition increased water‐extractable low molecular mass and less humified OM. This work evaluated the level and nature of water‐extractable soil organic matter (WEOM) from a 6‐yr potato (Solanum tuberosum L.) crop rotation field experiment. The content of WEOM was higher in continuous potato soils than in the 2‐yr and 3‐yr crop rotation soils except for those crop rotation soils with soil improvement management by composted manure. Irrigation increased the level of WEOM in crop rotation soils. Ultraviolet–visible and fluorescence parameters indicated that WEOM in continuous potato soil possessed a high degree of humification and that crop rotation increased the aromatic and low molecular mass portions. In most irrigated soils, WEOM contained less aromatic but higher humified components. Characterization of the WEOM samples with crop rotation and irrigation suggests that these management practices stimulate the decomposition of the humic fraction in soil organic matter pools, implying healthier soil conditions with these management practices than with continuous potato growth.

Evaluation of the Interactions between Water Extractable Soil Organic Matter and Metal Cations (Cu(II), Eu(III)) Using Excitation-Emission Matrix Combined with Parallel Factor Analysis.

The objectives of this study were to evaluate the binding behavior of Cu(II) and Eu(III) with water extractable organic matter (WEOM) in soil, and assess the competitive effect of the cations. Excitation-emission matrix (EEM) fluorescence spectrometry was used in combination with parallel factor analysis (PARAFAC) to obtain four WEOM components: fulvic-like, humic-like, microbial degraded humic-like, and protein-like substances. Fluorescence titration experiments were performed to obtain the binding parameters of PARAFAC-derived components with Cu(II) and Eu(III). The conditional complexation stability constants (logKM) of Cu(II) with the four components ranged from 5.49 to 5.94, and the Eu(III) logKM values were between 5.26 to 5.81. The component-specific binding parameters obtained from competitive binding experiments revealed that Cu(II) and Eu(III) competed for the same binding sites on the WEOM components. These results would help understand the molecular binding mechanisms of Cu(II) and Eu(III) with WEOM in soil environment.