Water-extractable Organic Matter Research Articles

Vulnerability of soil organic matter to microbial decomposition as a consequence of burning

Ecosystem fires are stochastic and anthropogenic phenomena that affect critical soil processes. Nevertheless, environmental managers, policy-makers, and even scientists have often overlooked the induced transformations that fire does to soil organic matter (SOM), which sustains an ecosystem’s overall health. Here, we investigated the effects of simulated fire conditions on bulk SOM, water-extractable organic matter (WEOM), and vulnerability to microbial degradation. Sequential thermal decomposition experiments were carried out to investigate the effect of increasing temperature on C, N, δ13C, and δ15N of bulk SOM and WEOM. A microbial decomposition experiment was also done to determine the effects of burning on SOM degradability. Intermediate-intensity burns caused significant decreases in C and N concentrations in soils, as well as alterations in bulk SOM and WEOM properties. Conversely, the effects of low-intensity burns were less apparent in terms of bulk SOM content and stable C and N isotope composition. However, the results of the microbial decomposition experiment revealed that low-intensity burning resulted in elevated CO2 emission that were significantly correlated with C concentration, δ13C values, and the C/N ratio of WEOM. These results provide evidence that low-intensity burning can have important consequences for soils, altering its organic components and releasing significant amounts of greenhouse gases. Thus, particular focus must be given to managing ecosystem fires and evaluating their impacts, especially because fire is still widely used in agriculture and forest management with frequency projected to increase in the coming years.

Impact of conventional agriculture on the concentration and quality of water-extractable organic matter (WEOM) in the surface horizons of Retisols—A case study from the Carpathian Foothills in Poland

In spite of many studies focused on the impact of land use changes on soil organic carbon (SOC) and the total nitrogen (TN) and soil organic matter (SOM) budget, less is known about the fate of water-extractable organic matter (WEOM) after the conversion of forestland to conventional farmland. The main aims of this study were: 1) to determine the concentration of water-extractable organic carbon (WEOC) and water-extractable total nitrogen (WETN) in topsoil horizons of forestland soils and cropland soils in the Carpathian Foothills in southern Poland and 2) to determine the quality of WEOM in topsoil horizons of forestland soils and cropland soils. Soil samples from topsoil horizons of forestland soils (Oi and Ah horizons) and cropland soils (Ap horizons) were collected and the concentration of WEOC and WETN was determined. In addition, the quality of WEOM was determined using ultraviolet-visible (UV–vis) spectroscopy and Fourier-transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy. The results of this comparative study clearly indicate that conventional agriculture significantly affects the concentration and quality of WEOM in surface soil horizons of Retisols in the Carpathian Foothills in southern Poland. Surface horizons of arable soils (Ap horizons) are characterized by a significantly lower mean concentration of WEOC and WETN in comparison with surface horizons of forestland soils (Oi and Ah horizons), and reflect changes in the content of bulk SOC and TN due to soil tillage. Mean concentrations of WEOC and WETN in the studied Ap horizons of arable soils are 1.98 mg l−1 and 0.15 mg l−1, respectively while the mean concentrations of WEOC and WETN in the Ah horizons of forestland soils are 15.53 mg l−1 and 0.84 mg l−1, respectively. The studied Oi horizons of forestland soils are characterized by the highest mean concentration of WEOC (44.53 mg l−1) and WETN (6.22 mg l−1). Mean specific UV absorbance values at wavelength of 254 nm (SUVA254) was the highest for WEOM from Ap horizons indicating the highest transformation of WEOM in these horizons. Mean FTIR-ATR absorption bands of WEOM obtained from Oi and Ah horizons are much more intensive than the bands of WEOM from Ap horizons, which is related to significantly lower concentration of WEOM in the latter horizons in comparison with Oi and Ah horizons of forestland soils.

Bulk and water-extractable organic matter from compost: evaluation of the selective dissolution in water using infrared absorbance ratios

Land application of composts affects concentration and composition of dissolved organic matter (OM) which plays important roles in soil functioning and may have effects on spreading of environmental pollution. Linking between the composition of bulk compost OM and its water-soluble fraction may, therefore, allow better understanding and prediction of the environmental impact of compost added to soil. The objectives of this study were to (i) examine composition-based links between bulk compost OM and water-extractable OM (WEOM), and (ii) evaluate and quantify selectivity of bulk compost OM dissolution, based on infrared (IR) absorbing functional groups. For that, 8 different composts and their freeze-dried WEOMs were characterized by mid-IR transmission spectroscopy. Compositions of compost OM and of WEOM were characterized in terms of ratios (R) defined on the basis of both areas and heights of specific IR absorbance bands in relation to absorbance by aliphatic CH groups. A simple novel approach is suggested, whereby selective dissolution of compost OM components is quantified by relating the R values determined for WEOM to those associated with compost OM. Significant similarities of IR spectra found in a series of WEOMs (and, to a lesser extent, in a series of compost OMs) suggest significant contributions of OM carboxylic groups to various bands. IR absorbance of compost OM contributed by hydrophilic and, specifically, carboxyl and carboxylate groups, when related to absorbance by aliphatic CH groups, can be used for predicting the indices characterizing WEOM composition, such as IR absorbance-based R values and aromaticity estimated from specific UV absorbance.

Rehabilitation of saline soil with biogas digestate, humic acid, calcium humate and their amalgamations

ABSTRACT Amelioration of saline soil is a requisite in order to increase crop productivity. A soil incubation study was performed for 60 days using digestate, humic acid, calcium humate and their combinations to investigate the influence on physical, chemical, microbial and enzyme activities of saline soil. Overall, digestate combined with calcium humate followed by humic acid treatments have shown their potency in decreasing the soil pH, electrical conductivity (EC), and sodium ion (Na+) concentration, and increase in potassium ion (K +), calcium ion (Ca 2+), magnesium ion (Mg 2+), mean weight diameter (MWD), soil enzyme activities, microbial biomass carbon (MBC), MBC: microbial biomass nitrogen (MBN) and soil respiration than control. The digestate, humic acid individually and their amalgamation evidenced greater MBN among all the treatments. The digestate alone efficiently improved the soil properties than humic acid and calcium humate individual groups except for the MWD where it is pronounced more in the latter groups. The greater metabolic quotient (qCO2) was observed in control than organic matter amended treatments indicating the stress conditions. The increase in water-extractable organic matter (WEOM) with minimal aromaticity (specific ultraviolet absorbance at 254 nm-Suva 254) in integrated amendments comprising groups, laid the ground reason to improve the properties of saline soil. Therefore, this study concludes that the fusion of fresh and humified substrates could facilitate reclamation.

Optical Properties of the Extractable Organic Matter Fractions in Typical Chernozems of Long-Term Field Experiments

The extractable fractions of organic matter (OM) differing in their mobility—water-extractable organic matter, labile humic substances, and humic substances—have been isolated from different-sized aggregates of a typical chernozem (Haplic Chernozem (Loamic, Pachic)). The first two fractions characterize the active OM pool, and the latter fraction characterizes the slow OM pool. Overall, the extractable OM fractions make it possible to assess the content and qualitative composition of the active and slow OM pools and, in an indirect manner, of the passive OM pool. The content of dissolved organic carbon in the extracts is used to quantify the yield of fractions, and the spectral characteristics of light absorption in the ultraviolet and visible ranges attest to their qualitative composition. The main attention is focused on the OM recovery in the chernozems after an extreme regime (permanent tilled fallow) and on the differences in the OM of soil aggregates in dependence on their size. A large part of plant residues entering the soil during the recovery of the chernozem after the extreme conditions of tilled fallow is fixed in the passive OM pool. The qualitative composition of active pools is restored much faster than their quantity typical of the undisturbed cenoses. The optical descriptors E2/E3 of labile humic substances and E4/E6 of humic substances significantly depends on the size of aggregates.

Potential denitrification stimulated by water-soluble organic carbon from plant residues during initial decomposition

Denitrification usually takes place under anoxic conditions and over short periods of time, and depends on readily available nitrate and carbon sources. Variations in CO2 and N2O emissions associated with plant residues have mainly been explained by differences in their decomposability. A factor rarely considered so far is water-extractable organic matter (WEOM) released to the soil during residue decomposition. Here, we examined the potential effect of plant residues on denitrification with special emphasis on WEOM. A range of fresh and leached plant residues was characterized by elemental analyses, 13C-NMR spectroscopy, and extraction with ultrapure water. The obtained solutions were analyzed for the concentrations of organic carbon (OC) and organic nitrogen (ON), and by UV-VIS spectroscopy. To test the potential denitrification induced by plant residues or three different OM solutions, these carbon sources were added to soil suspensions and incubated for 24 h at 20 °C in the dark under anoxic conditions; KNO3 was added to ensure unlimited nitrate supply. Evolving N2O and CO2 were analyzed by gas chromatography, and acetylene inhibition was used to determine denitrification and its product ratio. The production of all gases, as well as the molar (N2O + N2)–N/CO2–C ratio, was directly related to the water-extractable OC (WEOC) content of the plant residues, and the WEOC increased with carboxylic/carbonyl C and decreasing OC/ON ratio of the plant residues. Incubation of OM solutions revealed that the molar (N2O + N2)–N/CO2–C ratio and share of N2O are influenced by the WEOM's chemical composition. In conclusion, our results emphasize the potential of WEOM in largely undecomposed plant residues to support short-term denitrification activity in a typical ˈhot spot–hot momentˈ situation.

Differences in labile soil organic matter explain potential denitrification and denitrifying communities in a long-term fertilization experiment

Content and quality of organic matter (OM) may strongly affect the denitrification potential of soils. In particular, the impact of soil OM fractions of differing bioavailability (soluble, particulate, and mineral-associated OM) on denitrification remains unresolved. We determined the potential N2O and N2 as well as CO2 production for samples of a Haplic Chernozem from six treatment plots (control, mineral N and NP, farmyard manure - FYM, and FYM + mineral N or NP) of the Static Fertilization Experiment Bad Lauchstädt (Germany) as related to OM properties and denitrifier gene abundances. Soil OM was analyzed for bulk chemical composition (13C-CPMAS NMR spectroscopy) as well as water-extractable, particulate, and mineral-associated fractions. Soils receiving FYM had more total OM and larger portions of labile fractions such as particulate and water-extractable OM. Incubations were run under anoxic conditions without nitrate limitation for seven days at 25 °C in the dark to determine the denitrification potential (N2O and N2) using the acetylene inhibition technique. Abundances of nirS, nirK, and nosZ (I + II) genes were analyzed before and after incubation. The denitrification potential, defined as the combined amount of N released as N2O + N2 over the experimental period, was larger for plots receiving FYM (25.9–27.2 mg N kg−1) than pure mineral fertilization (17.1–19.2 mg N kg−1) or no fertilization (12.6 mg N kg−1). The CO2 and N2O production were well related and up to three-fold larger for FYM-receiving soils than under pure mineral fertilization. The N2 production differed significantly only between all manured and non-manured soils. Nitrogenous gas emissions related most closely to water-extractable organic carbon (WEOC), which again related well to free particulate OM. The larger contribution of N2 production in soils without FYM application, and thus, with less readily decomposable OM, coincided with decreasing abundances of nirS genes (NO2− reductase) and increasing abundances of genes indicating complete denitrifying organisms (nosZ I) during anoxic conditions. Limited OM sources, thus, favored a microbial community more efficient in resource use. This study suggests that WEOC, representing readily bioavailable OM, is a straightforward indicator of the denitrification potential of soils.

Influence of temperature and residence time on characteristics of biochars derived from agricultural residues: A comprehensive evaluation

In this study, the influence of production conditions (i.e., pyrolysis temperature and residence time) on the fundamental properties and the nutritional traits of biochars derived from different feedstocks (corn stalk, rape straw, wheat stalk, and peanut shell) were investigated. The results showed that pyrolysis temperature was a critical parameter affecting biochar properties, whereas the influences of residence time and feedstock were comparatively weaker. Biochar properties, including the pH, ash content, electrical conductivity (EC), oxidation stability, and the ability to retain nutrients, were positively related with increasing temperature and prolonged residence time. However, the water-extractable organic matter and available nutrients were negatively related with temperature and residence time. There was a minimal release of NH4+-N, NO3−-N, and PO43−-P from the generated biochars, and their concentrations were reduced through carbonization. By comparison, the water-soluble K+ content steadily increased with pyrolysis. Generally, the biochars showed a limited capacity for sorbing PO43− and NO3−, although the removal efficiencies were enhanced by pyrolysis. By comparison, higher NH4+ removal was achieved by biochars, and particularly for biochars with a higher pH. These findings suggest that the application of agricultural residue-derived biochars might not provide plants with sufficient nutrients (except for K). Additionally, the biochars generated at 500°C for a duration of 1h demonstrated higher potential for carbon sequestration.

Identifying eroded organic matter sources in sediments at fluvial system using multiple tracers on the Loess Plateau of China

Erosion is a fundamental phenomenon that governs the biogeochemical cycle of carbon (C) in the Earth system. Understanding the fate of eroded C at the watershed scale thus provides essential guidance for balancing the C budget and evaluating soil management strategies. In this study, we collected a total of 229 topsoils of source materials [land use types (forests, grassland, cropland, orchard, and fallow) and gully bank] and a total of 56 surface sediment samples [main river channel (18) and three tributaries (38) with different erosion intensity]. Multiple tracers (C and nitrogen (N) isotopes, molecular n-alkane biomarker, and spectroscopic indices) applied widely were then analyzed to determine relative source contributions to eroded organic matter (OM) (including bulk OM and water-extractable OM) in sediments at a broader watershed on the Loess Plateau of China. The results from an isotopic mixing model analysis showed that gully bank was the primary contributor of bulk OM in sediments, accounting for mean probability estimate (MPE) 55.3%, followed by “grassland” (MPE 21.0%), “cropland” (MPE 14.4%), and “forests” (MPE 9.3%). The contributions of grassland and cropland showed an increasing trend from upstream to downstream sections of the studied watershed. The contribution of forests was found higher in upstream than those of midstream and downstream. Along with previous evidences, our results show that erosion intensity mainly affects the proportion of C source, where gully bank was found as the highest contributor in three associated catchments, especially in region with light erosion intensity. We further demonstrated that the sediment bulk OM deposited was mainly derived from the region with light erosion intensity at the broader watershed scale. Moreover, results of n-alkane ratios and spectroscopic indices also implied that biological production of OM other than terrestrial sources (i.e., soil OM) might play a critical role in source contribution of water-extractable OM in sediments of main river channel.

Spectroscopic characteristics of water-extractable organic matter from different green manures1

ABSTRACT Water-extractable organic matter (WEOM) plays an important role in many chemical processes and in soil organic matter accumulation. Large amounts of WEOM can be released by green manure when being incorporated into the soil. However, the characteristics of WEOM extracted from different green manures (GMs) are unclear. In this study, WEOM samples were extracted from Chinese milk vetch (Astragalus sinicus L), radish (Raphanus sativus L), ryegrass (Lolium perenne L), hairy vetch (Vicia villosa L), February orchid (Orychophragmus violaceus) and rye (Secale cereale L) and their characteristics were studied by spectroscopic analysis. WEOM generated from legume GMs contained more water-extractable organic carbon (WEOC) than other GMs and Chinese milk vetch was the highest. UV-visible spectroscopy analysis revealed that all the samples were rich in C=C and C=O functional groups. Carboxylic acid, alcohol, phenol, fatty acids, aliphatic aromatic and amide compounds were found by FTIR spectroscopy in these WEOM samples. WEOM derived from Chinese milk vetch contained more carboxylic acid and inorganic sulphates only appeared in WEOM extracted from the radish. The aromaticity index, humification index, and FTIR absorption ratio 1650/2925 and 1650/2850 showed that WEOM extracted from Chinese milk vetch and hairy vetch had a relatively lower aromaticity and humification degree than other GMs. The fluorescence regional integration (FRI) analysis showed that Chinese milk vetch and hairy vetch had a higher portion of protein-like substance fractions than other GMs. We may deduce that WEOM generated from Chinese milk vetch and hairy vetch has higher chemical activity than other GMs.

High-pH and anoxic conditions during soil organic matter extraction increases its electron-exchange capacity and ability to stimulate microbial Fe(III) reduction by electron shuttling

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 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.

Climate, carbon content, and soil texture control the independent formation and persistence of particulate and mineral-associated organic matter in soil

Understanding the mechanisms controlling the formation and persistence of soil organic matter (SOM) is important for managing soil health and sustainable food production. The formation of SOM and the degree to which it is protected from decomposition are important for determining the long-term persistence of SOM. We used soils collected in a 13C-labelled litter decomposition study established at agricultural sites in Canada to understand the formation and persistence of newly-formed SOM. The ten agricultural sites spanned a wide range of soil carbon contents, texture, and climatic conditions. We fractionated the soil to isolate water extractable organic matter (WEOM), free light POM (fPOM), sand-sized and occluded particulate organic matter (oPOM), and silt and clay sized particles, referred to as mineral-associated organic matter (MAOM). Quantitative isotope tracing was used to determine the litter-derived C in all fractions. We performed these analyses early (six months after incubation) and later (five years after incubation) in the decomposition process to evaluate factors that control the formation and persistence of POM and MAOM. After six months litter-derived C was found in all fractions, but after five years it had declined in all fractions except the MAOM. Formation of MAOM was related to high mean annual precipitation and low sand content, whereas occluded POM formation was related to high soil C content. Persistence of MAOM and POM during the incubation were associated with low soil temperature and high soil C content. There was no consistent indication that formation of MAOM occurred from the decomposition of POM, suggesting that MAOM and POM are formed by two separate pathways. This has important implications for SOC models, which assume that plant-derived C passes through a sequence of pools, becoming more stable along the way.

Stable carbon isotope ratios of water-extractable organic carbon affected by application of rice straw and rice straw compost during a long-term rice experiment in Yamagata, Japan

ABSTRACT Hot-water- and water-extractable organic matter were obtained from soil samples collected from a rice paddy 31 years after the start of a long-term rice experiment in Yamagata, Japan. Specifically, hot-water-extractable organic carbon and nitrogen (HWEOC and HWEON) were obtained by extraction at 80°C for 16 h, and water-extractable organic carbon and nitrogen (WEOC and WEON) were obtained by extraction at room temperature. The soil samples were collected from surface (0–15 cm) and subsurface (15–25 cm) layers of five plots that had been treated with inorganic fertilizers alone or with inorganic fertilizers plus organic matter, as follows: PK, NPK, NPK plus rice straw (RS), NPK plus rice straw compost (CM1), and NPK plus a high dose of rice straw compost (CM3). The soil/water ratio was 1:10 for both extraction temperatures. We found that the organic carbon and total nitrogen contents of the bulk soils were highly correlated with the extractable organic carbon and nitrogen contents regardless of extraction temperature, and the extractable organic carbon and nitrogen contents were higher in the plots that were treated with inorganic fertilizers plus organic matter than in the PK and NPK plots. The HWEOC and WEOC δ13C values ranged from −28.2% to −26.4% and were similar to the values for the applied rice straw and rice straw compost. There were no correlations between the HWEOC or WEOC δ13C values and the amounts of HWEOC or WEOC. The δ13C values of the bulk soils ranged from −25.7% to −23.2% and were lower for the RS and CM plots than for the PK and NPK plots. These results indicate that HWEOC and WEOC originated mainly from rice plants and the applied organic matter rather than from the indigenous soil organic matter. The significant positive correlations between the amounts of HWEOC and HWEON and the amount of available nitrogen (P < 0.001) imply that extractable organic matter can be used as an index for soil fertility in this long-term experiment. We concluded that the applied organic matter decomposed more rapidly than the indigenous soil organic matter and affected WEOC δ13C values and amounts.

Water extractable organic matter and iron in relation to land use and seasonal changes

The soil carbon pool is an essential part of the global carbon cycle although it is sensitive to climatic changes and the local environment. Terrestrial areas are important sources of organic matter for aquatic ecosystems and the fluctuation of different soil minerals and elements is largely influenced by land use and season changes. We studied water-extractable organic matter (WEOM) properties including iron (Fe), water extractable organic carbon (WEOC) and spectral characteristics from forests and arable soils to evaluate the effects of land use and seasonal change on WEOM and Fe in terrestrial areas. We collected soil samples randomly from arable land (AR), broadleaf (BL) and aged needle leaf forests (NL) and extracted WEOM for analysis using rainwater. Results of WEOC and Fe showed similar trend seasonally and were higher in forest sample than in AR. WEOC was high in the upper layer while Fe was independent of the depth and higher in AR. On the other hand, specific ultra-violet absorbance at 254 nm (SUVA254) and a proxy for aromaticity significantly varied with both land use and season and was on average two times higher in arable land than forests during spring and summer. Humic-like components significantly varied between the studied sites seasonally while tyrosine-like was affected by season only. The relative abundance of both humic-like and tryptophan-like components were significantly affected by land use while [Fe]: [WEOC] ratio was also high in arable land and negatively correlated with humic-like components in forest sites. As observed from ratio and Fe oxidation rate constant, summer presented ideal conditions for WEOM interactions. The synchronized seasonal WEOC and Fe changes indicate an enhancement of Fe mobility by DOM whereas the differences observed between sites especially from the high humic- and protein-like components in NL and BL reflects the effect of land use.

Linking molecular and optical properties of dissolved organic matter across a soil-water interface on Akimiski Island (Nunavut, Canada)

Dissolved organic matter (DOM) plays a crucial role in terrestrial and aquatic carbon and biogeochemical cycles; however, molecular transformations between aquatic and terrestrial systems remain poorly understood due to the complexity and heterogeneity of DOM. In this study, we investigated the molecular diversity of aquatic DOM and adjacent soil derived water extractable organic matter (WEOM) from seven locations on Akimiski Island, Nunavut using a combination of absorbance spectroscopy and Fourier transform ion cyclotron mass spectrometry (FT-ICR-MS). Assigned elemental formula and Van Krevelen compositional analysis reveal compositional similarities in river, inland ponds and coastal pool sites for aquatic DOM and WEOM. More aromatic, oxygenated polyphenolic carbon rich molecules were found in aquatic DOM whereas WEOM was abundant in highly unsaturated aliphatic material. A total of 276 phenolic, unsaturated aliphatic, and vascular plant-derived polyphenolic molecules were identified as being conserved between WEOM and aquatic DOM at one river and two inland pond locations suggesting similar CHO sources from adjacent soils. Moreover, contributions of polyphenolic compounds in aquatic DOM and WEOM were greater at inland ponds than coastal pools, congruent with a greater aromaticity at inland sites. Our results highlight the similarities and differences in WEOM to aquatic DOM composition and how they range across surrounding watersheds that provide insight into the biogeochemical dynamics across a Canadian subarctic terrestrial-aquatic-continuum.

Effects of land use on characteristics of water-extracted organic matter in soils of arid and semi-arid regions.

The effects of land-use type on water-extracted organic matter (WEOM) concentration and composition and their spectral characteristics were investigated. Soil samples were collected across the Loess Plateau, which included 19 cropland samples (CL), 14 abandoned samples (AL), 10 grassland samples (GL), 5 forest samples (FR), and 4 orchard samples (Orch). The average WEOC concentration varied from 289.0 to 362.2 mg kg-1 among different land-use types, and there was no significant difference in WEOC content in topsoils (P ≤ 0.05). The amounts of four components identified by the EEM-PARAFAC method were in the order of C1 > C3 > C2 > C4 in all soils. The two humic acid-like components (C1 and C3) were the dominant fractions of WEOM in topsoils in the Loess Plateau area, irrespective of land-use types. The study showed that the EEM fluorescence spectroscopy can be successfully used to characterize WEOM fractions in soils. Our results showed that land-use type has no effects on WEOC concentration in topsoils. Humic acid-like substances dominated in the measured WEOM fractions, while the humic acid-like substances (C1) in Orch soils are the highest and fulvic acid-like and protein-like substances exist a relatively high content in GL soils.

Substitution of mineral fertilizers with biogas digestate plus biochar increases physically stabilized soil carbon but not crop biomass in a field trial

Various organic amendments are scrutinized as potential agricultural management strategies to ensure soil productivity while mitigating climate change due to the accumulation of soil organic matter (OM). The objectives of this experiment were to study the effects of biochar and biogas digestate versus mineral fertilizer on crop aboveground biomass as well as fractions and mineralization of soil organic carbon (SOC). Samples of a sandy Cambisol were taken 14 months after establishment of a field experiment in Germany. Treatments included application of equal nitrogen in the form of mineral fertilizer or liquid biogas digestate without biochar (B0), with 1 Mg biochar ha−1season−1 for two growing seasons (B2), or with 40 Mg biochar ha−1 application (B40). Soil fractionation in water separated water-extractable and free particulate (fPOM) OM, followed by sonification and sieving to isolate occluded particulate (oPOM) and < 20 μm aggregate-occluded and mineral-associated OM. CO2 emissions were measured during 92-day laboratory incubations at 10 and 20 °C. Analysis of variance found digestate lowered (p < 0.05) rye aboveground biomass compared to mineral fertilizer (9.3 vs. 10.6 Mg ha−1), while biochar had no effect. B40 treatments increased C mineralization during incubation by 16% and contained 3.8 times more SOC than B0 treatments. This additional SOC was allocated to fPOM (52%), oPOM (22%), and the <20 μm fraction (26%). Digestate application increased SOC content of oPOM by 11% compared to mineral fertilizer. Furthermore, combined application of 40 Mg biochar ha−1 with digestate resulted in 20% more SOC in the <20 μm fraction than biochar with mineral fertilizer. The lack of a significant fertilizer or biochar-fertilizer interaction effect on C mineralization during incubation demonstrates the stability of SOC from digestate alone or in combination with biochar. The absence of significant differences in SOC content between B0 and B2 treatments demonstrates the difficulty of documenting SOC sequestration in the field at low biochar application rates.

Dynamics, biodegradability, and microbial community shift of water-extractable organic matter in rice–wheat cropping soil under different fertilization treatments

Although fertilization plays an important role in determining the contents of soil dissolved organic matters or water-extractable organic matter (DOM, WEOM), knowledge regarding the dynamics, biodegradability, and microbial community shifts of WEOM in response to different fertilization treatments is very limited, particularly in rice–wheat cropping soil. Thus, in the present study, we performed biodegradation experiments using WEOM extracted from samples of soil that had been subjected to four different fertilization treatments: unfertilized control (CK), chemical fertilizer (CF), 50% chemical fertilizer plus pig manure (PMCF), and 100% chemical fertilizer plus rice straw (SRCF). UV spectrum and fluorescence 3D excitation–emission matrix analyses applied to investigate the chemical composition of WEOM revealed that all examined WEOMs were derived from microbial activity and the dominant portion comprised humic acid-like compounds. After the incubation, 31.17, 31.63, 43.47, and 33.01% of soil WEOM from CK, CF, PMCF, and SRCF treatments, respectively, were biodegraded. PMCF- derived WEOM had the highest biodegradation rate. High-throughput sequencing analyses performed to determine the microbial community before and after the incubation indicated that Sphingomonas, Bacillus, and Flavisolibacter were the predominant bacterial genera in the original inoculum derived from the four fertilization treatments. Following biodegradation, we observed that the dominant bacteria differed according to fertilization treatments: Curvibacter (43.25%) and Sphingobium (10.47%) for CK, Curvibacter (29.68%) and Caulobacter (20.00%) for CF, Azospirillum (23.68%) and Caulobacter (13.29%) for PMCF, and Ralstonia (51.75%) for SRCF. Canonical correspondence analysis revealed that, shifts in the microbial community were closely correlated with pH and specific UV absorbance at 254 nm. We speculated that the inherent traits of different WEOM and the properties of soil solutions under different fertilization treatments shaped the soil microbial community structure, thereby influencing the biodegradation of WEOM.

Water extractable organic carbon and nitrogen and their stable isotopes from long-term experiment in a Japanese rice paddy

Hot water- and water-extracted organic matter was extracted from soil samples collected after a 31-year long-term experiment which aimed to assess the effect of different fertilization strategies (inorganic fertilizers and organic matters) commonly used for paddy rice cultivation in Yamagata, northeastern Japan. The ratio of soil to extracted water was 2:3. The amounts of hot water-extracted organic carbon and nitrogen (HWEOC and HWEN) at 80 o C and 16 hours, water-extracted organic carbon and nitrogen (WEOC and WEN) at room temperature, and their δ 13 C and δ 15 N were measured from the five fertilizer treatment plots as [1) PK, 2) NPK, 3) NPK + 6 Mg ha -1 rice straw (RS), 4) NPK + 10 Mg ha -1 rice straw compost (CM1), and 5) NPK + 30 Mg ha -1 rice straw compost (CM3)], for surface (0-15 cm) and subsurface (15-25 cm) layers. HWEOC and WEOC accounted for an average of about 1.5 1 and 0.66% of SOC, while HWEN and WEN accounted for an average of about 1.09 and 0.40% of soil TN, respectively. About 90% of the extracted N was organic form among all treatments . The values of δ 13 C for HWEOC and WEOC ranged from -28.2 to -26. 5 ‰ and from -28.3 to -27.0‰, similar to the original rice straw and rice straw compost , and lower than the value of original soil at -22.5 ‰. The values of δ 15 N of HWEN, WEN and bulk soil ranged from 0. 8 to 3. 8 ‰, from 1.0 to 4.0‰, and from 0. 8 to 2. 8 ‰, respectively. It was clear that δ 15 N decreased in RS but increased in CM3 treatments. Our results indicated that the amounts of h ot water- and water-extracted organic matter were affected by long-term application of inorganic fertilizers and organic matter s remarkably. However, the values of δ 13 C for HWEOC and WEOC were not different among 5 treatments, but values of δ 15 N of HWEN and WEN were affected by RS and CM3 applications clearly .