Hot-water Extractable Organic Matter Research Articles

The effect of tree species on seasonal fluctuations in water-soluble and hot water-extractable organic matter at post-mining sites

Seasonal changes in the contents of water-soluble carbon (WSC) and hot water-extractable carbon (HWC) were investigated in the organic and underlying mineral horizons of soils in a common garden experiment at 40years old post-mining sites. The sites were planted with alder, oak, or spruce or overgrown by a spontaneous succession dominated by willow. Sampling was performed at 2-month intervals from April 2013 until March 2014. Two major peaks in WSC in the mineral horizons were evident: one in early summer and another in late autumn or winter. The peaks were more pronounced at sites with deciduous trees than with spruce. According to liquid-state 1H NMR spectroscopy, hot-water extracts (HWE) from the mineral horizons had a higher relative content of aliphatic components in summer, while the winter samples had a larger relative abundance of carbohydrates. The results suggest that the peaks had different sources of carbon. RDA with variation partitioning showed that both tree species and season significantly affected the composition of HWE, but that tree species was most important, explaining 67.3% of data variability. However, the trends in temporal variation in concentrations and chemical composition were remarkably similar at the studied sites suggesting the changes were mainly induced by seasons.

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.

Microbial Communities and Organic Matter Composition in Surface and Subsurface Sediments of the Helgoland Mud Area, North Sea.

The role of microorganisms in the cycling of sedimentary organic carbon is a crucial one. To better understand relationships between molecular composition of a potentially bioavailable fraction of organic matter and microbial populations, bacterial and archaeal communities were characterized using pyrosequencing-based 16S rRNA gene analysis in surface (top 30 cm) and subsurface/deeper sediments (30–530 cm) of the Helgoland mud area, North Sea. Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) was used to characterize a potentially bioavailable organic matter fraction (hot-water extractable organic matter, WE-OM). Algal polymer-associated microbial populations such as members of the Gammaproteobacteria, Bacteroidetes, and Verrucomicrobia were dominant in surface sediments while members of the Chloroflexi (Dehalococcoidales and candidate order GIF9) and Miscellaneous Crenarchaeota Groups (MCG), both of which are linked to degradation of more recalcitrant, aromatic compounds and detrital proteins, were dominant in subsurface sediments. Microbial populations dominant in subsurface sediments (Chloroflexi, members of MCG, and Thermoplasmata) showed strong correlations to total organic carbon (TOC) content. Changes of WE-OM with sediment depth reveal molecular transformations from oxygen-rich [high oxygen to carbon (O/C), low hydrogen to carbon (H/C) ratios] aromatic compounds and highly unsaturated compounds toward compounds with lower O/C and higher H/C ratios. The observed molecular changes were most pronounced in organic compounds containing only CHO atoms. Our data thus, highlights classes of sedimentary organic compounds that may serve as microbial energy sources in methanic marine subsurface environments.

Approaching a Standardized Method for the Hot-Water Extraction of Peat Material to Determine Labile SOM in Organic Soils

Peatland soils are the most effective and important long-term terrestrial carbon (C) storages. To estimate potential C loss, a valid characterization of soil decomposability, in particular the labile fraction, is of great interest. One of the most labile fractions is hot-water-extractable organic matter (HWOM), often measured as hot-water-extractable carbon (Chwe) and nitrogen (Nhwe). Various studies describe different extraction procedures for mineral soils. Because of methodical differences, it is difficult to compare extracted HWOM amounts directly to each other. For peatland soils, few studies exist. The aim of the present study is the development of a standardized method for the hot-water extraction of peat materials. Therefore, we extracted HWOM in various replicates from different peats on the basis of a standardized extraction method for mineral soils (1 h extraction at 100 °C). We tested how differences in soil/water ratios, extract treatment (filtering vs. not filtering), and sample pretreatment (freeze drying vs. air drying) influence HWOM amounts. The results clearly illustrated the influence of changing soil/water ratios on HWOM amounts. Mean Chwe concentrations ranged between 8 and 34 g kg−1 whereas Nhwe ranged between 0.2 and 2.6 g kg−1. We recommend the extraction under soil/water ratios of 1/800 to provide sufficient volume of solvent for Chwe. If relative differences for Nhwe amounts are greater than 15 percent, samples should be extracted again under soil/water ratios greater than 1/300 to avoid analytical errors due to unintended dilution effects. Filtering of centrifuged and decanted extracts before analysis is not necessary. Peat material should be either air dried or freeze dried before extraction.

Effects of calcium silicate treatment on the composition of forest floor organic matter in a northern hardwood forest stand

Calcium amendment can help improve forest sustainability in stands that have been impacted by chronic acid deposition. An important component of this improvement is the stimulation of the microbial activity that supports ecosystem nutrient cycling processes. To test the hypothesis that Ca treatment alters the structure and solubility of organic matter substrates, an important driver of microbial activity, we investigated the effect of wollastonite (CaSiO3) treatment on soil organic matter (SOM) and hot-water-extractable organic matter (HWEOM). We found a decrease in the HWEOM content of forest floor soils within 2 years of treatment with a high dosage of wollastonite (4,250 kg Ca/ha), but not at a low dosage (850 kg Ca/ha). High-dosage treatment did not reduce the biodegradability of HWEOM. Hence, a high dose of CaSiO3 appears to reduce the solubility of organic matter in the forest floor but not the bioavailability of the extracted SOM. Nuclear magnetic resonance spectroscopy revealed no significant changes in the O-alkyl C content of SOM in response to wollastonite addition, but a reduction in the O-alkyl C content of HWEOM suggests that the extractability of carbohydrate structures was reduced by added CaSiO3. Phosphorous treatment, when performed in combination with Ca, also decreased the O-alkyl C content of HWEOM, but had no effect when performed without Ca. The reduced solubility of SOM after Ca treatment may have been the result of bridging between Ca2+ and negatively charged sites on SOM, as suggested in other studies. Also, high concentrations of Si in soil solution, due to dissolution of the wollastonite, likely resulted in oversaturated conditions with respect to SiO2 or kaolinite, perhaps leading to co-precipitation of soluble organic matter. Overall, our results suggest that added Ca and/or Si may react with SOM to reduce the accessibility of labile C forms to soil microbes.

Effects of calcium treatment on forest floor organic matter composition along an elevation gradient

Calcium amendment is a restorative option for nutrient-depleted, acidic soils in the forests of the northeastern United States. We studied the effects of watershed-scale wollastonite (CaSiO3) application on the structural composition of soil organic matter (SOM) and hot-water-extractable organic matter (HWEOM) at the Hubbard Brook Experiment Forest in New Hampshire 7–9 years after treatment, along an elevation gradient. Soils in the high-elevation spruce–fir–birch (SFB) zone contained significantly greater amounts of HWEOM compared with lower elevation hardwood soils, likely due to differences in litter quality and slower decomposition rates in colder soils at higher elevation. The only significant difference in hot-water-extractable organic carbon concentration between reference and calcium-treated watersheds was in Oie horizons of the SFB zone, which also exhibited the greatest degree of soil chemical change after treatment. The 13C nuclear magnetic resonance (NMR) spectra showed no significant patterns in O-alkyl C abundance for either soil or HWEOM along the elevation gradient, suggesting that there were no elevation-related patterns in carbohydrate concentration. The general absence of long-term effects in this study suggests that effects of Ca amendment at this dosage on the composition of SOM were small or short-lived.

Effects of land use changes on the spectroscopic characterization of hot-water extractable organic matter along a chronosequence: Correlations with soil enzyme activity

Given the importance in soil C cycling, changes in the composition of hot-water extractable organic matter (HWEOM) due to the land use conversion are less known. The purposes of this study were to elucidate the structural characteristics of HWEOM from soils collected from a native wetland and five croplands which originated from the wetlands by conversions for 3, 8, 16, 22, and 24 years, and to relate the structural characteristics to soil enzyme properties. We examined the structural characteristics of HWEOM by UV absorption and fluorescence. The values of specific absorption at 254 nm, humification index and fluorescence efficiency index of HWEOM showed a tendency to increase initially compared to the values of wetland (the highest value was found in the cropland with cultivation-time of 16 years), and then the values tended to decline with time. Soil β-glucosidase and N-acetyl-β-glucosaminidase activities were negatively affected by the structural characteristics of soil HWEOM. Our results indicate that the HWEOM from the croplands contained more highly condensed aromatic compounds, and were less mobile than that from the native wetland. Furthermore, the stabilities of HWEOM in the croplands with short/long cultivated-time were different.

Compositional characterization of soil organic matter and hot-water-extractable organic matter in organic horizons using a molecular mixing model

Purpose Microbial decomposition of soil organic matter (SOM) is generally believed to be heterogeneous, resulting in the preferential loss of labile compounds such as carbohydrates and proteins and the accumulation of recalcitrant compounds such as lipids and lignin. However, these fractions are difficult to measure directly in soils. We examined patterns in the biomolecular composition of SOM and hot-water-extractable organic matter (HWEOM) by using a molecular mixing model (MMM) to estimate the content of carbohydrates, protein, lipids, and lignin.

Molecular weight separation of hot-water extractable soil organic matter using high-performance size exclusion chromatography with chemiluminescent nitrogen detection

To characterize soil organic nitrogen (N), we applied high-performance size exclusion chromatography (HPSEC) and chemiluminescent nitrogen detection (CLND) to the molecular weight separation of hot-water extractable organic matter (HWEOM) from soils that had been amended with or without plant residue compost applied at a rate of 2 kg m−2 y−1 for 25 years. The N chromatogram of the HWEOM showed that the HWEOM containing N had molecular weights ranging from 0.1 to 100 kDa. The N chromatograms of HWEOM from soils without compost differed among four soils (Cumulic Andosol, Low-humic Andosol, Gray Lowland soil, and Yellow soil). The N intensities in the HWEOM chromatograms were larger in the soils with compost amended than in those without compost over the whole range of molecular weights. These results demonstrate that HPSEC/CLND is a useful tool to investigate the structure and dynamics of water soluble organic N in soil.

Biodegradation and chemical characteristics of hot-water extractable organic matter from soils under four different vegetation types in the Wuyi Mountains, southeastern China

Water extractable organic carbon (WEOC) is the most active soil organic carbon and plays an important role in global C cycling. Our aim was to investigate the biodegradability and the chemical characteristics of hot-water extractable organic matter (HWEOM) from soils under different vegetation types, including evergreen broad-leaved forest (EBF), coniferous forest (CF), dwarf forest (DF) and alpine meadow (AM), across an altitude gradation in the Wuyi Mountains of southeastern China. We quantified the biodegradability in a 90 day incubation experiment by measuring the loss of hot-water extractable organic carbon (HWEOC) and calculated the sizes of labile and stable HWEOC pool and the respective decomposition rate constants by a double exponential model. We used UV and fluorescence spectroscopic techniques to characterize the structural properties of HWEOM and measured the contents of carbohydrates, phenols and hydrophilic carbon of HWEOM solution. The extent of HWEOC biodegradation was higher in EBF and DF soils with about 20% HWEOC loss, and the labile C accounted for 13–15% of the total HWEOC. HWEOC from CF soils was the least degradable with only 12% HWEOC loss. The estimated mean residence time of the labile HWEOC pool was 8–10 days, whereas that of the stable HWEOC pool ranged from 3 to 6 years. The biodegradability of HWEOC increased with decreasing soil C/N ratio, aromaticity and humification indices, proportion of phenols and increasing proportion of hydrophilic carbon.

Spectroscopic characterization of hot-water extractable organic matter from soils under four different vegetation types along an elevation gradient in the Wuyi Mountains

Water extractable organic carbon (WEOC) is the most active component in global carbon cycle and its chemical and structural characteristics most likely influence its biodegradation. Our objective was to investigate the structural characteristics of hot-water extractable organic matter (HWEOM) from soils collected at various depths (S1, S2, S3) under four different vegetation types, i.e. evergreen broad-leaved forest (EBF), coniferous forest (CF), dwarf forest (DF) and alpine meadow (AM) along an elevation gradient in the Wuyi Mountains, and to relate its structural characteristics to soil properties and biological properties. Here we examine the structural characteristics of HWEOM by UV, fluorescence and Fourier-transform infrared (FTIR) spectroscopic techniques. In the synchronous fluorescence spectra of all HWEOM samples, the main emission peaks were aromatic amino acid-like fluorophores, fulvic acid-like fluorophores and polycyclic aromatic structures with a high degree of conjugation. FTIR spectroscopy showed that HWEOM featured aliphatic C–H, aromatic C=C and carbohydrate C–O functional groups. The aromaticity, the humification indices (HIXem, HIXsyn) and the fluorescence efficiency of HWEOM from the S1 layer were significantly higher in CF than those in the other three vegetation types and, in all cases they decreased with depth in the soil profile. Soil C/N ratio was highly correlated with UV absorption, humification indices and fluorescence efficiency of HWEOM. Soil metabolic quotient (qCO 2) was highly negatively correlated with UV absorption and humification indices of HWEOM. The ratio of microbial biomass C to total organic C was significantly correlated with humification indices and fluorescence efficiency of HWEOM. Our results indicate that the HWEOM from CF soils contain more highly condensed aromatic and/or heterocyclic compounds and, at the same time, the high molecular weight and complex molecules are less mobile, leading to being preferentially retained in the upper soil layer.

Molecular‐Scale Characterization of Hot‐Water‐Extractable Organic Matter in Organic Horizons of a Forest Soil

Hot‐water‐extractable organic matter (HWEOM) has been shown to be highly correlated with microbial biomass in forest soils. We conducted elemental and 13C solid‐state nuclear magnetic resonance spectroscopic analyses to assess the composition and structural chemistry of HWEOM and its variations with soil depth within O horizons in a forest site in New Hampshire. The HWEOM fraction exhibited a higher H/C ratio and higher O‐alkyl C proportion than the soil from which it was extracted. It also had a 30 to 40% lower C/N ratio than the whole soil. The relative proportion of O‐alkyl C in the HWEOM increased with soil depth in the forest floor, while alkyl C decreased, contrary to the pattern observed for the whole soil. The spectral and elemental properties of HWEOM present in these acidic Spodosols support the hypothesis that HWEOM is largely a mixture of carbohydrates and proteins. We estimate that while HWEOM includes both labile C fractions and microbial biomass, microbial biomass can account for no more than 40% of the C extracted by hot water.

Effect of mulching on labile soil organic matter pools, microbial community functional diversity and nitrogen transformations in two hardwood plantations of subtropical Australia

The aims of this study were to: (1) assess the effects of mulching on labile soil organic matter (SOM) pools (water-soluble organic matter, WSOM; hot water extractable organic matter, HWEOM; microbial biomass, MB; acid hydrolysable organic matter, AHOM), microbial community functional diversity and soil nitrogen (N) transformations and (2) quantify the relationships among labile SOM pools, soil N transformations and microbial community functional diversity. Soil (0–10 cm) was collected from two hardwood plantations in subtropical Australia to which plant residues (1.57 kg m −1, C/N ratio of 80) had previously been applied as mulch for 12 months. Both in southeast Queensland, Australia, one plantation was located in dry area (Pechey, average annual rainfall 851 mm) and the other at Proston, drier by comparison (average annual rainfall 613 mm). At both locations, soil WSOM-carbon (C), HWEOM-C, MB-C, and AHOM-C were significantly higher in the mulch treatment than in the non-mulch treatment. The soil microbial community functional diversity (Shannon's diversity index, H′) measured by BIOLOG GN2 micro-plates was significantly greater in the mulch treatment than in the non-mulch treatment. The mulch treatment had significantly higher soil gross N mineralization and gross N immobilization rates than the non-mulch treatment. However, there was no significant difference in soil gross nitrification between the two treatments. Stepwise multiple regressions revealed that HWEOM-C had significant correlation with soil microbial functional diversity and explained more of the variation in microbial functional diversity than any other labile SOM pools. The MB-N explained more of the variations in gross N transformations than other labile SOM pools.

Soil organic matter under different forest types in Southern China

Conversion of native broadleaved forests to pure coniferous plantation is a common management practice driven by an increasing demand for timber production. A study was conducted to evaluate the impact of land-use change from native broadleaved forest to pure coniferous plantation on soil organic matter quantity and quality in San Menjiang Forest, in southern China. Additionally, the potential of hot-water extractable organic matter as an indicator of changes in SOM was also assessed. Significant differences in labile SOM fractions (hot-water extractable organic C and total N, cold-water extractable organic C and total N, and microbial biomass C and N) between pure Cunninghamia lanceolata Hook plantation and native broadleaved forest were tested using ANOVA. The contents of soil organic C, total N, and labile organic matter such as hot-water extractable organic C and total N, cold-water extractable organic C and total N, and microbial biomass was significantly lower in pure C. lanceolata plantation than in native broadleaved forest. The percentage ratios of hot-water extractable organic C and N, cold-water extractable organic C, and microbial biomass C and N to soil organic C or total N were also significantly reduced in C. lanceolata plantation. Hot-water extractable organic C and total N were significantly correlated with cold-water extractable organic C and total N and microbial biomass C and N in both native broadleaved forest and pure coniferous plantation ( r > 0.608, P < 0.01). The correlation coefficients between hot-water extractable organic matter and soil organic C or total N ( r > 0.694, P < 0.01) were higher than that between cold-water extractable organic matter, microbial biomass and soil organic C or total N ( r > 0.525, P < 0.05). Therefore, land-use change from native broadleaved forest to coniferous plantation reduced soil organic matter quantity and quality, and hot-water extractable organic matter can be used as an indicator of changes in SOM quality in forest soils.

Labile, recalcitrant, and inert organic matter in Mediterranean forest soils

The biochemical quality of soil organic matter (SOM) was studied in various profiles under Quercus rotundifolia Lam. stands on calcareous parent material. Special attention was paid to the question of how biochemical quality is affected by position within the soil profile (upper versus lower horizons). The following global SOM characteristics were investigated: (a) overall recalcitrance, using hydrolysis with either hydrochloric or sulphuric acid; (b) hydrolyzable carbohydrates and polyphenolics; (c) extractability by hot water and quality of the extract; and (d) abundance of inert forms of SOM: charcoal and soot-graphite. The recalcitrance of soil organic carbon (OC) decreases with depth, following the order: H horizons>A horizons>B horizons. In contrast, the recalcitrance of nitrogen is roughly maintained with depth. The ratio carbohydrate C to total OC increases from H to B horizons, due to the increasing importance of cellulosic polysaccharides in B horizons, whereas other carbohydrates are maintained throughout the soil profile at a relatively constant level, 12–15% of the total OC in the horizon. Whereas the quality of the hydrolyzable carbon (measured by the carbohydrate to polyphenolic C ratio) decreases with depth from H to B horizons, the quality of the hot-water extractable organic matter is much higher in B horizons than in A or H horizons. The relative importance of both charcoal and soot-graphitic C and N tends to increase with depth. The ratio black/total is usually higher for N than for C, a result that suggests that inert SOM may represent a relevant compartment in the nitrogen cycle. Overall, our data suggest that in Mediterranean forest soils the organic matter in B horizons could be less stable than often thought.

Cold and hot water–extractable organic matter as indicators of litter decomposition in forest soils

AbstractMild extractions were used as indicators of easily decomposable organic matter (OM). However, the chemical composition of extracted OM often remained unclear. Therefore, the composition of cold and hot water–extractable OM was investigated in the O horizons (Oi, Oe, Oa) of a 170 y old beech stand (Fagus sylvatica) in the Ore Mtns., SE Germany. To simulate litter decomposition, the O horizon samples were incubated for 1 week under defined conditions. Cold‐ and hot‐water extracts were analyzed and chemically characterized by pyrolysis–field ionization mass spectrometry (Py‐FIMS). The C and N concentrations were always lower in the cold‐(C: 2.69 to 3.95 g kg–1; N: 0.14 to 0.29 g kg–1) than in the hot‐water extracts (C: 13.77 to 15.51 g kg–1; N: 0.34 to 0.83 g kg–1). The C : N ratios of both extracts increased with increasing depth. Incubation increased the concentrations of C and N in all water extracts, while C : N ratios of extracts decreased. The molecular‐chemical composition of cold and hot water–extracted OM revealed distinct differences. Generally, cold water–extracted OM was thermally more stable than hot water–extracted OM. The mass spectra of the hot water–extracted organic matter revealed more intensive signals of carbohydrates, phenols, and lignin monomers. Additionally, the n‐C28 fatty acid and the n‐C38–to–n‐C52 alkyl monoesters clearly distinguished the hot‐ from the cold‐water extract. A principle‐component analysis visualized (1) alterations in the molecular‐chemical composition of cold‐ and hot‐water extracts due to previous incubation of the solid O horizon samples and (2) a decomposition from the Oi to the Oh horizon. This provides evidence that the macromorphological litter decomposition was reflected by the chemical composition of water extracts, and that Py‐FIMS is well‐suited to explain at the molecular level why OM decomposability is correlated with water‐extracted C.

Relations entre stabilité de l'agrégation et matière organique totale et soluble à l'eau chaude dans des sols ferrallitiques argileux (Congo, Brésil)

Organic matter (OM) generally plays an important role in soil aggregate stability. The objective of this work was to characterize the hot water-extractable OM and its role in the aggregate stability of clayey ferrallitic soils under different land use management. The macroaggregate (&gt;200 µm) stability of these soils was determined before (AS) and after hot-water extraction (ASe). The contents in total organic carbon and in carbon present as carbohydrates, as well as the sugar composition, were determined on the bulk soils and their hot-water extracts.The carbon and macroaggregate contents decreased upon land-clearing and cultivation, but to a lesser extent when some cultural practices were used. Whatever the situation considered, the hot-water extract always presented a higher carbohydrate content than the bulk soil. The high values of ratios r = (galactose + mannose)/(arabinose + xylose) suggested that a large proportion of carbohydrates was of microbial origin. There were significant correlations between AS and the carbon content of bulk soils or hot-water extracts, and between AS and the soil carbohydrate content. On the opposite, whatever the soil sample studied, no significant difference could be found between AS and ASe, showing that hot-water extractable OM had thus no clear aggregating role in these soils. Key words: Organic matter, macroaggregate stability, hot-water extraction, carbohydrates, ferrallitic soils