Differences In Soil Organic Matter Composition Research Articles

Decreased Soil Organic Matter in a Long‐Term Soil Warming Experiment Lowers Soil Water Holding Capacity and Affects Soil Thermal and Hydrological Buffering

AbstractLong‐term soil warming can decrease soil organic matter (SOM), resulting in self‐reinforcing feedback to the global climate system. We investigated additional consequences of SOM reduction for soil water holding capacity (WHC) and soil thermal and hydrological buffering. At a long‐term soil warming experiment in a temperate forest in the northeastern United States, we suspended the warming treatment for 104 days during the summer of 2017. The formerly heated plot remained warmer (+0.39 °C) and drier (−0.024 cm3 H2O cm−3 soil) than the control plot throughout the suspension. We measured decreased SOM content (−0.184 g SOM g−1 for O horizon soil, −0.010 g SOM g−1 for A horizon soil) and WHC (−0.82 g H2O g−1 for O horizon soil, −0.18 g H2O g−1 for A horizon soil) in the formerly heated plot relative to the control plot. Reduced SOM content accounted for 62% of the WHC reduction in the O horizon and 22% in the A horizon. We investigated differences in SOM composition as a possible explanation for the remaining reductions with Fourier transform infrared (FTIR) spectra. We found FTIR spectra that correlated more strongly with WHC than SOM, but those particular spectra did not differ between the heated and control plots, suggesting that SOM composition affects WHC but does not explain treatment differences in this study. We conclude that SOM reductions due to soil warming can reduce WHC and hydrological and thermal buffering, further warming soil and decreasing SOM. This feedback may operate in parallel, and perhaps synergistically, with carbon cycle feedbacks to climate change.

Soil carbon characterization and nutrient ratios across land uses on two contrasting soils: Their relationships to microbial biomass and function

Soil carbon (C) plays a central role in the global biogeochemical cycles of most major nutrients, but the degree to which the quality versus quantity of C controls microbial abundance and function across land uses is still somewhat uncertain. We measured soil organic matter (SOM) concentration and composition as well as nutrient ratios and other soil characteristics on two contrasting soil types across three land uses (forest, pasture, maize cropping), to determine their relationships to microbial abundance and specific measures of microbial activity (e.g. qCO2, the ratio of respiration rate to microbial biomass, and net laboratory N mineralization). Although there was significant variability in bulk SOM composition (by 13C NMR spectroscopy), we could detect differences between broad-leaved forest and pasture/maize systems on a landscape scale, primarily attributable to differences in aryl C content. Variability in O-alkyl C between sites correlated strongly with the soil C:N ratio, but variability in alkyl C (which was particularly evident in pasture sites) could not be adequately explained by measured environmental or soil characteristics. Soil C:P and N:P ratios followed similar patterns with forest > pasture > maize. Bulk soil C:N, hot-water extractable C:N and particulate C:N all followed similar patterns with forest > pasture ≈ maize cropping. Microbial biomass C:N followed a different pattern, however, with forest ≈ pasture > maize. Despite the differences in SOM composition and nutrient ratios, anaerobically mineralizable N and hot-water extractable C (as a measures of available C) best explained the variation in microbial biomass and function across sites. Anaerobically mineralizable N generally explained the most variation for microbial biomass and qCO2 and had the smallest soil or land use effect. Hot-water extractable C explained the most variance for net N mineralization. Addition of stoichiometric measures and other soil attributes (e.g. soil C:N, C:P, δ15N) in a multiple regression model explained more of the variation than a single factor plus the land use effect (though soil order still explained a small, but significant amount of variance for measures of microbial biomass). A measure of available C, however, was needed to explain the maximum amount of variance in microbial biomass and function across sites (i.e. total C plus nutrient status and other soil attributes could not explain as much of the variance). Our data suggest that nutrient content/stoichiometry does assist in defining the quality of SOM, but a measure of available C (similar to the “active” pool in C models) is also needed. Anaerobically mineralizable N and/or hot water–water extractable C appear to be adequate measures of available C that relate to “active” C, but C functional groups (from 13C NMR spectroscopy) were not particularly useful for this purpose.

Soil organic matter composition and quality across fire severity gradients in coniferous and deciduous forests of the southern boreal region

AbstractRecent patterns of prolonged regional drought in southern boreal forests of the Great Lakes region, USA, suggest that the ecological effects of disturbance by wildfire may become increasingly severe. Losses of forest soil organic matter (SOM) during fire can limit soil nutrient availability and forest regeneration. These processes are also influenced by the composition of postfire SOM. We sampled the forest floor layer (i.e., full organic horizon) and 0–10 cm mineral soil from stands dominated by coniferous (Pinus banksiana Lamb.) or deciduous (Populus tremuloides Michx.) species 1–2 months after the 2011 Pagami Creek wildfire in northern Minnesota. We used solid‐state 13C NMR to characterize SOM composition across a gradient of fire severity in both forest cover types. SOM composition was affected by fire, even when no statistically significant losses of total C stocks were evident. The most pronounced differences in SOM composition between burned and unburned reference areas occurred in the forest floor for both cover types. Carbohydrate stocks in forest floor and mineral horizons decreased with severity level in both cover types, whereas pyrogenic C stocks increased with severity in the coniferous forest floor and decreased in only the highest severity level in the deciduous forest floor. Loss of carbohydrate and lignin pools contributed to a decreased SOM stability index and increased decomposition index. Our results suggest that increases in fire severity expected to occur under future climate scenarios may lead to changes in SOM composition and dynamics with consequences for postfire forest recovery and C uptake.

Biome-scale characterisation and differentiation of semi-arid and arid zone soil organic matter compositions using pyrolysis–GC/MS analysis

Soil organic matter (SOM) is a major pool within the global carbon cycle and understanding its composition (and compositional variability) is critical for predicting its response to future climatic change. Arid and semi-arid regions are typified by relatively low soil organic matter concentrations, but their vast area means that they still represent an important component of the global soil carbon pool. Recent studies have demonstrated the potential of pyrolysis–gas chromatography/mass spectrometry (py–GC/MS) to elucidate basic inter-biome differences in SOM composition. This study provides a first systematic application of py–GC/MS to the analysis of macromolecular SOM across a semi-arid/arid environment; specifically the Fynbos and Succulent Karoo Biomes of South Africa.Data from 27 sites across the region reveal considerable plot-scale variability in SOM quantity and quality, but also demonstrate distinct inter-biome differences in SOM composition. Overall, the pyrolysis data suggest that SOM in the Fynbos Biome contains a higher abundance of minimally/unaltered plant-derived structures, such as lignin and cellulose, in contrast with the more variable, more microbially-altered SOM of the Succulent Karoo. Pyrolysis of soils in the latter biome generally produces higher proportions of simple aromatic and certain nitrogen-bearing pyrolysis products. The majority of soils associated with lowest total organic carbon contents tend to produce a much more limited number of aromatic pyrolysis products. Principal component analyses (PCA) suggest that the overall extent of organic matter degradation is the primary driver of compositional variability across the study area and regardless of biome/ecoregion, the pathways of degradation are essentially similar. Additional affects are also observed, notably in the Succulent Karoo where some soils are relatively enriched in short to mid chain length aliphatic pyrolysis products. Across the whole study area, the extent of degradation, which is broadly represented by the PCA axis 1 scores, is correlated with mean annual rainfall, although the relationship is relatively weak, highlighting the potential significance of other factors, such as local edaphic conditions, soil mineralogy and other mechanisms of dryland SOM degradation such as photo-degradation. Overall, the study demonstrates that despite some known limitations, the systematic application of py–GC/MS can yield insights into large/landscape-scale variability in SOM quality and quantity.

Tillage Management Alters Surface Soil Organic Matter Composition: A Pyrolysis Mass Spectroscopy Study

We examined the effects of 10 yr of reduced tillage (RT) management, compared with conventional tillage (CT), on the chemical composition of soil organic matter (SOM) using a combination of physical fractionation and pyrolysis–field ionization mass spectroscopy (Py–FIMS). Surface soil samples (0–10 cm), which were collected from two adjacent CT and RT managed fields, were separated into particulate organic matter (POM) fractions and sand, silt, and clay size separates. Higher amounts of C (77%) and N (64%) were observed in all size separates of the RT soil and, in particular, the proportion of intramicroaggregate POM was two times as large as in the CT soil. The Py–FIMS spectra showed that larger proportions of free fatty acids, sterols, and N‐containing compounds were present in the RT soil's sand fraction than in the CT soil. Similar observations were also made for both free and intramicroaggregate POM fractions. In the silt and clay size fractions, no substantial differences in SOM composition were measured between tillage treatments, except that the RT soil's clay fraction had a higher proportion of N‐containing compounds and a lower proportion of alkylaromatics. Ratios of carbohydrate hexose to pentose subunits and of lignin monomers + phenols to lignin dimers confirmed a less decomposed character of the SOM in the silt and clay size fractions of the RT soil. These observations can be logically linked to the reduced mixing of crop residues and manure in the soil profile and higher surface soil microbial biomass under RT.

Comparison of organic compounds in the particle-size fractions of earthworm casts and surrounding soil in humid Laos

Storage of soil organic matter (SOM) in earthworm casts contributes to an improvement of soil fertility, especially in the tropics. This study was conducted to elucidate the incorporation of microbe- and plant-derived organic compounds into earthworm casts in humid Laos. Composite samples of earthworm casts and surrounding topsoil (0–10 cm) were fractionated into the following size fractions: coarse sand (250–2000 μm), fine sand (20–250 μm), silt (2–20 μm), and clay (<2 μm). Thereafter, lignin-derived phenols, neutral, acidic, and amino sugars were determined in bulk samples and the fractions. Organic C and N were enriched in earthworm casts, exceeding the respective C and N contents of the surrounding soil by a factor of 1.5, and 1.3, respectively. This enrichment was apparent in all particle-size fractions, and was not restricted to certain organic compounds. Plant-derived lignin, however, was accumulated in preference to microbe-derived amino sugars. The latter were rather redistributed among different size fractions of the casts. As differences in SOM composition between casts and surrounding soil were reflected most clearly for the silt fractions, it is concluded that organic matter storage in the casts of humid Laos is accompanied by the incorporation of SOM into (pseudo) stable silt-sized microaggregates.

Solid-state 13C NMR spectroscopy: characterization of soil organic matter under two contrasting residue management regimes in a 2-year-old pine plantation of subtropical Australia

Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy, with cross-polarisation (CP) and magic angle spinning (MAS), was used to characterize soil organic matter (SOM) in a 2-year-old exotic pine plantation of subtropical Queensland, Australia, under two contrasting harvest residue management regimes. Soil samples were collected from the 0–10 cm depth of experimental plots receiving either no harvest residues (no harvest residues) or the double quantity of harvest residues applied (double harvest residues). Carbon-13 CP and dipolar dephasing (DD) NMR techniques were able to detect differences in SOM composition and quality under the two contrasting residue treatments. The SOM under no harvest residues displayed an increased extent of decomposition, as determined by the alkyl C/ O-alkyl C (A/O-A) ratio, and lower potentially mineralizable nitrogen (PMN), organic C, total P and total N contents. The CP spectra displayed little evidence of strong aromatic signals derived from lignin or tannin structures. This was confirmed by the DD spectra, which rapidly lost signal in the methoxyl and alkyl C regions, indicating protein and amide structures with little mobility might be dominant in the aromatic spectral region. The DD spectra also indicated that SOM under double harvest residues might have a small amount of condensed tannin structures, which did not exist in the SOM under no harvest residues. The carbonyl C region displayed resonances indicative of oxalate, carboxyl, amide and ester C in both treatments. Overall, the results of this study indicate that residue removal following harvest of exotic pine plantations on low-fertility soils in subtropical Australia can remove valuable nutrients from the site, which in turn may increase the extent of decomposition, leading to decreased SOM quality in subsequent rotations.

A direct link between forest vegetation type and soil organic matter composition

Total carbon storage and turnover in soils can be simulated as a series of pools with different turnover rates, ranging from seasonal to millennial. This approach has emphasized the importance of climatic controls on soil organic matter (SOM) dynamics, but implicitly assumes that SOM is minimally influenced by the nature of the plant material from which it is derived. Here we test this assumption by contrasting the influence of climate and vegetation (oak, manzanita, and conifers) on SOM composition in granitic-derived soils from California. Soils developed under the same climate in the San Gabriel Mountains were compared to soils with varying climate along an elevational transect in the Sierra Nevada range. Solid state TOSS CPMAS 13C NMR was used to semiquantitatively characterize the chemical structure of organic matter in litter layers, and low-density and fine silt fractions isolated from sampled A horizons. For all soils, there was a progressive decrease in O-alkyl C, and an increase in alkyl and carbonyl C from the litter to the low-density and fine silt fractions. The NMR spectra of the low-density fractions, and even more so of the fine silt fractions exhibited clear differences in SOM composition associated with different plant genera, regardless of climate. The carbonyl C dominated under oak, O-alkyl C prevailed under manzanita, and alkyl C was prominent under coniferous vegetation. These results indicate that vegetation, not climate, was the factor controlling SOM composition in these soils, and should be taken as a caution against a simplistic climatic interpretation of storage and turnover rate of carbon in soils.