Changes In Soil Organic Matter Composition Research Articles

Long-term ley and manure managements have consistent effects on microbial functional profiles and organic C groups across soils from a latitudinal gradient

Soil organic matter (SOM) is important in maintaining soil fertility and other ecosystem functions. Yet, land management in intensive agriculture has caused SOM level to decrease, with knock-on effects for soil fertility and quality. Therefore, land management options that ensure that SOM is not depleted and that soil functions are better sustained are of increasing interest. However, there is limited knowledge on how different land managements affect the composition of SOM and associated microbial functional profiles. Twelve long-term field experiments, covering a wide range of climatic zones and soil types, were selected in Sweden. They focused on the role of combining ley in crop rotations with the manure application (livestock farm), as opposed to the management without ley and receiving only inorganic fertilizer (arable farm). In ten out of the 12 study sites, livestock farm management tended to have higher proportions of aliphatic and double bonded C groups, as estimated by mid-infrared spectroscopy. This was further confirmed by 13C NMR analysis, which found greater proportions of O-alkyl and di-O-alkyl groups and less aromatic C in livestock farm than arable farm management in five of the eight sites analyzed. The changes in SOM composition were reflected in microbial functional profiles across many sites: soils from livestock farm management utilized more carbohydrates and amino acids, while polymer and aromatic compounds were associated with arable farm management. Overall, shifts in both microbial functional profiles and SOM composition showed great consistency across geographical and climatic zones. Livestock farm management maintained higher levels of microbial functional diversity and were associated with higher proportions of “reactive” C functional groups. Our investigation demonstrates that livestock farm management could maintain soil fertility over the long-term via the changes in SOM composition and the regulation of microbial functional profiles.

Long-term crop rotation and different tillage practices alter soil organic matter composition and degradation

Conservation agriculture such as conservation tillage and crop rotation can alter soil organic matter (OM) storage and cycling. However, little is known about how these methods alter the molecular-level soil OM composition and turnover. We collected soil samples (0−10 cm) from a 37-year experiment with contrasting diversity crop rotations under conventional tillage and conservation tillage. Soil organic carbon concentrations, molecular-level OM composition and microbial biomass and community structure were measured. Soil organic carbon concentrations were similar between the two tillage practices, likely due to the balance between carbon loss via degradation and carbon gains through incorporation of crop residues in soil. We observed significant changes in soil OM composition between conventional tillage and conservation tillage despite similar soil organic carbon concentrations. Under long-term conservation tillage practice, lower suberin-derived compounds and higher lignin-derived compounds were observed. The degradation of cutin-, lignin-derived compounds and soil OM were lower under conservation tillage among most rotations. The changes in lignin-derived compounds and degradation were microbially driven and likely controlled by substrate utilization. In contrast, the differences in suberin-derived compounds were likely associated with crop residue-induced OM inputs and/or the differential introduction of OM (e.g. root materials) to the upper layer of soil due to conventional tillage vs. conservation tillage managements. These findings reveal that microbial degradation, OM inputs from crop residues and management-induced OM distribution in soil layers may alter agricultural soil OM composition. Additionally, we found that the differences in some specific groups of compounds between conventional tillage and conservation tillage were lower in crop rotations with higher diversity, highlighting the benefits of the interaction between diverse rotations and conservation tillage.

Stabilization of organic C in an Indo-Gangetic alluvial soil under long-term manure and compost management in a rice–wheat system

The nature of carbon input to soil, its transformation to soil organic matter (SOM), and the degree of protection from decomposition are important for long-term persistence of SOM. The effect of exogenous addition of organic matter on SOM build-up is well known; however, changes in quality vis-à-vis composition of added organic matter have been less investigated. It is necessary to ascertain the form under which C accumulates and how the composition of added organic matter influences the stabilization of soil organic C (SOC). Changes in composition of SOM were studied by fractionating the soil into coarse (cPOC; >250 µm) and fine particulate organic C (fPOC; 53–250 µm) and mineral-associated organic C (MinOC; <53 µm) and measuring the acid non-hydrolyzable C (NHC) as well as mineralizable C following repeated applications of farmyard manure (FYM), rice straw compost (RSC) and vermicompost (VC) in a rice (Oryza sativa L.)–wheat (Triticum aestivum L.) system field experiment. Addition of organic amendments for 10 years significantly increased SOC stocks and the increase was greatest with RSC (12.2 t ha−1) followed by FYM (9.1 t ha−1), and lowest with VC (8.5 t ha−1), compared with solitary application of NPK. The C accumulation in separated pools was influenced by the chemical composition of the organic source. Compost applications accumulated C preferentially in the MinOC pool, whereas the FYM accumulated a greater proportion of C as POC, particularly as fPOC. The differences were attributed to the lower C/N ratio and higher lignin content of RSC than FYM. The RSC application increased NHC stocks to a significantly greater extent than FYM and VC, indicating recalcitrance of the accumulated C. This was substantiated by the significantly lower C mineralization potential of RSC-amended soils compared to the FYM- and VC-amended soils. The results showed that addition of compost, besides leading to C build-up, improved C stabilization by imparting recalcitrance to SOC.

Molecular-level changes in soil organic matter composition after 10 years of litter, root and nitrogen manipulation in a temperate forest

With climate change, forests are expected to receive increased inputs of carbon (C) and nitrogen (N) but it is unclear how this will modify forest C cycling and storage at the molecular-level. To investigate the response of forest soil organic matter (SOM) to changes in soil inputs, a study area was established in a Michigan hardwood forest as part of the Detrital Input and Removal Treatments (DIRT) network. Experimental treatments were comprised of both exclusions of detrital inputs (No Litter, No Roots, No Inputs) and additions of C and N (Double Litter, N-Addition, Double Litter + N, Wood). After 10 years of treatment, the soils were characterized using elemental analysis, molecular biomarker techniques, nuclear magnetic resonance spectroscopy, and microbial biomass C measurements. Although manipulation of detrital inputs did not significantly change the soil C and N content after 10 years, alterations in the cycling and distribution of SOM components were observed. Root exclusion enhanced SOM degradation, while doubling litter favoured the degradation of more labile forms of soil C such as unsaturated n-alkanoic acids and simple sugars. N-Addition and Double Litter + N increased the concentrations of extractable biomarkers, including aliphatic and cyclic lipids and compounds derived from cutin, suberin, and lignin. Microbial biomass C also varied with experimental litter input manipulations and N addition, and these data were consistent with the observed changes in SOM composition. Overall, the observed shifts in SOM chemistry after 10 years of manipulating ecosystem inputs highlight the sensitivity of natural systems to changes in amounts of C and N inputs from roots and litter, and N inputs from external sources.

Distinct changes in composition of soil organic matter with length of cropping time in subsoils of a Phaeozem and Chernozem

SummaryDeeper soil horizons might provide an opportunity to enhance C sequestration because soil organic matter (SOM) at depth is assumed to be stable. However, it is unknown whether the stable composition of organic carbon in subsoils changes with the length of cropping time and the type of soil. The objectives of this study were to determine the effects on the chemical structures of SOM of cropping time after conversion from grassland to arable land under a Phaeozem and Chernozem in northeast China. Near‐quantitative multiple cross‐polarization (multiCP)13C nuclear magnetic resonance (NMR) spectroscopy was applied, and 11 types of carbon (C) functional groups were identified. Principal component analysis of these functional groups showed that the chemical composition of SOM was differentiated by soil type and depth. The Phaeozem and Chernozem profiles differed mainly in their relative proportions of aromatic C–C and (CH2)ngroups: the Phaeozem contained relatively more aromatic C–C, whereas the Chernozem contained relatively more (CH2)ngroups. The fused‐ring aromatic C–C carbon was probably derived from char‐like organic matter generated by burning of plant litter or from SOM humification, whereas the (CH2)ngroups were likely to be from plant‐ or microbially‐derived residues. The main differences between top‐ and sub‐soils were the occurrence of more protonated C in the topsoils and more non‐protonated C in the subsoils. With increasing length of cropping time, aromatic C–C and C–O groups and COO/N–C=O groups increased, but (CH2)ngroups decreased in the Phaeozem subsoils and increased in the Chernozem subsoils. Our findings suggested that leaching and soil moisture might influence the origin, redistribution and transformation of the recalcitrant components of SOM in the soil profile, resulting in changes in SOM composition under different climates and soil types.HighlightsCharacterization of chemical structures of soil organic matter (SOM) in soil profiles by solid‐state13C NMRSOM composition varied between a Phaeozem and Chernozem and to a larger extent in the subsoilsFused‐ring aromatics and (CH2)ngroups accumulated in subsoils of the Phaeozem and Chernozem, respectivelySOM composition of subsoil changed probably with precipitation‐controlled translocation or transformation

Coupling pyrolysis with mid-infrared spectroscopy (Py-MIRS) to fingerprint soil organic matter bulk chemistry

A novel method coupling pyrolysis with mid-infrared spectroscopy (Py-MIRS) was developed to characterize soil organic matter (SOM) chemistry in soils. The pyrolyzer was interfaced to the MIR spectrometer by means of a Brill cell™ (CDS Analytica). The set-up generates pyrolysis fingerprint spectra from which individual pyrolysis products can be related to SOM bulk chemistry. Py-MIRS development involved the testing of experimental conditions like pyrolysis temperature (550, 700, 1000 °C), heating rate (20 °C s−1 and 20 °C ms−1) and time (15, 30 and 60 s) using reference standard compounds ranging from carbohydrates to phenols varying in chemical and structural composition like levoglusogan, gluten, tannin, syringol, pectin and leucine falling within different compound categories (carbohydrates, amino acids, proteins, phenols, etc.) as well as soil samples. Pyrolysis yields of prominent specific functional groups, like aliphatics (CH stretching at 2930 cm−1) and CC aromatics (1510 cm−1), varied with pyrolysis temperature, heating rate and time. The preferred settings for high pyrolysis yield and minimized secondary reactions were obtained at a pyrolysis temperature of 700 °C, heating rate of 20 °C ms−1 and heating time of 30 s. The suitability of Py-MIRS to detect changes in SOM composition was evaluated by comparing Py-MIRS results to Diffuse Reflectance Fourier Transform mid-Infrared Spectroscopy (DRIFTS) results. Soil samples taken from the Static Fertilization Experiment, Bad Lauchstädt, Germany (Chernozem) revealed a major SOM contribution of the peak at 1750 cm−1 (CO), followed by peaks at 950 (CH), 1510 (CC), 1176 (CH, OH) cm−1, with smaller contributions from the 2930 (CH) and 3015 (CH4) cm−1 peaks, apart from a dominant CO2 peak. Using the preferred pyrolysis settings, Py-MIRS as well as DRIFTS results further indicated that soils receiving organic (e.g. farmyard manure) inputs were highly enriched in aliphatic groups, while their absence favored the accumulation of carboxyl and aromatic groups as well as polysaccharides. Py-MIRS allowed via semi-quantification of pyrolysis products a rapid monitoring of SOM bulk chemistry with a high degree of reproducibility. It was concluded that Py-MIRS represents a fast, effective and reproducible technique to characterize changes in the SOM bulk chemistry as a result of management practices. It also allows to minimize acknowledged constraints of other analytical techniques used to characterize SOM bulk chemistry such as mineral interferences and associated secondary reactions.

Nitrogen-rich compounds constitute an increasing proportion of organic matter with depth in Oi-Oe-Oa-A horizons of temperate forests

Soil organic matter (SOM) in O horizons of temperate forests forms a natural gradient of increasing decomposition, as operationally separated into the Oi (leaf litter), Oe (partially decomposed), and Oa (highly decomposed) sub-horizons. This study determines the contribution of extractable nitrogen (N)-rich biomolecules and characterizes changes in SOM composition within O horizon decomposition gradients and in underlying A horizons of three temperate forest stands. Four biomolecules were chosen for this investigation: chlorophyll, protein, DNA, and chitin; combined, they represent the majority of identifiable N-rich organic compound classes present in soils. Selective chemical extractions and purification methods provide estimates of concentrations of each distinct biomolecule class, while short-term respiration measurements provide insight into possible relationships with microbial activity. Changes in the molecular character of SOM are probed using Fourier Transform Infrared (FTIR) and Carbon K-edge X-ray Absorption Near Edge Structure (C-XANES) spectroscopies. Differences in biomolecule concentrations and respiration by horizon reveal two contrasting trends with increasing decomposition and depth: chlorophyll and respiration decrease, whereas protein, DNA and chitin increase. Increasing contribution of these three extractable N-rich biomolecules with progressive decomposition and depth at all sites correspond to decreasing concentrations of C and N and to increases in C:N ratio. These trends are consistent with FTIR spectral results, which indicate a relative increase of NH bending and aromatic CN vibrations, and with C-XANES spectral results, which show a relative increase of aromatic CN transitions. These multiple lines of evidence demonstrate that extractable N-rich biomolecules constitute a growing proportion of the organic C and N content within an organic matter decomposition gradient and that this trend continues into the underlying mineral soil. We posit that the persistence of these intact N-containing biomolecules represents a growing contribution from the microbial biomass pool, but also from non-specific interactions among biomolecules in the Oe and Oa horizons and from biomolecule-mineral interactions in mineral soils. The results of this investigation (increases in the proportion of extractable N-rich biomolecules) also provide valuable insight into the potential bioavailability and biochemical structure of SOM in relation to degree of decomposition.

13C-NMR, PAHs, WSOC and water repellence of fire-affected soils (Albic Podzols) in lichen pine forests, Russia

Soil organic matter (SOM) in boreal forest ecosystems can be strongly transformed by fires which, in turn, can affect soil physical and biological properties such as water repellency and nutrient cycling. The article contains the results of a study on SOM content in fire-affected lichen pine forests of the middle taiga subzone of the European part of Russia (Komi Republic). Soils which survived surface fires 2, 10, 16 years ago and unburned area were studied. We investigated changes in SOM composition using 13C NMR spectroscopy, and we measured water-soluble organic carbon (WSOC) content, polycyclic aromatic hydrocarbons (PAHs) concentrations, and contact angles of water droplets on soils. Forest fires increased pyrogenic organic carbon content and also largely transformed SOM composition. 13C NMR spectroscopy showed that the SOM composition of densimetric fractions in pyrogenic and unburned soil greatly differs. Fires increased the aromatic content in SOM composition of light fractions. The highest amount of aromatic carbon probably pyrogenic origin accumulated in fractions of free and occluded organic matter. According to 13C NMR spectroscopy, the aromatic component of organic matter increased in the pyrogenic horizons. The strongest PAHs concentrations occurred in the fractions of free particulate and occluded organic matter, in reference and fire-affected soils. The presence of PAHs in light densimetric fractions may serve as markers of past fire and fire intensity. In the initial years after fire, there was a decrease in the content of WSOC compounds; however, the WSOC content recovered with the recovery of ground vegetation. Our results showed that the greatest changes of contact angles were in upper pyrogenic soil horizons, whereas fires did not modify the contact angle value of lower mineral horizons.

Thermal alteration of soil organic matter properties: a systematic study to infer response of Sierra Nevada climosequence soils to forest fires

Abstract. Fire is a major driver of soil organic matter (SOM) dynamics, and contemporary global climate change is changing global fire regimes. We conducted laboratory heating experiments on soils from five locations across the western Sierra Nevada climosequence to investigate thermal alteration of SOM properties and determine temperature thresholds for major shifts in SOM properties. Topsoils (0 to 5 cm depth) were exposed to a range of temperatures that are expected during prescribed and wild fires (150, 250, 350, 450, 550, and 650 °C). With increase in temperature, we found that the concentrations of carbon (C) and nitrogen (N) decreased in a similar pattern among all five soils that varied considerably in their original SOM concentrations and mineralogies. Soils were separated into discrete size classes by dry sieving. The C and N concentrations in the larger aggregate size fractions (2–0.25 mm) decreased with an increase in temperature, so that at 450 °C the remaining C and N were almost entirely associated with the smaller aggregate size fractions ( &lt; 0.25 mm). We observed a general trend of 13C enrichment with temperature increase. There was also 15N enrichment with temperature increase, followed by 15N depletion when temperature increased beyond 350 °C. For all the measured variables, the largest physical, chemical, elemental, and isotopic changes occurred at the mid-intensity fire temperatures, i.e., 350 and 450 °C. The magnitude of the observed changes in SOM composition and distribution in three aggregate size classes, as well as the temperature thresholds for critical changes in physical and chemical properties of soils (such as specific surface area, pH, cation exchange capacity), suggest that transformation and loss of SOM are the principal responses in heated soils. Findings from this systematic investigation of soil and SOM response to heating are critical for predicting how soils are likely to be affected by future climate and fire regimes.

Tillage-induced short-term soil organic matter turnover and respiration

Abstract. Tillage induces decomposition and mineralisation of soil organic matter (SOM) by the disruption of macroaggregates and may increase soil CO2 efflux by respiration, but these processes are not well understood at the molecular level. We sampled three treatments (mineral fertiliser: MF; biogas digestate: BD; unfertilised control: CL) of a Stagnic Luvisol a few hours before and directly after tillage as well as 4 days later from a harvested maize field in northern Germany and investigated these samples by means of pyrolysis-field ionisation mass spectrometry (Py-FIMS) and hot-water extraction. Before tillage, the Py-FIMS mass spectra revealed differences in relative ion intensities of MF and CL compared to BD most likely attributable to the cattle manure used for the biogas feedstock and to relative enrichments during anaerobic fermentation. After tillage, the CO2 effluxes were increased in all treatments, but this increase was less pronounced in BD. We explain this by restricted availability of readily biodegradable carbon compounds and possibly an inhibitory effect of sterols from digestates. Significant changes in SOM composition were observed following tillage. In particular, lignin decomposition and increased proportions of N-containing compounds were detected in BD. In MF, lipid proportions increased at the expense of ammonia, ammonium, carbohydrates and peptides, indicating enhanced microbial activity. SOM composition in CL was unaffected by tillage. Our analyses provide strong evidence for significant short-term SOM changes due to tillage in fertilised soils.

Changes in SOM composition and stability to microbial degradation over time in response to wood chip ash fertilisation

Recirculation of nutrients from wood chip combustion by ash fertilisation of forest systems can be used to re-introduce nutrients that are otherwise lost, counteracting nutrient depletion due to whole tree harvesting. However, the effects of ash application on soil organic matter (SOM) composition, turnover and stability are unknown. The aim of the study was to investigate how ash fertilisation of forest soils affects SOM composition and stability to microbial degradation over time. O-horizon and 0–5 cm mineral soil samples were collected from two coniferous forest sites, one in Finland and one in Denmark, where ash had been spread at different times. Changes in SOM biodegradability were estimated based on an incubation experiment, expressed as percentage of initial carbon. Changes in SOM composition were characterised using thermal analysis and Fourier transform mid-infrared photoacoustic spectroscopy (FTIR-PAS) analysis of bulk soil samples. Ash fertilisation of forest soils affected SOM composition in the O-horizon, but not in the top 5 cm of the mineral soil. The pH and biodegradability of SOM were increased in the O-horizon. The changes in SOM composition consisted of enrichment of Fe- and Al-oxides/hydroxides, depletion of carboxylic and aromatic groups and lower thermal stability in soils with older and greater ash application. Ash fertilisation increased soil pH, either right after ash application or through a buffering effect of the ash on acidification caused by decomposing needles over time. The increased pH due to ash fertilisation together with the nutrient inputs from the ash most likely stimulated SOM turnover. This in turn increased the labile fraction of SOM, whereby the thermal stability of SOM decreased as simpler compounds were formed.

Effects of Throughfall Exclusion, Soil Texture and Spatial Continuity on Soil Water Repellency in Fichtel Mountains, Germany

Core Ideas Drought reduces soil wettability even after eliminating the effect of soil moisture Sand and clay content significantly decreased, while silt content increased WDPT WDPT shows low spatial dependence on mesoscale at lags from 0.6 m to 5.2 m The occurrence of soil water repellency (SWR) in soil is controlled by soil organic matter (SOM) composition and is strongly soil‐moisture dependent. During drying the reduction of water content in soil has been shown to induce the outward orientation of nonpolar ends of organic compounds and hence the increase in SWR. A prolonged drought can however also induce changes in SOM composition which in turn can affect SWR. In this study, we eliminate differences in water content after prolonged throughfall exclusion and a control treatment by oven‐drying of the soil samples, to test if a prolonged drought affects SWR even after excluding the direct effect of soil moisture. In addition, the relevance of soil texture variability and spatial dependence of SWR for prediction of soil wettability distribution over the study area was explored. The samples of the upper mineral soil horizon were taken from six plots in Fichtel Mountains, subjected to a throughfall exclusion or control treatments, oven‐dried and analyzed for soil texture and water drop penetration time (WDPT). A linear model with spatially correlated random effects was used to quantify the effects of soil texture and treatment on the persistence of the SWR and to simultaneously evaluate the spatial structure of the SWR. Based on estimated parameters the persistence of SWR was calculated on unsampled locations by robust kriging with external drift. The throughfall exclusion treatment significantly increased the log(WDPT) (p &lt; 0.01) of the oven‐dried soil by 0.46. The clay content and the sand content had highly significant (p &lt; 0.001) negative effects, while silt content had positive effects on the log(WDPT). The variogram parameter with a range of 5.2 m, a nugget of 0.25, and a sill of 0.45 indicated a rather low degree of spatial dependence of log(WDPT). The main outcome of this study is that the positive effect of throughfall exclusion on SWR cannot be fully attributed to water content reduction. Most probably the drought‐induced changes in SOM composition and microbial community were responsible for the observed increase in SWR.

Reclamation of intensively managed soils in temperate regions by addition of wood bottom ash containing charcoal: SOM composition and microbial functional diversity

Wood bottom ash (BA), a complex mixture of mineral ash and moderate amounts of charcoal, can be used as a fertilizer in agricultural and forestry systems. BA is generated as a by-product of biomass combustion and may become a major source of pyrogenic organic matter (OM) as a result of the current high demand for biomass energy. However, very few studies have investigated how the application of BA to land affects soil organic matter (SOM) quality and soil biological properties.In the present study, changes in soil microbial community and community level physiological profile were related to changes in SOM composition (element analysis and 13C CP–MAS NMR) and thermal stability (DSC). The parameters were monitored throughout 3 years in a field experiment in which a degraded soil (acidic, poorly drained and low SOM content) was treated with different doses (16 and 32Mgha−1) of each of two types of BA (fly ash and mixed ash). The treatments led to changes in SOM composition that affected soil microbial communities. The data suggest that some of the pyrogenic OM in the BA may have been mineralized by soil microorganisms. The relatively low recalcitrance of the charcoal, as well as the priming effect of labile organic compounds and the higher nutrient availability may have favoured the process. Although application of BA to land may be useful for restoring fertility and the C-sequestration capacity of soil, the charcoal contained in this type of ash may be susceptible to microbial degradation in humid temperate climate zones and is not stable in the long term.

Long-term doubling of litter inputs accelerates soil organic matter degradation and reduces soil carbon stocks

Soil organic matter (SOM) constitutes more than two-thirds of terrestrial carbon stocks yet there are many uncertainties about the fate of soil carbon reserves with global environmental change. Moisture, altered nutrient cycles, species shifts, growing season length or rising temperatures may alter forest primary productivity and the proportions of above and belowground biomass entering soil. We investigated SOM composition using molecular-level techniques after 20 years of detrital input and removal treatment (DIRT) at the Harvard Forest. SOM biomarkers (solvent extraction, base hydrolysis and cupric(II) oxide oxidation) and nuclear magnetic resonance (NMR) spectroscopy were used to quantify changes in SOM composition and microbial activity and community composition was assessed using phospholipid fatty acid (PLFA) analysis. Doubling aboveground litter inputs decreased soil carbon content, increased the degradation of labile SOM and enhanced the sequestration of aliphatic compounds in soil. The exclusion of belowground inputs resulted in a decrease in root-derived components and enhanced the degradation of stable SOM components such as leaf-derived aliphatic structures (cutin). The DIRT manipulations resulted in soil microbial community shifts that were attributed to the accelerated processing of specific SOM components. These results collectively reveal that a detailed molecular-level characterization of SOM can provide information on SOM compositional changes and transformations after 20 years of input manipulation in a temperate forest.

Soil organic matter characterization of temperate peatland soil with FTIR‐spectroscopy: effects of mire type and drainage intensity

SummaryPeatlands are an important component of the global carbon cycle because they comprise huge amounts of terrestrial carbon (C). Different conditions during peat formation and secondary peat decomposition affect the quantity and composition of soil organic matter (SOM) in peats. There are few comparative studies on the chemical composition of SOM in temperate peatland soil. This study investigates compositional changes of SOM functional groups in peats and corresponding peat‐forming plants by Fourier transform infrared (FTIR) spectroscopy. Three plant samples and 29 peat samples were taken from seven temperate peatland sites with different genesis and land‐use intensity. Site‐specific differences, such as genesis of the peat, were found to be reflected in the FTIR spectra. In general, there was more variation in FTIR spectra in samples from fens than in those from bogs and peat‐forming plants. The samples from fens have a smaller C–H absorption band than those from bogs and plants, which reflects greater biochemical activity in the minerotrophic than ombrotrophic environments. In addition to peat genesis, drainage and secondary peat decomposition also affect SOM composition substantially. The larger amounts of aliphatic compounds in undrained peats could be explained by selective preservation caused by anaerobic conditions. With increasing drainage of the sites, there was a decrease in the C–H absorption that was accompanied by a relative increase in C=O absorption. These changes in absorption intensities reflect the enhanced aerobic decomposition and mineralization that accompanies drainage and land‐use intensity. However, the ‘degree of peat decomposition’, a diagnostic tool used in the field, is not reflected by OM composition determined by FTIR spectroscopy. Our results contribute to further understanding of changes in SOM composition during peat formation and processes of secondary decomposition caused by drainage.

Long-Term Grazing Exclusion Improves the Composition and Stability of Soil Organic Matter in Inner Mongolian Grasslands.

Alteration of the composition of soil organic matter (SOM) in Inner Mongolian grassland soils associated with the duration of grazing exclusion (GE) has been considered an important index for evaluating the restoring effects of GE practice. By using five plots from a grassland succession series from free grazing to 31-year GE, we measured the content of soil organic carbon (SOC), humic acid carbon (HAC), fulvic acid carbon (FAC), humin carbon (HUC), and humic acid structure to evaluate the changes in SOM composition. The results showed that SOC, HUC, and the ratios of HAC/FAC and HAC/extractable humus carbon (C) increased significantly with prolonged GE duration, and their relationships can be well fitted by positive exponential equations, except for FAC. In contrast, the HAC content increased logarithmically with prolonged GE duration. Long-term GE enhanced the content of SOC and soil humification, which was obvious after more than 10 years of GE. Solid-state 13C nuclear magnetic resonance spectroscopy showed that the ratios of alkyl C/O-alkyl C first decreased, and then remained stable with prolonged GE. Alternately, the ratios of aromaticity and hydrophobicity first increased, and then were maintained at relatively stable levels. Thus, a decade of GE improved the composition and structure of SOM in semiarid grassland soil and made it more stable. These findings provide new evidence to support the positive effects of long-term GE on soil SOC sequestration in the Inner Mongolian grasslands, in view of the improvement of SOM structure and stability.

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.

Comparison of soil organic matter composition after incubation with maize leaves, roots, and stems

As plants are major contributors to soil organic matter (OM), the chemical composition of plant tissues entering the soil should be related to the soil OM composition. We examined the OM composition of maize leaves, stems, and roots, to determine if differences in plant tissue composition altered the soil OM composition during a 36-week degradation experiment. Lignin phenols were measured by gas chromatography/mass spectrometry (GC/MS), and soil OM and humic substances were characterized using solid-state 13C and solution-state 1H nuclear magnetic resonance (NMR) spectroscopy respectively. Lignin phenol composition, oxidation, and depletion relative to cutin-derived OH-acids changed less in leaf- compared to stem- and root-amended soils over time, and may be due to greater vanillyl concentrations in leaves. Soil amended with stems had higher concentrations of carbohydrates in soil OM. Humic substances from leaf-amended soils had higher concentrations of aliphatic components, likely due to higher concentrations of aliphatic compounds in leaf tissues, which suggests that compounds derived from leaves are potential contributors to the stable pool of soil OM. After 36-weeks of incubation, the contribution of microbial-derived OM was greatest in humic extracts from root-amended soils, and increased contribution from these compounds was detected earlier in these soils than stem- and leaf-amended soils. This indicates that root amendment may enhance contributions from microbial-derived OM. Our study suggests that changes in soil OM composition over time was related to the chemical composition of the plant tissue, and demonstrates the important link between plant chemistry and soil OM turnover.

Molecular-level methods for monitoring soil organic matter responses to global climate change

Soil organic matter (SOM) is one of the most complex natural mixtures on earth. It plays a critical role in many ecosystem functions and is directly responsible for sustaining life on our planet. However, due to its chemical heterogeneity, SOM composition at molecular-level is still not completely clear. Consequently, the response of SOM to global climate change is difficult to predict. Here we highlight applications of two complementary molecular-level methods (biomarkers and nuclear magnetic resonance (NMR)) for ascertaining SOM responses to various environmental changes. Biomarker methods that measure highly specific molecules determine the source and decomposition stage of SOM components. However, biomarkers only make up a small fraction of SOM components because much of SOM is non-extractable. By comparison, (13)C solid-state NMR allows measurement of SOM in its entirety with little or no pretreatment but suffers from poor resolution (due to overlapping of SOM components) and insensitivity, and thus subtle changes in SOM composition may not always be detected. Alternatively, (1)H solution-state NMR methods offer an added benefit of improved resolution and sensitivity but can only analyze SOM components that are fully soluble (humic type molecules) in an NMR compatible solvent. We discuss how these complementary methods have been employed to monitor SOM responses to: soil warming in a temperate forest, elevated atmospheric CO(2) and nitrogen fertilization in a temperate forest, and permafrost thawing in the Canadian High Arctic. These molecular-level methods allow some novel and important observations of soil dynamics and ecosystem function in a changing climate.

Changes in Soil Organic Matter Pools and Carbon Mineralization as Influenced by Fire Severity

Fire transforms soil organic matter (SOM) to recalcitrant forms of C. The degree to which SOM is altered is dependent on fire severity. This study investigated changes in SOM composition and mineralization by controlling the fire severity of laboratory burns on reconstructed soil profiles (O, A1 [0–1 cm], and A2 [1–2 cm] horizons). Burning simulated low‐, moderate‐, and high‐severity fires. Organic and mineral soils were incubated for 180 d and CO2 production was measured with soda lime traps. Soils were analyzed for SOM composition pre‐ and post‐incubation using an alkaline extraction. Higher severity burning resulted in lower O horizon decomposition rates on a C basis. Fulvic acid C in the control and low‐severity O horizons was reduced by 13% by incubation, which was negatively correlated with cumulative C mineralized (r = −0.564). After incubation, the SOM composition of the burned O horizons remained different from the control. Higher severity burning caused mineral soil to initially have higher C mineralization rates, which disappeared by the end of the incubation. Fulvic acid concentration was reduced by 61 and 38% during incubation of the A1 and A2 horizons, respectively, returning the SOM composition to control levels. The sum of alkaline insoluble forms (e.g., humin) of SOM accounted for 74 and 61% of total soil C and N, respectively, which was not significantly different between treatments. The fire severities examined show that the major impacts to SOM occurred to the O horizon and any fire‐related changes to the mineral soil were not persistent.