Soil Organic Matter In Subsoil Research Articles

Impact of Climate on Soil Organic Matter Composition in Soils of Tropical Volcanic Regions Revealed by EGA-MS and Py-GC/MS.

Soil organic matter (SOM) crucially influences the global carbon cycle, yet its molecular composition and determinants are understudied, especially for tropical volcanic regions. We investigated how SOM compounds change in response to climate, vegetation, soil horizon, and soil properties and the relationship between SOM composition and microbial decomposability in Tanzanian and Indonesian volcanic regions. We collected topsoil (0-15 cm) and subsoil (20-40 cm) horizons (n = 22; pH: 4.6-7.6; SOC: 10-152 g kg-1) with undisturbed vegetation and wide mean annual temperature and moisture ranges (14-26 °C; 800-3300 mm) across four elevational transects (340-2210 m asl.). Evolved gas analysis-mass spectrometry (EGA-MS) documented a simultaneous release of SOM compounds and clay mineral dehydroxylation. Subsequently applying double-shot pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) at 200 and 550 °C, we detailed the molecular composition of topsoil and subsoil SOM. A minor portion (2.7 ± 1.9%) of compounds desorbed at 200 °C, limiting its efficacy for investigating overall SOM characteristics. Pyrolyzed SOM closely aligns with the intermediate decomposable SOM pool, with most pyrolysates (550 °C) originating from this pool. Pyrolysates composition suggests tropical SOM is mainly microbial-derived and subsoil contains more degraded compounds. Higher litter inputs and attenuated SOM decomposition due to cooler temperatures and lower soil pH (<5.5) produce less-degraded SOM at higher elevations. Redundancy analyses revealed the crucial role of active Al/Fe (oxalate-extractable Al/Fe), abundant in low-temperature/high-moisture conditions, in stabilizing these less-degraded components. Our findings provide new insights into SOM molecular composition and its determinants, critical for understanding the carbon cycle in tropical ecosystems.

Land Use Changes Affecting Soil Organic Matter Accumulation in Topsoil and Subsoil in Northeast Thailand

The objectives of this study were to investigate effects of land use on accumulation of soil organic matter (SOM) in the soil profile (0–100 cm) and to determine pattern of SOM stock distribution in soil profiles. Soil samples were collected from five soil depths at 20 cm intervals from 0 to 100 cm under four adjacent land uses including forest, cassava, sugarcane, and paddy lands located in six districts of Maha Sarakham province in the Northeast of Thailand. When considering SOM stock among different land uses in all locations, forest soils had significantly higher total SOM stocks in 0–100 cm (193 Mg·C·ha−1) than those in cassava, sugarcane, and paddy soils in all locations. Leaf litter and remaining rice stover on soil surfaces resulted in a higher amount of SOM stocks in topsoil (0–20 cm) than subsoil (20–100 cm) in some forest and paddy land uses. General pattern of SOM stock distribution in soil profiles was such that the SOM stock declined with soil depth. Although SOM stocks decreased with depth, the subsoil stock contributes to longer term storage of C than topsoils as they are more stabilized through adsorption onto clay fraction in finer textured subsoil than those of the topsoils. Agricultural practices, notably applications of organic materials, such as cattle manure, could increase subsoil SOM stock as found in some agricultural land uses (cassava and sugarcane) in some location in our study. Upland agricultural land uses, notably cassava, caused high rate of soil degradation. To restore soil fertility of these agricultural lands, appropriate agronomic practices including application of organic soil amendments, return of crop residues, and reduction of soil disturbance to increase and maintain SOM stock, should be practiced.

Warming enhances old organic carbon decomposition through altering functional microbial communities

Soil organic matter (SOM) stocks contain nearly three times as much carbon (C) as the atmosphere and changes in soil C stocks may have a major impact on future atmospheric carbon dioxide concentrations and climate. Over the past two decades, much research has been devoted to examining the influence of warming on SOM decomposition in topsoil. Most SOM, however, is old and stored in subsoil. The fate of subsoil SOM under future warming remains highly uncertain. Here, by combining a long-term field warming experiment and a meta-analysis study, we showed that warming significantly increased SOM decomposition in subsoil. We also showed that a decade of warming promoted decomposition of subsoil SOM with turnover times of decades to millennia in a tall grass prairie and this effect was largely associated with shifts in the functional gene structure of microbial communities. By coupling stable isotope probing with metagenomics, we found that microbial communities in warmed soils possessed a higher relative abundance of key functional genes involved in the degradation of organic materials with varying recalcitrance than those in control soils. These findings suggest warming may considerably alter the stability of the vast pool of old SOM in subsoil, contributing to the long-term positive feedback between the C cycle and climate.

Biochemically altered charcoal residues as an important source of soil organic matter in subsoils of fire-affected subtropical regions

Although climatic conditions of subtropical regions support fast biochemical degradation of soil organic matter (SOM), some of their soils reveal dark umbric horizons with considerably high organic C contents. Since such soils can reach a depth of several meters a better understanding of the processes involved in the humification of its SOM is essential for an improved elucidation of the impact of the hot and humid conditions of subtropical regions on global C cycling. Therefore, the SOM of three Umbric Ferralsols from Brazil was characterized for its quantity and chemical composition as a function of soil depth. Solid-state 13C nuclear magnetic resonance (NMR) spectra revealed an increase of the aromaticity with soil depth to exceptionally high values between 40 and 70% of the SOM. Chemical oxidation of the SOM with acid potassium dichromate confirmed that a major part of this C group is attributable to pyrogenic organic matter (PyOM). High carboxyl-C contents between 13 and 21% of the total organic C in the subsoils showed that their PyOM is strongly oxidized, most likely due to previous intense biochemical reworking. Since the latter improves the water solubility of the formerly hydrophobic charcoal residues, such oxidized PyOM can be transported with the soil solution into deeper soil horizons. As indicated by the pattern of the 13C NMR spectra and the high C contents of the subsoil, the partially degraded PyOM seems to have lost its attractiveness to microbial attack with ongoing downward movement and accumulation. Correlating parameters of organic matter with those of soil mineralogy suggested that in addition to stabilization by the mineral phase other mechanisms must have contributed to the persistence of PyOM in the studied soils. Considering that in soils, microbial degradation of aromatic structures is mainly performed by lignolytic and aerobic organisms, we suggest that increasing oxygen depletion and unfavorable living conditions in the deeper soil regions decreased the activity of those organisms. This enhanced the biochemical recalcitrance of the aromatic PyOM whereas non-pyrogenic SOM was still subjected to degradation although most likely at a slower rate. As a consequence, the selective preservation of PyOM shifted not only the quality of the subsoil SOM toward higher aromaticity but also increased its quantity. Based on our results, we concluded that downward translocation of partially degraded PyOM and its subsequent accumulation represent important processes in soil systems which are frequently affected by fires.

Deep soil organic matter—a key but poorly understood component of terrestrial C cycle

Despite their low carbon (C) content, most subsoil horizons contribute to more than half of the total soil C stocks, and therefore need to be considered in the global C cycle. Until recently, the properties and dynamics of C in deep soils was largely ignored. The aim of this review is to synthesize literature concerning the sources, composition, mechanisms of stabilisation and destabilization of soil organic matter (SOM) stored in subsoil horizons. Organic C input into subsoils occurs in dissolved form (DOC) following preferential flow pathways, as aboveground or root litter and exudates along root channels and/or through bioturbation. The relative importance of these inputs for subsoil C distribution and dynamics still needs to be evaluated. Generally, C in deep soil horizons is characterized by high mean residence times of up to several thousand years. With few exceptions, the carbon-to-nitrogen (C/N) ratio is decreasing with soil depth, while the stable C and N isotope ratios of SOM are increasing, indicating that organic matter (OM) in deep soil horizons is highly processed. Several studies suggest that SOM in subsoils is enriched in microbial-derived C compounds and depleted in energy-rich plant material compared to topsoil SOM. However, the chemical composition of SOM in subsoils is soil-type specific and greatly influenced by pedological processes. Interaction with the mineral phase, in particular amorphous iron (Fe) and aluminum (Al) oxides was reported to be the main stabilization mechanism in acid and near neutral soils. In addition, occlusion within soil aggregates has been identified to account for a great proportion of SOM preserved in subsoils. Laboratory studies have shown that the decomposition of subsoil C with high residence times could be stimulated by addition of labile C. Other mechanisms leading to destabilisation of SOM in subsoils include disruption of the physical structure and nutrient supply to soil microorganisms. One of the most important factors leading to protection of SOM in subsoils may be the spatial separation of SOM, microorganisms and extracellular enzyme activity possibly related to the heterogeneity of C input. As a result of the different processes, stabilized SOM in subsoils is horizontally stratified. In order to better understand deep SOM dynamics and to include them into soil C models, quantitative information about C fluxes resulting from C input, stabilization and destabilization processes at the field scale are necessary.

The distribution and degradation of biomarkers in Alberta grassland soil profiles

Recent studies have shown that subsoil (B and C) horizons can store significant amounts of soil organic matter (SOM). Yet the quantity, source, turnover, and chemical composition of subsoil SOM are largely unknown. Biomarker methods were employed in this study to investigate the vertical distribution and degradation of SOM in Alberta grassland soils. Specifically, the composition of solvent extracts, bound lipids, and lignin compounds in the A h, B m, and C ca horizons of four chernozemic soils was analyzed using chemolysis and gas chromatography–mass spectrometry (GC–MS) techniques. Suberin, cutin, and lignin compounds were observed to degrade with soil depth. Aliphatic molecules (such as hydroxyalkanoic acids) from suberin and cutin were preferentially preserved in the deeper horizons in comparison to lignin compounds. Trehalose, a carbohydrate found in high abundance in fungal tissues, was detected in significant abundance in the B m and C ca horizons of three grassland soils, suggesting that non-plant biomass may strongly contribute to the deposition of carbon into the subsoil. It was also demonstrated that soil-forming processes (such as eluviation) played a role in the composition of organic carbon in the lower soil horizons.