Soil Organic Matter Formation Research Articles

Linkages between soil organic matter and magnetic mineral formation in agricultural fields in southeastern Minnesota, USA

Magnetic properties of soil are widely utilized to study soil development in a variety of settings due to the formation of strongly magnetic iron oxides during pedogenesis. Similarly, soil organic matter (SOM) is commonly measured in soil surveys conducted on agricultural lands due to the essential role SOM plays in the soil ecosystem. Here, we present data from two agricultural fields in southeastern Minnesota that demonstrate a relationship between soil magnetic properties and SOM. In each field, we collected 100 topsoil samples along a 40 m by 20 m grid to determine spatial variability in soil magnetic properties and SOM, as well as two soil cores to constrain variability with depth (∼0–60 cm). Magnetic susceptibility, low-field remanence, and hysteresis properties were used to characterize magnetic mineral abundance and grain-size in the soils. There are strong positive correlations between SOM and three magnetic properties: the frequency dependence of susceptibility (χfd), anhysteretic remanent magnetization (ARM), and the ratio of ARM to isothermal remanent magnetization (ARM/IRM). All three of these magnetic properties (χfd, ARM, and ARM/IRM) are sensitive to the concentration (or relative abundance) of fine-grained (<75 nm) magnetite/maghemite known to form in well-drained soils during pedogenesis. Correlation between SOM and magnetic properties persist in each field despite differences in the management strategy over the past three decades. Our results support a functional link between SOM and soil-formed magnetite/maghemite, where increasing SOM (up to a threshold) enhances the production and stability of soil-formed magnetite due to its role in soil redox processes and iron-organic complexes. Agricultural soils seem particularly well suited to demonstrate correlations between SOM and pedogenic magnetic minerals due to their relatively low SOM and typically well-drained environments, supporting the utility of soil magnetism in agricultural soil survey studies.

Root Carbon Interaction with Soil Minerals Is Dynamic, Leaving a Legacy of Microbially Derived Residues.

Minerals preserve the oldest, most persistent soil carbon, and mineral characteristics appear to play a critical role in the formation of soil organic matter (SOM) associations. To test the hypothesis that roots, and differences in carbon source and microbial communities, influence mineral SOM associations over short timescales, we incubated permeable mineral bags in soil microcosms with and without plants, inside a 13CO2 labeling chamber. Mineral bags contained quartz, ferrihydrite, kaolinite, or soil minerals isolated via density separation. Using 13C-nuclear magnetic resonance, Fourier transform ion cyclotron resonance mass spectrometry, and lipidomics, we traced carbon deposition onto minerals, characterizing total carbon, 13C enrichment, and SOM chemistry over three growth stages of Avena barbata. Carbon accumulation was rapid and mineral-dependent but slowed with time; the accumulated amount was not significantly affected by root presence. However, plant roots strongly shaped the chemistry of mineral-associated SOM. Minerals incubated in a plant rhizosphere were associated with a more diverse array of compounds (with different functional groups-carbonyl, aromatics, carbohydrates, and lipids) than minerals incubated in an unplanted bulk soil control. We also found that many of the lipids that sorbed to minerals were microbially derived, including many fungal lipids. Together, our data suggest that diverse rhizosphere-derived compounds may represent a transient fraction of mineral SOM, rapidly exchanging with mineral surfaces.

Filling gaps in models simulating carbon storage in agricultural soils: the role of cereal stubbles

Carbon (C) input is a prerequisite for the formation of soil organic matter and thus for soil organic C (SOC) sequestration. Here we used the C-TOOL model to simulate SOC changes in a long-term field experiment (1932–2020) at Askov, Denmark, which involved four different levels of nutrients added in mineral fertilizer (0, 0.5, 1, 1.5 NPK) and a four-crop rotation. The C input into soils consists of belowground and aboveground plant biomass and was estimated using allometric functions. The simulation showed that modelled SOC based on standard allometric functions of C input from crop residues did not adequately matched measured SOC contents. However, applying modified allometric functions based on current and the previously measured results for aboveground and belowground C inputs in winter wheat and grass clover in rotations provided much better match between simulated and measured SOC contents for fertilized treatments at normal and high level of fertilization. This improved indicators of C-TOOL model performance (e.g. yielding RMSE of 2.24 t C ha−1 and model efficiency of 0.73 in 1.5 NPK treatment). The results highlight that standard allometric functions greatly overestimates the amount of C in winter wheat stubble left after harvest in treatments dressed with NPK compared with modified functions. The results also highlight further needs for improvement of allometric functions used in simulation models for C-accounting in agroecosystems.

Plant residue-derived hydrophilic and hydrophobic fractions contribute to the formation of soil organic matter

Plant residue-derived hydrophilic and hydrophobic fractions contribute to the formation of soil organic matter

How vegetation patches drive soil development and organic matter formation on polar islands

As Antarctica is strongly affected by climate change and global warming, the factors that mainly determine soil development might also shift from the dominance of physical to biochemical processes. Vegetation is restricted to the margins of the Antarctic continent with the Antarctic Peninsula being a region of patchily distributed vegetation. While on James Ross Island in the Weddell Sea only cryptogams can be found, on King George Island in the Southern Ocean also vascular plants are present. As rates of soil development and the build-up of soil organic matter are very low in these polar conditions, it can be hypothesized that vegetation patches comprise hot spots for biogeochemical soil processes. To analyze the effect of vegetation on soils in maritime Antarctica, we investigated vegetated and vegetation-free soils from both islands. On both islands, we found clearly higher carbon and nitrogen contents in vegetated soils. Using physical fractionation, we could demonstrate that the amount of free and occluded particulate organic matter is also higher in soils under vegetation, but at the same time, that clay-sized mineral-associated organic matter contributes most to carbon storage in all soils. The dominance of aromatic compounds in vegetation-free soils was disclosed by 13C NMR spectroscopy as well as a larger proportion of compounds with a lower molecular weight in vegetated soils. Thus, vegetation patches lead to soil organic matter containing higher amounts of bioavailable substrates that can be assumed to foster microbial activity and thus drive further soil development in a warmer future. However, in the cold arid environments a propagation of aridity might result in vegetation dieback and thus in a ceasing of biological soil activity driving a slowing of soil development.

The effects of tree-mycorrhizal type on soil organic matter properties from neighborhood to watershed scales

A large portion of terrestrial carbon (C) is stored as soil organic matter (SOM) so the balance between C inputs to SOM and soil C loss through respiration has important implications for global climate change. Tree-mycorrhizal association is a promising predictor of SOM dynamics, with the effect of ectomycorrhizal (ECM) trees and associated fungi scaling with the percent basal area of ECM-associated trees in a forest stand. However, uncertainty remains in the generality of ECM effects across the different spatial scales at which soil properties are known to vary. To determine the spatial variability of ECM effects on SOM pools within a forest stand, we used likelihood modelling techniques to compare non-spatial models, which average out ECM effects at the stand scale, against spatial models, which account for the size and spatial distribution of ECM-associated trees within a stand (i.e., the tree neighborhood). To investigate how watershed-scale variability in soil fertility and pH mediate ECM effects, we quantified forest floor leaf litter and soil properties in forest stands along mycorrhizal gradients within four adjacent watersheds that varied in parent material and rainfall. We found that ECM effects on SOM pools scaled with stand-level ECM dominance rather than varying based on individual trees at the neighborhood scale. Although forest floor leaf litter and mineral C:N ratios consistently increased with ECM dominance across watersheds, ECM effects on O horizon formation and the composition of SOM at depth differed among watersheds. Specifically, watersheds with lower soil pH and fertility exhibited greater changes along mycorrhizal gradients in O horizon depth, leaf litter δ15N and in the difference between δ15N of leaf litter and mineral soil fractions. Overall, this study shows the potential for intrinsic soil properties to mediate the effects of ECM trees and associated fungi on SOM formation and persistence in the tropics.

Effects of conservation tillage on soil microbial community and the function of soil carbon cycling

Agricultural tillage practices significantly affect the structure and function of soil micro-bial community, as well as its control over soil carbon cycling. Conservation tillage practice based on no-tillage and crop straw returning is an important measure to improve soil carbon sequestration and fertility, in which soil microorganisms play a key role. Although many previous studies focus on the structure and function of microbial communities under conservation tillage, our overall understanding of soil microbial responses at community level upon conservation tillage is still lacking, due to the complexity of the soil, environmental factors and the different selections of microbial research methods. Furthermore, previous studies paid more attention to the role of soil microorganisms as decomposers and the contribution of plant-derived carbon to the formation of soil carbon pool, but ignored the contribution of microbial-derived carbon to the formation and stability of soil carbon pool. We summarized the paradigm shift in soil organic matter formation and stability theories, reviewed the research methods of soil microbial community, focused on the effects of conservation tillage on soil microbial biomass, community diversity and composition, carbon metabolism, as well as microbial-derived carbon storage, and proposed suggestions for future study, aiming to provide support for future studies regarding microbial responses and its control over soil carbon dynamics in agroecosystem.

Visualizing the transfer of organic matter from decaying plant residues to soil mineral surfaces controlled by microorganisms

The interface between decaying plant residues and soil minerals represents an essential soil microenvironment at which soil organic matter forms. The high amount of microbial products and residues within this hot spot of microbial activity fosters the formation of mineral-associated organic matter. Besides classical quantitative analyses, our understanding of processes controlling soil organic matter formation greatly benefits from microscopic observations and measurements, which provide spatially resolved information at a meaningful scale for microbial processes and for the association between organic and mineral particles. We studied carbon and nitrogen transfer from fresh-plant residues to the mineral soil, through a litter decomposition experiment in an artificial soil mixture. Needles of Norway spruce (Picea abies L.) were placed in microbatch containers filled with an artificial soil mixture free of soil organic matter. Containers were buried in fresh organic layer material from a Norway spruce stand and incubated for 14 and 42 days. We applied nanoscale secondary ion mass spectroscopy (NanoSIMS) to investigate the spatial distribution of mineral and organic compounds at the needle vicinity and into the mineral soil (0–550 μm from the needle). After 14 days, we depicted the formation of mineral-associated organic matter in the surrounding of the decaying needles. After 42 days, we observed substantial colonization of the needles and the detritusphere by saprotrophic fungi. The fungal hyphae extended into the mineral matrix of the artificial soil acting as vectors for the transfer of litter-derived carbon and nitrogen into the bulk soil. This resulted in an increase of the area covered by organic matter in the detritusphere, with up to 10% of the total investigated area classified as organic matter closely associated with mineral surfaces. Our results provide evidence that the carbon and nitrogen derived from litter decomposition transformed by microorganisms is transferred as mineral-associated organic matter, heterogeneously distributed from the litter source, and still detected 550 μm away from the latter. The close association of newly formed soil organic matter and fine sized minerals suggests that the formation of mineral-associated OM and likely also microaggregates is directly driven by microbial activity in the vicinity of hot spots for plant carbon input (e.g. the detritusphere).

Worlds apart: Location above‐ or below‐ground determines plant litter decomposition in a semi‐arid Patagonian steppe

AbstractWhile considerable attention has been devoted to how precipitation modulates net primary productivity in arid and semi‐arid ecosystems, the emergence of multi‐faceted controls on carbon (C) turnover suggests that there is much to be understood with respect to the mechanistic controls on plant litter decomposition.In the Patagonian steppe, we conducted a long‐term factorial experiment, evaluating the importance of position, litter quality, tissue origin and soil resources on rates of C turnover under natural field conditions. Leaf and root litter of dominant grass species were placed in litterbags in different positions, on the soil surface and buried at 5‐cm depth, with soil treatments of labile C, nitrogen (N) and their combination (C + N) over a 3‐year period.As predicted, leaf litter decomposed significantly (nearly sixfold) faster above‐ground than did root litter below‐ground (p &lt; 0.001). Surprisingly, root litter decomposed significantly faster than leaf litter above‐ground (p &lt; 0.001), and above‐ground decomposition was not strongly affected by soil resource additions. Below‐ground decomposition was largely determined by the interaction of litter quality and soil resource availability. Determining a C balance by integrating biomass allocation and primary productivity from this field site, combined with the data from this study, suggests large differences between the contribution of the above‐ and below‐ground biomass to soil organic matter (SOM) pools and a long residence time of undecomposed root litter.Synthesis. Litter position clearly emerged as the predominant variable determining C turnover in this semi‐arid steppe ecosystem, with litter quality and soil resources having significant, but more modest, effects. The near complete independence of above‐ground litter decomposition from soil resources and rapid decomposition of surface litter, coupled with the counterintuitive relationships with litter quality, suggests that, in the long term, C loss from photodegradation may result in a minimal contribution of above‐ground litter to SOM formation. These results have mechanistic implications for the distinct functionality of litter decomposition above‐ and below‐ground in semi‐arid ecosystems, and how these differential controls may alter the C balance due to future changes in climate and land use.

Low-intensity frequent fires in coniferous forests transform soil organic matter in ways that may offset ecosystem carbon losses.

The impact of shifting disturbance regimes on soil carbon (C) storage is a key uncertainty in global change research. Wildfires in coniferous forests are becoming more frequent in many regions, potentially causing large C emissions. Repeated low-intensity prescribed fires can mitigate wildfire severity, but repeated combustion may decrease soil C unless compensatory responses stabilize soil organic matter. Here, we tested how 30years of decadal prescribed burning affected C and nitrogen (N) in plants, detritus, and soils in coniferous forests in the Sierra Nevada mountains, USA. Tree basal area and litter stocks were resilient to fire, but fire reduced forest floor C by 77% (-36.4 Mg C/ha). In mineral soils, fire reduced C that was free from minerals by 41% (-4.4 Mg C/ha) but not C associated with minerals, and only in depths ≤ 5cm. Fire also transformed the properties of remaining mineral soil organic matter by increasing the proportion of C in a pyrogenic form (from 3.2% to 7.5%) and associated with minerals (from 46% to 58%), suggesting the remaining soil C is more resistant to decomposition. Laboratory assays illustrated that fire reduced microbial CO2 respiration rates by 55% and the activity of eight extracellular enzymes that degrade cellulosic and aromatic compounds by 40-66%. Lower decomposition was correlated with lower inorganic N (-49%), especially ammonium, suggesting N availability is coupled with decomposition. The relative increase in forms of soil organic matter that are resistant to decay or stabilized onto mineral surfaces, and the associated decline in decomposition suggest that low-intensity fires may promote mineral soil C storage in pools with long mean residence times in coniferous forests.

Soil texture affects the coupling of litter decomposition and soil organic matter formation

Incomplete knowledge on the environmental factors linking litter decomposition and the formation of soil organic matter (SOM) hampers the sustainable management of soil as a carbon (C) sink. Here, we explored the effect of soil texture on the fate of C from decomposing litter (Indiangrass; Sorghastrum nutans (L.) Nash) and the concurrent formation of SOM in mineral soils of different textures (sand- and clay-rich) and forest floor material. We quantified the amount of litter C respired, C remaining in the litter, and litter C retained in the soil/forest floor in a 186-day incubation employing stable isotope analyses (13C). We complemented our isotopic approach with the extraction of microbial biomarkers from the litter and soils/forest floor material and spectroscopic studies into the compositional changes of the incubated materials.We found that soil texture affected both the decomposition of litter and the retention of litter-derived C in the soil. The soil rich in clay provided conditions favorable for a more efficient microbial utilization of the litter material (high pH and high C use efficiency) as compared to the sand-rich soil and the forest floor. This resulted in lower amounts of litter C respired as CO2 (25.0%, vs. 55.6 and 56.1% in clay vs. sand and forest floor material, respectively) and higher amounts of litter C retained in the clay-rich soil (12.6% vs. 3.5 and 5.3% in clay vs. sand and forest floor material, respectively). High contents of silt- and clay-sized mineral particles in the clay-rich soil likely resulted in the ability to stabilize litter C in aggregates and organo-mineral associations, perhaps as microbial residues. This ability was low in the sand-rich soil and virtually absent in the forest floor, where the recalcitrance of the litter and native SOM was probably more relevant, and a larger portion of litter C may have been retained in the soil as relatively untransformed plant compounds. We emphasize that litter decomposition, the formation of SOM, and soil texture are tightly linked, such that any differences in soil texture alter litter decomposition and SOM formation patterns for the same litter.

Plant rhizodeposition: A key factor for soil organic matter formation in stable fractions.

Soil organic carbon formation remains poorly understood despite its importance for human livelihoods. Uncertainties remain for the relative contributions of aboveground, root, and rhizodeposition inputs to particulate (POC) and mineral-associated (MAOC) organic carbon fractions. Combining a novel framework with isotope tracer studies, we quantified POC and MAOC formation efficiencies (% of C-inputs incorporated into each fraction). We found that rhizodeposition inputs have the highest MAOC formation efficiency (46%) as compared to roots (9%) or aboveground inputs (7%). In addition, rhizodeposition unexpectedly reduced POC formation, likely because it increased decomposition rates of new POC. Conversely, root biomass inputs have the highest POC formation efficiency (19%). Therefore, rhizodeposition and roots appear to play opposite but complementary roles for building MAOC and POC fractions.

Different decomposition metrics of root xylem and root tissues outside xylem: an 8-year-long root decomposition study in an alpine shrubland

Both root xylem (Rxy) and tissues outside the xylem (Rtox) are involved in root decomposition, a critical process for greenhouse gas emissions and soil organic matter formation. However, differences in decomposition and responses to global environmental change, e.g., nitrogen (N) deposition, between them remain unknown. We conducted an 8-year-long decomposition experiment and quantified the remaining mass, total carbon (C), and C components of Rxy and Rtox of roots across four levels of N addition using 13C nuclear magnetic resonance. After 8 years of decomposition, roots lost a total of 61–64% (mean of 62%) of their initial mass across the four treatments. The average remaining mass of Rxy was 12.4% higher than that of Rtox. Contrary to previous studies, we did not observe significant effects of N addition on the decomposition of mass, total C, and C components of Rxy, Rtox, or whole roots. However, compared to Rtox, Rxy contributed 84% more labile C compounds (i.e., O-alkyl and O2-alkyl C) to the remaining root C, suggesting that Rxy is a mechanically protected and persistent source of labile C. Despite the greater C loss, Rtox still retained more recalcitrant C compounds (i.e., aromatic and alkyl C) than Rxy after 8 years of decomposition. These findings highlight the role of mechanical protection in regulating root tissue decomposition and point to the necessity of considering decomposition rates of different root components and their respective responses to global environmental change.

The molecular composition of extractable soil microbial compounds and their contribution to soil organic matter vary with soil depth and tree species

Recent findings on soil organic matter (SOM) revealed that soil microorganisms are not only crucial for SOM formation through plant litter degradation but soil microbial biomass (SMB) may also directly contribute to SOM and its composition. However, the role and interactions of litter quality, microbial turnover and composition of SMB and SOM remain unclear. Hence, soil profiles (organic forest floor and mineral soil layers) at a recultivated and afforested post-mining site were investigated for the influence of litter quality from different tree species (Fagus sylvatica, Pseudotsuga menziesii, Pinus nigra) and soil depth – representing different degrees of organic matter (OM) turnover – on the molecular composition of chloroform fumigation extracted SMB-derived compounds in comparison with easily extractable (non-fumigated) SOM-derived compounds. The SMB extracts were analyzed for microbial biomass carbon (MBC), nitrogen (MBN) and phosphorus (MBP). The molecular composition of SMB and SOM compounds were determined by electrospray ionization Fourier transformation ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS) complemented by the determination of nine monosaccharides representing microbial or plant origin.Van Krevelen diagrams obtained from the ESI-FT-ICR mass spectra revealed the substantial contribution of microbial-derived compounds to extractable SOM that further increased with soil depth. Analysis of the easily extractable monosaccharides implied that >99% were of microbial origin. Microbial sugars as well as MBC, MBN and MBP explained best depth-related variations of extractable SMB compounds indicating that supply and availability of C-rich OM drive these parameters. Furthermore, the contribution of microbial sugar C to MBC increased with depth, suggesting that recycling of carbohydrates is an adaptation strategy of microorganisms in C-limited environments. The supply of tree species-specific substrates resulted in different chemical composition of SMB with largest differences between deciduous and coniferous stands and vice versa, microorganisms contributed to SOM resulting in large similarity in the composition of extractable SOM and SMB.

Variable root:shoot ratios and plant nitrogen concentrations discourage using just aboveground biomass to select legume service crops

Comparing different winter legume species by plant traits that determine their contributions to ecosystems as service crops. Since root biomass determines soil organic matter formation and biological nitrogen fixation (BNF) increases soil organic nitrogen stock, those plant traits are more suitable than shoot biomass to select service crops. We evaluated shoot and root biomass and BNF (using 15N natural abundance method) of 10 winter legume service crop species under field conditions, in two consecutive years. The legumes significantly varied in shoot (3.4 to 9.1 Mg ha−1 year−1) and root biomass (0.8 to 1.3 Mg ha−1 year−1) and in the proportion and the amount of biologically fixed nitrogen (36–61% and 43–121 kg of N ha−1, respectively). Whereas shoot biomass varied between years, root and BNF were rather constant determining a low correlation between the variables. Among the evaluated species, Trifolium pratense and T. alexandrinum were the top-ranked species for the evaluated conditions, showing the largest amount of root biomass (~ 1.3 Mg ha−1 year−1) and BNF (~ 112 kg of N ha−1 year−1). Selecting service crops species aimed to increase soil organic matter based only on shoot biomass may be insufficient. As a consequence, we highlight the importance of performing breeding programs aimed to increase, in addition to crop yield or shoot biomass production, other plant traits such as root biomass or biological nitrogen fixation that provide key regulating and supporting ecosystem services.

Does biochar enhance soil organic matter formation in tropical soils?

Tropical soils are often deeply weathered and vulnerable to degradation. Biochar is a promising means to improve physico-chemical characteristics such as pH or bulk density. Sustainable soil amelioration is best achieved by improving biological activity, resulting in enhanced soil organic matter (SOM) stocks. In a meta-analysis, we investigated, if biochar amendment to tropical soils led to native SOM stock increases. We found a mean additional C accumulation (MAC) of 0.29% soil dry weight (% dw). MAC was independent of study duration, climate, and biochar addition rate, but strongly linked to soil type and nutrient status prior to the experiment: In Nitisols, MAC was highest (0.99% dw) and initial C and N contents were higher in these soils. MAC was negative in Ferralsols and Oxisols (–0.01% dw and –0.2% dw respectively). MAC as a percentage of initial C content was <50% for most soil types, but –50% in Ferralsols, Oxisols and Ultisols. We conclude that while biochar can enhance SOM stocks, attention has to be paid to the soil environment it is amended to. In low-activity clay soils, biochar amendment can lead to C mining and should therefore be co-amended with nutrient-rich organic amendments.

Soil organic matter formation is controlled by the chemistry and bioavailability of organic carbon inputs across different land uses

Soil organic matter (SOM) formation involves microbial transformation of plant materials of various quality with physico-chemical stabilisation via soil aggregation. Land use and vegetation type can affect the litter chemistry and bioavailability of organic carbon (OC), and consequently influence the processing and stabilisation of OC into SOM. We used 13C nuclear magnetic resonance (13C NMR) and hot-water extraction to assess the changes in chemical composition and labile OC fractions during the transformation processes from leaf to litter to SOM depending on land use and vegetation type. The hot-water-extractable OC (HWEOC) decreased from leaf (43–65 g kg−1) to litter (19–23 g kg−1) to SOM (8–16 g kg−1) similar in four land use types: grassland, sugarcane, forest and banana. These trends demonstrated the uniform converging pathways of OC transformation and increasing stability by SOM formation. The preferential decomposition and decrease of labile OC fractions (∑% di-O-alkyl, O-alkyl and methoxyl) from leaf (54–69%) to SOM (41–43%) confirmed the increasing stability of the remaining compounds. Despite differences in the biochemical composition of the leaf tissues among the vegetation types, the proportions of labile OC fractions in SOM were similar across land uses. The OC content of soil was higher in forest (7.9%) and grassland (5.2%) compared to sugarcane (2.3%) and banana (3.0%). Consequently, the HWEOC per unit of soil weight was higher in forest and grassland (2.0 and 1.2 g kg−1 soil, respectively) compared to sugarcane and banana (0.3 and 0.4 g kg soil−1, respectively). The availability of labile SOM is dependent on the quantity of SOM not the chemical composition of SOM. In conclusion, labile OC fractions in SOM, as identified by 13C NMR, were similar across land use regardless of vegetation type and consequently, SOM formation leads to convergence of chemical composition despite diversity of OC sources.

Millennial-age glycerol dialkyl glycerol tetraethers (GDGTs) in forested mineral soils: &amp;lt;sup&amp;gt;14&amp;lt;/sup&amp;gt;C-based evidence for stabilization of microbial necromass

Abstract. Understanding controls on the persistence of soil organic matter (SOM) is essential to constrain its role in the carbon cycle and inform climate–carbon cycle model predictions. Emerging concepts regarding the formation and turnover of SOM imply that it is mainly comprised of mineral-stabilized microbial products and residues; however, direct evidence in support of this concept remains limited. Here, we introduce and test a method for the isolation of isoprenoid and branched glycerol dialkyl glycerol tetraethers (GDGTs) – diagnostic membrane lipids of archaea and bacteria, respectively – for subsequent natural abundance radiocarbon analysis. The method is applied to depth profiles from two Swiss pre-Alpine forested soils. We find that the Δ14C values of these microbial markers markedly decrease with increasing soil depth, indicating turnover times of millennia in mineral subsoils. The contrasting metabolisms of the GDGT-producing microorganisms indicates it is unlikely that the low Δ14C values of these membrane lipids reflect heterotrophic acquisition of 14C-depleted carbon. We therefore attribute the 14C-depleted signatures of GDGTs to their physical protection through association with mineral surfaces. These findings thus provide strong evidence for the presence of stabilized microbial necromass in forested mineral soils.

Root decomposition in silvopastures is influenced by grazing, fertility, and grass species

AbstractGrass root production and decomposition is a major source of C entering soils, although rates are largely unknown based on edaphic and management factors. Therefore, study objectives were to evaluate four explanatory variables including forage species (native and nonnative), fertility (poultry litter and a control), soil moisture (udic and aquic), and pasture management (grazed and an ungrazed control) in order to evaluate driving factors for root turnover and subsequent soil organic matter formation in silvopastoral systems using the root litter bag technique. Native grass root decomposition was accelerated relative to the nonnative forage based on root mass balance, as well as the exponential decay function, likely owing to greater five‐ and six‐C sugars and more digestible root tissues of native grasses. These physiochemical results suggest more favorable microbial food sources, which culminate in faster decomposition and greater microbially derived organic matter. Overall, there was greater root sloughing and subsequent soil organic matter formation potential with native grass species and poultry litter applications, with soil moisture affecting decomposition to a lesser extent. This study contributes to the understanding of complex interactions of grass species, soil moisture, nutrients, and grazing, which controls primary productivity, as well as nutrient cycling and C sequestration in silvopastures.

Changes of carbon-isotope ratios in soil organic matter relative to parent vegetation and site specificity

Investigating the correlation between biodiversity and ecosystem function in natural environments using carbon-isotope composition (δ13C) allows distinguishing the nutrient cycling pattern and anthropogenic effects incorporation in plants and soil processes. The mechanisms behind the isotopic composition of soil organic matter (SOM) and parent vegetation in relation to the context of site-specificity was approached in this work. Formation of SOM can be affected by the presence of a high concentration of heavy metals in soils. Still, no systematic studies were performed in most of the industrial sites to support this hypothesis. In order to explore this incomplete understood influence, investigation of carbon isotope signatures (d13C) variations in soil organic matter were performed in two industrial areas from Romania (Copșa Mică industrial platform and Baia Sprie mining zone). The current study, also, investigated the C:N ratio variation, as well as the influence of N speciation regarding d13C values of SOM. The decrease in C:N ratio indicated an increasing effect of the microbial products on SOM matter at increasing depth, for both regions, while an increase of the denitrification processes with depth was found for both areas. For the most appropriate depth (20-40 cm), the soil from Baia Sprie region was more enriched in 13C comparing with the soil from Copsa Mica region, and this higher isotope fractionation of SOM might be due to a higher carbon content, respectively a higher nitrogen content of Baia Sprie soil. It was concluded that the SOM of the surface soil in the two investigated regions has an 13C isotopic composition similar to the plant remains from which it was formed, offering an integrated value of plant material, time and the local origin and providing useful markers of tree isotopic composition.