Soil Density Fractions Research Articles

Stabilization of exogenous carbon in soil density fractions is affected by its chemical composition and soil management

Low molecular weight organic carbon (LMWOC) can be physiochemically protected by soil particles via sorption and enfolding. Soil management usually changed the soil particle distribution (e.g. upland vs paddy soil). Higher organic carbon stocks in paddy soil than upland soil could partially be because of the greater immobilization and inaccessibility of LMWOC in paddy soil. To assess the effects of soil management and types of LMWOC on its carbon stabilization in soil density fractions, a simulated study was conducted to investigate the distribution of LMWOC (i.e., glucose, acetic acid, and oxalic acid) in soil density fractions based on 13C-labeling experiments in both soils. The results showed that the total 13C recovery was determined by LMWOC types, following the order of glucose> acetic acid > oxalic acid (p < 0.05). Both soil management and types of LMWOC significantly affect carbon distribution in soil density fractions. The 13C from LMWOC was predominant in the 1.85–2.4 g.cm−3 fraction of upland soil, while the highest concentration of LMWOC 13C was found in mineral grains (2.4–2.65 g.cm−3) in paddy soil, suggesting that more LMWOC was preserved in high density fractions in paddy soil than in upland soil. Our results highlighted that the LMWOC was mainly associated with aggregates in upland soil while subjected to mineral adsorption in paddy soil.

Effect of straw amendment modes on soil organic carbon, nitrogen sequestration and crop yield on the North‐Central Plain of China

AbstractWe quantified the effects of different straw return modes on soil organic carbon (SOC), total nitrogen content (TN) and C:N ratios in a wheat/maize double‐cropping agricultural system by analysing their content in different soil aggregate sizes and density fractions under four modes of straw return: (a) no return/retention of wheat and maize straw (Control); (b) retention of long wheat stubble only (Wheat Stubble); (c) retention of long wheat stubble and return of chopped maize straw (Mixed); and (d) return of chopped wheat and maize straw (Both Chopped). The Mixed and Both Chopped straw return modes produced the highest crop yields. Relative to the Control, SOC stock was 9.6% greater with the Mixed treatment and 14.5% greater with the Both Chopped treatment, whereas the Wheat Stubble treatment had no effect on SOC. Mixed and Both Chopped significantly enhanced TN stock relative to the Wheat Stubble and Control treatments. Compared with the Control, the Mixed and Both Chopped treatments increased the mass proportions of large macroaggregates and reduced the silt plus clay fraction; Mixed and Both Chopped caused a significant increase in SOC and TN in large and small macroaggregates; the Mixed treatment significantly increased SOC content in the coarse and fine intra‐aggregate particulate organic matter (iPOM) density fractions of large macroaggregates, whereas Both Chopped increased SOC in the coarse iPOM, fine iPOM and mineral‐associated organic matter (mSOM) density fractions of both large and small macroaggregates; and Mixed and Both Chopped enhanced TN content in coarse iPOM and fine iPOM within small macroaggregates. Although the Mixed treatment was slightly less effective at improving C sequestration in agricultural fields than the Both Chopped treatment, the Mixed treatment may nonetheless be the optimal plant residue management mode in terms of minimizing time and labour due to its ability to improve soil structure, maintain organic carbon levels and provide a means of sustainable crop production in intensive wheat/maize double‐cropping systems.

13C isotopic signature and C concentration of soil density fractions illustrate reduced C allocation to subalpine grassland soil under high atmospheric N deposition

We followed soil C fluxes in a subalpine grassland system exposed to experimentally increased atmospheric N deposition for 7 years. Earlier we found that, different from the plant productivity response, the bulk soil C stock increase was highest at the medium, not the high N input as hypothesized. This implies that a smaller N-deposition rate has a greater potential to favor the biological greenhouse gas-sink. To help elucidate the mechanisms controlling those changes in SOC in response to N deposition, we produced four soil density fractions and analyzed soil organic C concentration [SOC], as well as δ13C signatures (δ13CSOC) of SOC components. Soil respired CO2 (δ13CCO2) was analyzed to better distinguish seasonal short term dynamics from N-deposition effects and to identify the predominant substrate of soil respiration.Both at the start of the experiment and after 7 years we found a strong, negative correlation between [SOC] and δ13CSOC of the soil density fractions in the control treatment, consistent with an advanced stage of microbial processing of SOC in fractions of higher density. During the experiment the [SOC] increased in the two lighter density fractions, but decreased in the two heavier fractions, suggesting a possible priming effect that accelerated decomposition of formerly recalcitrant (heavy) organic matter pools. The seasonal pattern of soil δ13CCO2 was affected by weather and canopy development, and δ13CCO2 values for the different N treatment levels indicated that soil respiration originated primarily from the lightest density fractions.Surprisingly, [SOC] increases were significantly higher under medium N deposition in the <1.8 fraction and in bulk soil, compared to the high N treatment. Analogously, the depletion of δ13CSOC was significantly higher in the medium compared to the high N treatment in the three lighter fractions. Thus, medium N deposition favored the highest C sequestration potential, compared to the low N control and the high N treatment.Clearly, our results show that it is inappropriate to use plant productivity N response as an indicator for shifts in SOC content in grassland ecosystems. Here, isotopic techniques illustrated why atmospheric N deposition of 14 kg N ha−1 yr−1 is below, and 54 kg N ha−1 yr−1 is above a threshold that tips the balance between new, assimilative gains and respiratory losses towards a net loss of [SOC] for certain soil fractions in the subalpine grassland.

Conversion of a Semiarid Nevada Soil to Irrigated Agriculture Preferentially Removes Labile Carbon

Due to the scarcity of arable land, semiarid rangelands are often converted to irrigated croplands, which is likely to affect soil organic carbon (SOC) due to changes in C inputs into the soil and environmental factors regulating decomposition. In this study, soil density and particle size fractions as well as their C and N contents, stable isotopic composition, and chemical characterization by mid-infrared spectroscopy were measured in a native shrubland and an adjacent agricultural site under alfalfa cultivation for at least 50 years in western Nevada. Cultivation significantly reduced the amount of C and N in the surface soils and the proportion of C present in the labile fractions. The δ13C and δ15N values of the SOC reflected dominant vegetation types at each site, and suggested most SOC was root-derived. The potential decomposition rate of SOC was higher in the shrubland than in the alfalfa surface soil reflecting the larger amount of labile C present in the shrubland soils. Spectroscopy results suggested that the greater recalcitrance of the alfalfa soils was due to insoluble SOC moieties. Additional analyses of buried, SOC-rich, A horizons at both sites showed that slower decomposition of ‘deep’ SOC was due to lower substrate quality supported by fractionation and spectroscopy data. The results of this study showed that converting a semiarid shrubland into irrigated cropland significantly reduced SOC content but increased overall stability of residual SOC.

Distinct bioenergetic signatures in particulate versus mineral-associated soil organic matter

Physical and chemical stabilization, environmental conditions, and organic matter composition all play vital roles in determining the persistence of soil organic matter (SOM). Fundamentally, SOM stability depends on the balance of microbial bioenergetics between the input of energy needed to decompose it (i.e., activation energy; Ea) and the net energy gained (i.e., energy density; ED) from its decomposition. This relationship is complicated in soils by chemical and physical protection mechanisms, which require additional energies to overcome for decomposition to occur. In this study, we analyze the bioenergetics of soil density fractions, which vary in their degrees of organic matter-mineral association, and show that the relationship of ED and Ea has the ability to provide information about relative differences in SOM chemical composition and stability. Our results demonstrate distinct bioenergetic signatures between particulate, light (free and occluded) fractions versus mineral-associated, heavy fractions isolated from soil samples collected at two depths from a climosequence along an elevation gradient in the Sierra Nevada, California. While there were no significant differences in ED and Ea within each fraction across climates, the light fractions (LF) were characterized by larger ED and Ea values, whereas the heavy fractions (HF) were characterized by smaller ED and Ea values. Combined with CHN analyses, we conclude that SOM in HF pools is likely comprised of relatively simple organic compounds that have long turnover rates because of chemical association with soil minerals, whereas the LF pools are comprised of more chemically complex molecules with low chemical reactivity and high Ea.

The contribution of various organic matter fractions to soil–water interactions and structural stability of an agriculturally cultivated soil

AbstractThe presence and mutual interactions of soil organic matter (SOM) and clay particles are major factors determining soil structural stability. In the scope of agricultural management and environmental sustainability, it remains unclear how various mineral and organic matter (OM) fractions, OM–clay interactions and swelling processes in the interparticle space determine soil–water interactions and thus soil structural stability. To investigate this issue, we isolated the mineral and OM fractions of an agriculturally cultivated silty loam soil by soil density fractionation and assessed their hydration characteristics and effects on soil structural stability combining 1H‐NMR relaxometry, soil rheology and single wet‐sieving of soil aggregates. The results showed that agricultural management practices, in particular compost and ploughing, as well as various OM–clay interactions significantly affected soil–water interactions and soil structural stability. On the one hand, ploughing reduced soil structural stability by promoting clay swelling as a result of disrupted soil structures and reduced SOM content. On the other hand, compost treatment and reduced tillage increased soil structural stability. In all cases, soil density fractionation showed that compost‐derived particulate organic matter (POM) and mineral‐associated organic matter (MAOM) restricted clay swelling and resulted in a highly porous and mechanically stable soil matrix. In particular, POM increased soil structural stability by acting as nucleus for soil aggregation and by restricting clay swelling via its presence as solid, granular interparticulate material. In contrast, MAOM seemed to restrict clay swelling via clay surface covering and the formation of viscous interparticulate hydrogel structures.

Distribution of radiocesium in different density fractions of temperate forest soils in Fukushima

Abstract In forested areas of Fukushima, Japan, radiocesium emitted by the Fukushima Daiichi nuclear power plant accident has accumulated on the forest floor and in surface mineral soils. To estimate and model the mobility of radiocesium in forest soils, the concept of physical variations in soil organic matter is useful. We applied density fractionation to soil samples collected in two different forest stands in Fukushima; a deciduous broadleaf forest (DBF) and an evergreen needleleaf forest (ENF). To determine the relative contributions of the two density fractions to retention of radiocesium in surface mineral soils in a temperate forest region we investigated the concentrations and stocks of radiocesium in the low- and high-density fractions (LF and HF, respectively) of the soils, as well as in the litter, and temporal changes therein between 2012 and 2014. The 137Cs stock in the litter decreased from 2012 to 2014, whereas in the soil, as the sum of the LF and HF, it increased. On average, more than 40% of the 137Cs stock in surface mineral soil (depth of 0–5 cm) was retained in LF particles, which were composed mainly of particulate organic matter. Although the LF accounted for only 10.3% of the dry mass of the soil on average, it was also characterized by an average concentration of 137Cs that was 7.8 times higher than that of the HF, which was composed of organo-mineral particles. The mean increase in the 137Cs stock in the LF from 2012 to 2014 was equivalent to 70.1% and 52.5% of those in the soils in the DBF and ENF, respectively. The relationship between loss on ignition and 137Cs content indicated that the spatial heterogeneity of radiocesium in bulk soil was substantially affected by the heterogeneity in particulate organic matter. The coarse-size (>250 μm) of the LF suggested that it was less physically protected against microbial attack. Accordingly, the LF was considered to act as a large, but temporal reservoir of radiocesium that was originally retained in the litter layer. In contrast, the accumulation of 137Cs in the HF was low, especially in 2013–2014. The HF (or organo-mineral particles) might have a more important role than the LF in 137Cs fixation over a long period, and should be continuously studied as part of the current radiocesium monitoring network.

Fate of carbohydrates and lignin in north-east Siberian permafrost soils

Permafrost soils preserve huge amounts of organic carbon (OC) prone to decomposition under changing climatic conditions. However, knowledge on the composition of soil organic matter (OM) and its transformation and vulnerability to decomposition in these soils is scarce. We determined neutral sugars and lignin-derived phenols, released by trifluoroacetic acid (TFA) and CuO oxidation, respectively, within plants and soil density fractions from the active layer and the upper permafrost layer at three different tundra types (shrubby grass, shrubby tussock, shrubby lichen) in the Northeast Siberian Arctic. The heavy fraction (HF; >1.6 g mL−1) was characterized by a larger enrichment of microbial sugars (hexoses vs. pentoses) and more pronounced lignin degradation (acids vs. aldehydes) as compared to the light fraction (LF; <1.6 g mL−1), showing the transformation from plant residue-dominated particulate OM to a largely microbial imprint in mineral-associated OM. In contrast to temperate and tropical soils, total neutral sugar contents and galactose plus mannose to arabinose plus xylose ratios (GM/AX) decreased in the HF with soil depth, which may indicate a process of effective recycling of microbial biomass rather than utilizing old plant materials. At the same time, lignin-derived phenols increased and the degree of oxidative decomposition of lignin decreased with soil depth, suggesting a selective preservation of lignin presumably due to anaerobiosis. As large parts of the plant-derived pentoses are incorporated in lignocelluloses and thereby protected against rapid decomposition, this might also explain the relative enrichment of pentoses with soil depth. Hence, our results show a relatively large contribution of plant-derived OM, particularly in the buried topsoil and subsoil, which is stabilized by the current soil environmental conditions but may become available to decomposers if permafrost degradation promotes soil drainage and improves the soil oxygen supply.

Organic carbon characteristics in density fractions of soils with contrasting mineralogies

This study was aimed to evaluate the role of minerals in the preservation of organic carbon (OC) in different soil types. Sequential density fractionation was done to isolate particulate organic matter (POM, <1.8gcm−3) and mineral associated OM (MOM: 1.8–2.2, 2.2–2.6 and >2.6gcm−3) from four soils, i.e., a Ferralsol, a Luvisol, a Vertisol and a Solonetz. Organic matter (OM) in the density fractions was characterised using diffuse reflectance Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and mass spectroscopy in the original states (i.e., without any chemical pre-treatment), and after 6% sodium hypochlorite (NaOCl) and 10% hydrofluoric acid (HF) treatments. The NaOCl oxidation resistant fraction was considered asa relatively stable pool of OC and the HF soluble fraction was presumed as the mineral bound OC. Phyllosilicate-dominated soils, i.e., Vertisol, Luvisol and Solonetz, contained a greater proportion of POM than Fe and Al oxide-dominated Ferralsol. Wider C:N ratio and lower δ13C and δ15N in POM suggest the dominance of labile OC in this fraction and this was also supported by a greater proportion of NaOCl oxidised OC in the same fraction that was enriched with aliphatic C. The sequential density fractionation method effectively isolated OM into three distinct groups in the soils: (i) OM associated with Fe and Al oxides (>1.8gcm−3 in the Ferralsol); (ii) OM associated with phyllosilicates (1.8–2.6gcm−3) and (iii) OM associated with quartz and feldspar (>2.6gcm−3) in the other three soils. Greater oxidation resistance, and more dissolution of OC during the HF treatment in the Fe and Al oxides dominated fractions suggest a greater potential of these minerals to protect OC from oxidative degradation as compared to the phyllosilicates, and quartz and feldspar matrices. OM associated with Fe and Al oxides was predominantly aromatic and carboxylate C. Decreased C:N ratio in the NaOCl oxidation resistant OM and HF soluble OM of phyllosilicates, and quartz and feldspars dominant fractions compared to their untreated fractions indicate a preferred retention of N rich organic compounds by these minerals. OM associated with phyllosilicates was enriched with protonated amide N and aromatic C. Quartz and feldspars associated OM comprised of N containing organic compounds and polysaccharides, although we don’t expect any role of these minerals in their preservation. Our results imply that the abundance and surface properties of minerals in the soil largely control the dynamics of OC and subsequently protect OC from microbial cycling.

Effect of land use change on contents and distribution of monosacharides within density fractions of calcareous soil

In the last century, many calcareous soils in Castilla León (northwestern Spain) have been transformed from natural Quercus ilex forest to cropped land. During the past 40 years, cropping was abandoned and some of these soils has been reforested with Pinus halepensis. We studied how thEse land use changes affected the soil organic matter amount (C stocks) and characteristics. Density fractionation combined with ultrasonic dispersion enables separation and study of soil OM fractions, considered on the basis of the mechanisms of physical protection: non-physically protected OM (FF), OM occluded into aggregates (OF), and OM stabilized in organo-mineral complexes (DF). In the present study, whole soils and density fractions were analyzed for organic C amount (OC) and for carbohydrate content. The study aimed to describe the carbohydrate content in whole soils and its SOM density fractions to find out whether the carbohydrates can be used to explain the origin of SOM fractions and if they can depict its response to land management. We found a loss of 67% of the topsoil OC as a result of converting the natural Quercus ilex forest to cropped land, ∼100 years ago. After crop abandonment and soil reforestation with Pinus halepensis, the stock of topsoil OC increased. After ∼40 years of pine plantation, an average recovery of 71% of the previously lost OC had been recovered (43% loss of reference topsoil). The changes in OC stocks affected mainly the free fraction (FF) but also the organo-mineral (DF) one. Our results point to the role of physical protection in the C stocks but also that organo-mineral associations could be disrupted under a land use change. Monosaccharide content was significantly different (p < 0.01) among land uses. Gas chromatography analysis revealed significant differences in monosaccharide composition between land uses and also among density fractions. Whatever the fraction and land use considered, glucose was the dominant sugar monomer, followed by mannose and xylose.

Little effects on soil organic matter chemistry of density fractions after seven years of forest soil warming

Rising temperatures enhance microbial decomposition of soil organic matter (SOM) and thereby increase the soil CO2 efflux. Elevated decomposition rates might differently affect distinct SOM pools, depending on their stability and accessibility. Soil fractions derived from density fractionation have been suggested to represent SOM pools with different turnover times and stability against microbial decomposition.To investigate the effect of soil warming on functionally different soil organic matter pools, we here investigated the chemical and isotopic composition of bulk soil and three density fractions (free particulate organic matter, fPOM; occluded particulate organic matter, oPOM; and mineral associated organic matter, MaOM) of a C-rich soil from a long-term warming experiment in a spruce forest in the Austrian Alps. At the time of sampling, the soil in this experiment had been warmed during the snow-free period for seven consecutive years. During that time no thermal adaptation of the microbial community could be identified and CO2 release from the soil continued to be elevated by the warming treatment. Our results, which included organic carbon content, total nitrogen content, δ13C, Δ14C, δ15N and the chemical composition, identified by pyrolysis-GC/MS, showed no significant differences in bulk soil between warming treatment and control. Surprisingly, the differences in the three density fractions were mostly small and the direction of warming induced change was variable with fraction and soil depth. Warming led to reduced N content in topsoil oPOM and subsoil fPOM and to reduced relative abundance of N-bearing compounds in subsoil MaOM. Further, warming increased the δ13C of MaOM at both sampling depths, reduced the relative abundance of carbohydrates while it increased the relative abundance of lignins in subsoil oPOM. As the size of the functionally different SOM pools did not significantly change, we assume that the few and small modifications in SOM chemistry result from an interplay of enhanced microbial decomposition of SOM and increased root litter input in the warmed plots. Overall, stable functional SOM pool sizes indicate that soil warming had similarly affected easily decomposable and stabilized SOM of this C-rich forest soil.

Maize root decomposition in subsoil horizons of two silt loams differing in soil organic C accumulation due to colluvial processes

To analyse mechanisms controlling sequestration of organic C in subsoil, a field experiment was carried out for two years. Soilbags with a mesh size of 100μm, containing original soil material and maize root residues (C4 plant) were buried at three different depths (35, 45, and 65cm) at two neighbouring arable sites and were sampled after 12, 18 and 24months. The sites were a Colluvic Cambisol with high soil organic carbon (SOC) contents in the subsoil (12mgg−1 soil), and the other a Haplic Luvisol with low SOC contents (4mgg−1 soil) below 30cm depth. We determined the effects of the site, depths, and time on bulk SOC, organic C associated with soil density fractions, and microbial biomass C (MBC) in the soilbags. MBC increased to a similar extent (2.5 fold) from the initial content to its maximum at all sites and depths. This increase relied largely on the added maize root residues, as about 50% of the MBC was maize-derived after two years. However, we detected distinct differences in the substrate use for anabolism compared to catabolism, which decreased with depth and was lower in the Haplic Luvisol than in the Colluvic Cambisol. Freshly added plant material seems to be highly accessible to microorganisms in subsoil, but its metabolic use was determined by the soil properties of the two sites. The addition of plant residues also had an impact on aggregation dynamics, resulting in an almost complete replacement of formerly aggregate occluded material (i.e., occluded light fraction) by maize derived material after 24months.

Land use change exerts a strong impact on deep soil C stabilization in subtropical forests

Purpose It has been widely recognized that land use changes can cause significant alterations of soil organic matter (SOM) of various ecosystems. Forest conversion, a common land use change, and its effects on SOM have been a hot research topic during the past two decades. However, the mechanisms of the effects of forest conversion on SOM dynamics, particularly in deep soils, largely remain uncertain. This study aimed to examine the impacts of forest conversion on SOM stabilization through the analysis of soil aggregate and density fractionation, microbial composition, and functions in deep soils.

Chronic nitrogen fertilization and carbon sequestration in grassland soils: evidence of a microbial enzyme link

Chronic nitrogen (N) fertilization can greatly affect soil carbon (C) sequestration by altering biochemical interactions between plant detritus and soil microbes. In lignin-rich forest soils, chronic N additions tend to increase soil C content partly by decreasing the activity of lignin-degrading enzymes. In cellulose-rich grassland soils it is not clear whether cellulose-degrading enzymes are also inhibited by N additions and what consequences this might have on changes in soil C content. Here we address whether chronic N fertilization has affected (1) the C content of light versus heavier soil fractions, and (2) the activity of four extracellular enzymes including the C-acquiring enzyme β-1,4-glucosidase (BG; necessary for cellulose hydrolysis). We found that 19 years of chronic N-only addition to permanent grassland have significantly increased soil C sequestration in heavy but not in light soil density fractions, and this C accrual was associated with a significant increase (and not decrease) of BG activity. Chronic N fertilization may increase BG activity because greater N availability reduces root C:N ratios thus increasing microbial demand for C, which is met by C inputs from enhanced root C pools in N-only fertilized soils. However, BG activity and total root mass strongly decreased in high pH soils under the application of lime (i.e. CaCO3), which reduced the ability of these organo-mineral soils to gain more C per units of N added. Our study is the first to show a potential ‘enzyme link’ between (1) long-term additions of inorganic N to grassland soils, and (2) the greater C content of organo-mineral soil fractions. Our new hypothesis is that the ‘enzyme link’ occurs because (a) BG activity is stimulated by increased microbial C demand relative to N under chronic fertilization, and (b) increased BG activity causes more C from roots and from microbial metabolites to accumulate and stabilize into organo-mineral C fractions. We suggest that any combination of management practices that can influence the BG ‘enzyme link’ will have far reaching implications for long-term C sequestration in grassland soils.

NanoSIMS investigation of glycine-derived C and N retention with soil organo-mineral associations

While microbial-mineral-organic matter interactions are key features controlling the fates of low molecular-weight compounds in soils, direct investigations of how they control their fine-scale spatial distribution are scant. Here, we addressed how microbial transformations affect the retention of 13C/15N-labeled glycine in a forest topsoil 8 h after application. We assessed the contribution of soil microorganisms to glycine-derived 13C and 15N retention using γ-irradiated and non-irradiated soils. We tracked down the glycine-derived 13C and 15N at the surface of particles randomly isolated from soil density fractions using nano-scale secondary ions mass spectrometry (NanoSIMS) imaging. Eight hours after addition, 7 % of the glycine-derived 13C and 15N initially applied was recovered among soil density fractions, mainly via the activity of soil microorganisms (>85 % of total retention). Glycine-derived 13C and 15N distribution among density fractions was correlated with that of soil organic matter (SOM) determined by NanoSIMS (R ≥ 0.85), suggesting that the spatial patterns of the mineral-attached SOM controls the spatial distribution of the glycine-derived 13C and 15N. NanoSIMS images showed largely decoupled glycine-derived 13C and 15N spots preferentially attached to aggregated particles. We speculate that the glycine-derived 13C was principally found within or in the vicinity of microbial cells, whereas the glycine-derived 15N was mostly found as NH4 + and/or exoenzymes spread across soil surfaces. The C:N ratios determined by NanoSIMS suggest that local chemical properties of mineral-attached SOM drive glycine-derived 15N attachment, with the preferential attachment to mineral-attached SOM rich in N (C:N ratios mostly 2.65 g cm−3).

Carbon pool size and stability are affected by trees and grassland cover types within agroforestry systems of western Canada

Agroforestry systems are common land uses across Canada and could play a substantial role in sequestering carbon (C) as part of efforts to combat climate change. We studied the impact of component land cover types (forested vs. adjacent herbland) in three agroforestry systems (hedgerow, shelterbelt and silvopasture) on organic C and nitrogen (N) distribution in three density fractions of soils at the 0–10 and 10–30cm layers. The study evaluated 36 sites (12 hedgerows, 12 shelterbelts and 12 silvopastures) in central Alberta, Canada, distributed along a soil/climate gradient of increasing moisture availability. At the 0–10cm layer, soil organic C (SOC) stock in the bulk soil was significantly greater in the silvopasture system (101) than in either the hedgerow (77) or shelterbelt system (67Mg Cha−1). Soil organic C stock in both soil layers (0–10 and 10–30cm) was also significantly greater in the forested land cover (89 and 119Mg Cha−1, respectively) than in adjacent herblands (76 and 77Mg Cha−1). Across all sites, 31.5, 29.1, and 35.5% of SOC was found in the light fraction (<1.6gcm−3), occluded fraction (ultrasonic dispersion at 360W for 5min, <1.6gcm−3), and heavy fraction (>1.6gcm−3) of soils, respectively. The largest pool of SOC in the more labile light fraction of the 0–10cm layer was in the silvopasture system (50Mg Cha−1), whereas the smallest labile light fraction component of SOC was in the shelterbelt system (17Mg Cha−1). The largest pool of SOC in the more stable heavy fraction of both the 0–10 and 10–30cm depth classes was in the shelterbelt (33 and 35Mg Cha−1, respectively), while the least SOC was in the silvopasture system (26 and 20Mg Cha−1, respectively). We conclude that the presence of Populus based silvopasture system can increase C storage in surface mineral soils, and that the establishment of Picea based shelterbelts in an otherwise annually cropped agricultural landscape enhances the size of the stable SOC pool.

Soil extracellular enzyme activities are sensitive indicators of detrital inputs and carbon availability

In a litter manipulation experiment in a temperate deciduous oak forest in Central Europe, we examined soil carbon contents and density fractions, as well as β-glucosidase and polyphenol oxidase enzyme activities, which play central roles in the degradation of litter and soil organic matter. Our measurements were taken in the DIRT (detritus input and removal treatments) plots, where manipulations include doubling of leaf litter or woody debris inputs, as well as removal of litter, trenching to prevent root inputs, and removal of all litter inputs. After 10 years of manipulation, soil C content did not vary predictably among plots, although the amount of light fraction material was greater in control and litter addition plots compared to litter removal plots. Even after 10 years of litter addition, there were no significant differences in activities of either enzyme in double litter plots compared to control plots, a result consistent with other observed measures of microbial activity. However, removal of roots and litter caused significant decreases in β-glucosidase activities very quickly, and these differences increased over time. However, polyphenol oxidase activities were not significant different among treatments. Enzyme activities were not correlated with total soil carbon contents, but activities of both enzymes were significantly and positively related to the amount of light fraction carbon, suggesting that enzymes respond to increases in labile carbon availability.

Carbon Accumulation in the Bulk Soil and Different Soil Fractions During the Rehabilitation of Desertified Grassland in Horqin Sandy Land (Northern China)

Desertification, which affects more than two-thirds of the world's arid and semi-arid regions, is a significant global ecological and environmental problem. There is a strong link between desertification of the drylands and emission of CO2 from soil and vegetation to the atmosphere. The Horqin Sandy Land is a severely desertified area in China's agro-pastoral ecotone due to its fragile ecology, combined with unsustainable land management. We estimated changes of organic carbon content in the bulk soil (0–5 cm), in the light-fraction of soil organic matter (based on density fractionation), and in the various particle-size fractions in areas with mobile sand dunes after implementing grazing exclusion (12 and 27 years) and tree and shrub planting (22 and 24 years). Carbon stocks in the bulk soil and all soil density and particle-size fractions increased significantly in the exclosure and plantation plots. The average rates of carbon accumulation in the bulk soil in the exclosure and plantation plots were 16.0 and 17.8 g m-2 y-1, respectively, versus corresponding values of 2.3 and 7.1 g m-2 y-1 for the light fraction, 4.3 and 8.0 g m-2 y-1 for the coarse fraction, 5.0 and 3.4 g m-2 y-1 for the fine sand, 4.5 and 4.2 g m-2 y-1 for the very fine sand, and 1.8 and 1.8 g m-2 y-1 for the silt clay fraction. The older the exclosure and plantation, the more carbon accumulated in the bulk soil and in each fraction. The carbon pool exceeded the level in non-desertified grasslands after 27 years of grazing exclosure and 24 years of the shrub plantation. Our results suggest that both grazing exclusion and planting trees and shrubs can restore desertified grassland, creating a high potential for sequestering soil carbon, but that the plantations appeared to accumulate soil carbon faster than the exclosures.

Microbial contribution to SOM quantity and quality in density fractions of temperate arable soils

The formation of soil organic matter (SOM) very much depends on microbial activity. Even more, latest studies identified microbial necromass itself being a significant source of SOM and found microbial products to initiate and enhance the formation of long-term stabilized SOM. The objectives of this study were to investigate the microbial contribution to SOM in pools of different stability and its impact on SOM quality. Hence, four arable soils of widely differing properties were density-fractionated into free and occluded particulate organic matter (fPOM, oPOM < 1.6 g cm−3 and oPOM < 2.0 g cm−3) and mineral associated organic matter (MOM > 2.0 g cm−3) by using sodium polytungstate. These fractions were characterized by in-source pyrolysis-field ionization mass spectrometry (Py-FIMS). Main SOM compound classes of the fractions were determined and further SOM properties were derived (polydispersity, thermostability). The contribution of microbial derived input to arable soil OM was estimated from the hexose to pentose ratio of the carbohydrates and the ratio of C4–C26 to C26–C36 fatty acids. Additionally, selected samples were investigated by scanning electron microscopy (SEM) for visualizing structures as indicators for the origin of OM. Results showed that, although the samples differed significantly regarding soil properties, SOM composition was comparable and almost 50% of identifiable SOM compounds of all soils types and all density fractions were assigned to phenols, lignin monomers and alkylaromatics. Most distinguishing were the high contents of carbohydrates for the MOM and of lipids for the POM fractions. Qualitative features such as polydispersity or thermostability were not in general assignable to specific compounds, density fractions or different mean residence times. Only the microbial derived part of the soil carbohydrates could be shown to be correlated with high SOM thermostability (r2 = 0.63**, n=39). Microbial derived carbohydrates and fatty acids were both enriched in the MOM, showing that the relative contribution of microbial versus plant-derived input to arable SOM increased with density and therefore especially increased MOM thermostability. Nevertheless, the general microbial contribution to arable SOM is suggested to be high for all density fractions; a mean proportion of about 1:1 was estimated for carbohydrates. Despite biomolecules released from living microorganisms, SEM revealed that microbial mass (biomass and necromass) is a considerable source for stable SOM which is also increasing with density.

Assimilation and accumulation of C by fungi and bacteria attached to soil density fractions

Soil microorganisms play a key role in soil organic matter (SOM) dynamics, but little is known about the controls affecting the distribution of microbial biomass and their residues in soil. Here, a forested Cambisol topsoil was incubated with 13C-labeled glycine or beech leaves for 12 weeks prior to sequential density fractionation. The incorporation of the 13C label in amino sugars (AS) was used to gain insight into bacterial and fungal assimilation of the substrates. AS derived from glycine or leaves were compared to total AS to investigate how microbial residues and active communities were distributed among soil density fractions.Bacteria slightly dominated leaf C assimilation, while a pronounced fungal dominance was observed for glycine. The glycine-derived AS and original AS were similarly distributed among the soil density fractions, both peaking in microbial aggregates (1.8–2.4 g cm−3). Leaf-derived AS were mostly found in association with the plant debris (<1.65 g cm−3). The ratios of substrate-derived AS C to substrate-derived C increased with soil fraction density for both glycine and leaves. The same pattern was observed with original AS C to soil fraction C ratios. We concluded that bacteria and fungi were most active where the resource was even though their residues accumulate mostly in microbial aggregates (1.8–2.4 g cm−3). We suggest that such accumulation might be attributed to (1) an increasing stabilization efficiency of microbial residues and (2) the progressive SOM transfer, from plant debris to microbial aggregates (1.8–2.4 g cm−3).