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Soil carbon transformation and sequestration have received significant interest in recent years due to a growing need for quantitating its role in mitigating climate change. Even though our understanding of the nature of soil organic matter has recently been substantially revised, fundamental uncertainty remains about the quantitative importance of microbial necromass as part of persistent organic matter. Addressing this uncertainty has been hampered by the absence of quantitative assessments whether microbial matter makes up the majority of the persistent carbon in soil. Direct quantitation of microbial necromass in soil is very challenging because of an overlapping molecular signature with nonmicrobial organic carbon. Here, we use a comprehensive analysis of existing biomarker amino sugar data published between 1996 and 2018, combined with novel appropriation using an ecological systems approach, elemental carbon–nitrogen stoichiometry, and biomarker scaling, to demonstrate a suit of strategies for quantitating the contribution of microbe‐derived carbon to the topsoil organic carbon reservoir in global temperate agricultural, grassland, and forest ecosystems. We show that microbial necromass can make up more than half of soil organic carbon. Hence, we suggest that next‐generation field management requires promoting microbial biomass formation and necromass preservation to maintain healthy soils, ecosystems, and climate. Our analyses have important implications for improving current climate and carbon models, and helping develop management practices and policies. [ABSTRACT FROM AUTHOR]

Abstract In Bavarian alpine forests, long-term input of organic matter from conifer species has built up thick organic surface layers, thus immobilized substantial soil organic phosphorus upon calcareous parent materials. The composition of these phosphorus stocks, however, remained unrevealed. Therefore, we analyzed organic phosphorus species in three so called Tangelhumus (Histosol according to World Reference Base) profiles with different radiocarbon ages using alkaline extraction followed by one-dimension and two-dimension 31P nuclear magnetic spectroscopy in order to understand changes in organic phosphorus composition with soil organic matter genesis. A diverse phosphorus pool composed of monoesters, diesters, and phosphonates was detected. However, in these Tangelhumus soils only monoesters accumulated overtime and eventually dominated the organic phosphorus pool after 1558 years. Diesters and phosphonates did not show any significant accumulation pattern. The origin of monoesters varied with soil depth and soil age: monoesters in young surface soils were actually hydrolysis products of diesters released by plant and microbial cells during alkaline extraction; whereas in older soils monoesters originated largely from Ca-bound precipitates formed after long-term in situ decomposition of organic matter and stabilization by Ca2+. Myo -inositol hexakisphosphate was below detection limit in all soils. The presence of its scyllo and other isomers likely pointed to direct synthesis by microorganisms instead of microbial epimerization from myo -inositol hexakisphosphate. In general, the overall picture of phosphorus speciation manifested as the continuous degradation of organic matter and accumulation of monoesters. Therefore, the current data indicated no involvement of humification process but favored the soil continuum model. Highlights • Only monoesters showed dramatic accumulation after 1500 years of Tangelhumus formation. • Origin of monoesters varied with soil age/depth. • Ca2+ acted the stabilizer for SOP accumulation. • Myo -inositol hexakisphosphate was missing in all Tangelhumus soils. • Current data favored soil continuum model rather than humification. [ABSTRACT FROM AUTHOR]

Drained peatlands are hotspots of carbon dioxide (CO2) emissions from agriculture. As a consequence of both drainage-induced mineralisation and anthropogenic mixing with mineral soils, large areas of former peatlands under agricultural use now contain soil organic carbon (SOC) at the boundary between mineral and organic soils and/or underwent a secondary transformation of the peat (e.g. formation of aggregates). However, low carbon organic soils have rarely been studied since previous research has mainly focused on either mineral soils or true peat soils. The aim of the present study was to evaluate the soil organic matter (SOM) vulnerability of the whole range of organic soils including very carbon rich mineral soils (73 g kg-1 < SOC < 569 g kg-1) and to identify indicators for mineralisation of such anthropogenically disturbed organic soils. Using a large sample pool from the German Agricultural Soil Inventory, 91 soil samples were selected covering a broad range of soil and site characteristics. Fen and bog samples were grouped into disturbance classes according to their pedogenetic features. Potential CO2 production by aerobic incubation was then measured. Specific basal respiration rates (SBR) per unit SOC showed the highest potential emissions for heavily disturbed fen (12.1 ± 5.0 μg CO2-C g SOC-1 h-1) and moderately disturbed bog samples (10.3 ± 5.2 μg CO2-C g SOC-1 h-1). Surprisingly, SOM vulnerability increased with an increasing degree of disturbance and a decreasing SOC content, indicating positive feedback mechanisms as soon as peat soils are disturbed by drainage. Furthermore, with increasing degree of disturbance the variability of the SBR increased drastically, but correlations between soil properties and SBR could not be identified. Respiration rates increased more strongly with an increasing degree of disturbance in bog than in fen samples. Peat properties that positively influenced the turnover of SOM in less disturbed soil samples were mainly pH value and nitrogen content, while phosphorus was important for the mineralisation of increasingly disturbed samples and bog peat in general. Furthermore, a narrow carbon-to-nitrogen ratio correlated strongly with potential emissions. Given the high potential of CO2 emissions from organic soils with a low SOC content, mixing with mineral soil does not seem to be a promising option for decreasing emissions. [ABSTRACT FROM AUTHOR]

Nitrite (NO ) as an important intermediate of the biological nitrogen cycle is particularly reactive in acidic soils and acts as a source of NO and NO (NO and NO). However, abiotic and biotic pathways of NO -driven NO and NO production in forest soil and the role of soil organic matter (SOM) in these processes are still unclear. In this study, NO was applied to both unsterile and sterilized soil samples as well as to different SOM fractions from a Norway spruce forest. Biotic and abiotic NO emission was measured with an infrared absorption analyzer and gas chromatography, while NO emission was quantified with a chemiluminescence analyzer. Isotopic signatures of NO (δN, δO, and N-NO site preference) were analyzed with an isotope ratio mass spectrometer. After NO addition, a large amount of NO was emitted immediately, while NO emission occurred 15-60 min later and was much lower compared to NO. Sterilization of soil decreased NO emission significantly, but not NO emission. The N site preference of NO ranged from 7.98 to 11.58‰ for abiotic and 4.69-7.42‰ for biotic sources. The fulvic acid fraction contributed the most to abiotic NO emission, while the fastest NO and NO emission occurred after NO application to the humin fraction, followed by the humic acid fraction. These results are important for the future understanding of NO and NO sources, as well as the use of isotopic signatures for source-partitioning NO emission from soil. Graphical Abstract: [ABSTRACT FROM AUTHOR]

Past land-use changes, intensive cropping with large proportions of root crops, and preferred use of mineral fertilizer have been made responsible for proceeding losses of soil organic C (SOC) in the plough layer. We hypothesized that in intensive agriculturally managed regions changes in SOC stocks would be detectable within a decade. To test this hypothesis, we tracked the temporal development of the concentrations and stocks of SOC in 268 arable sites, sampled by horizon down to 60 cm in the Cologne-Bonn region, W Germany, in 2005 and in 2013. We then related these changes to soil management data and humus balances obtained from farmers' surveys. As we expected that changes in SOC concentrations might at least in part be minor, we fractionated soils from 38 representative sites according to particle size in order to obtain C pools of different stability. We found that SOC concentrations had increased significantly in the topsoil (from 9.4 g kg−1 in 2005 to 9.8 g kg−1 in 2013), but had decreased significantly in the subsoil (from 4.1 g kg−1 in 2005 to 3.5 g kg−­1 in 2013). Intriguingly, these changes were due to changes in mineral-bound SOC rather than to changes in sand-sized organic matter pools. As bulk density decreased, the overall SOC stocks in the upper 60 cm exhibited a SOC loss of nearly 0.6 t C (ha · y)−1 after correction by the equivalent soil mass method. This loss was most pronounced for sandy soils [−0.73 t SOC (ha · y)−1], and less pronounced for loamy soils [−0.64 t SOC (ha · y)−1]; silty soils revealed the smallest reduction in SOC [−0.3 t SOC (ha · y)−1]. Losses of SOC occurred even with the overall humus balances having increased positively from about 20 kg C (ha · y)−1 (2003-2005) to about 133 kg C (ha · y)−1 (2005-2013) due to an improved organic fertilization and intercropping. We conclude that current management may fail to raise overall SOC stocks. In our study area SOC stocks even continued to decline, despite humus conservation practice, likely because past land use conversions (before 2005) still affect SOC dynamics. [ABSTRACT FROM AUTHOR]

Wastewater irrigation is often performed by flood irrigation, leading to changes in redox potential (Eh) of irrigated soils. In addition to soil organic matter, Fe-(hydr)oxides are important sorbents for pollutants, and biotransformation of pollutants can be accelerated under reducing conditions. Here, the influence of reducing conditions on the release of sorbed pharmaceuticals from soil and their potential accelerated dissipation was investigated in a microcosm study. Samples of a soil from the Mezquital Valley (Mexico) irrigated for 85 yr with untreated wastewater were incubated under oxidizing (Eh of 500 ± 20 mV), weakly reducing (Eh of 100 ± 20 mV), and moderately reducing (Eh of -100 ± 20 mV) soil conditions for 30 to 31 d. The concentrations of nine pharmaceuticals (bezafibrate, carbamazepine, ciprofloxacin, sulfamethoxazole, trimethoprim, enrofloxacin, clarithromycin, diclofenac, and naproxen) were extracted via solid-phase extraction from soil slurries and analyzed by liquid chromatography-tandem mass spectrometry. Low Eh did not lead to a release of formerly sorbed pharmaceuticals from the wastewater irrigated soil. High pH values (>8) of the examined soil resulting from denitrification under reducing conditions prevented the dissolution of Fe-(hydr) oxides and, hence, the potential release of pharmaceuticals. A trend of decreasing concentrations of sulfamethoxazole and bezafibrate with time under moderately reducing conditions supports previous findings of a transformation of these compounds under anaerobic conditions. [ABSTRACT FROM AUTHOR]

Leaf area index ( LAI), a measure of canopy density, is a key variable for modelling and understanding primary productivity, and also water use and energy exchange in forest ecosystems. However, LAI varies considerably with phenology and disturbance patterns, so alternative approaches to quantifying stand-level processes should be considered. The carbon isotope composition of soil organic matter (δ13 CSOM) provides a time-integrated, productivity-weighted measure of physiological and stand-level processes, reflecting biomass deposition from seasonal to decadal time scales., Our primary aim was to explore how well LAI correlates with δ13 CSOM across biomes., Using a global data set spanning large environmental gradients in tropical, temperate and boreal forest and woodland, we assess the strength of the correlation between LAI and δ13 CSOM; we also assess climatic variables derived from the World Clim database., We found that LAI was strongly correlated with δ13 CSOM, but was also correlated with Mean Temperature of the Wettest Quarter, Mean Precipitation of Warmest Quarter and Annual Solar Radiation across and within biomes., Synthesis. Our results demonstrate that δ13 CSOM values can provide spatially explicit estimates of leaf area index ( LAI) and could therefore serve as a surrogate for productivity and water use. While δ13 CSOM has traditionally been used to reconstruct the relative abundance of C3 versus C4 species, the results of this study demonstrate that within stable C3- or C4-dominated biomes, δ13 CSOM can provide additional insights. The fact that LAI is strongly correlated to δ13 CSOM may allow for a more nuanced interpretation of ecosystem properties of palaeoecosystems based on palaeosol 13 C values. [ABSTRACT FROM AUTHOR]

Highlights: [•] Soil texture and soil organic carbon content do not affect 15N dynamic. [•] 15N release depends on fertilizer (chicken manure, mushroom compost, mineral N). [•] Mustard intercropping is a tool to enhance N efficiency. [Copyright &y& Elsevier]

Abstract: In arable farming systems, the term ‘subsoil’ refers to the soil beneath the tilled or formerly tilled soil horizon whereas the latter one is denoted as ‘topsoil’. To date, most agronomic and plant nutrition studies have widely neglected subsoil processes involved in nutrient acquisition by crop roots. Based on our current knowledge it can be assumed that subsoil properties such as comparatively high bulk density, low air permeability, and poverty of organic matter, nutrients and microbial biomass are obviously adverse for nutrient acquisition, and sometimes subsoils provide as little as less than 10% of annual nutrient uptake in fertilised arable fields. Nevertheless, there is also strong evidence indicating that subsoil can contribute to more than two-thirds of the plant nutrition of N, P and K, especially when the topsoil is dry or nutrient-depleted. Based on the existing literature, nutrient acquisition from arable subsoils may be conceptualised into three major process components: (I) mobilisation from the subsoil, (II) translocation to the shoot and long-term accumulation in the Ap horizon and (III) re-allocation to the subsoil. The quantitative estimation of nutrient acquisition from the subsoil requires the linking of field experiments with mathematical modelling approaches on different spatial scales including Process Based Models for the field scale and Functional–Structural Plant Models for the plant scale. Possibilities to modify subsoil properties by means of agronomic management are limited, but ‘subsoiling’ – i.e. deep mechanical loosening – as well as the promotion of biopore formation are two potential strategies for increasing access to subsoil resources for crop roots in arable soils. The quantitative role of biopores in the nutrient acquisition from the subsoil is still unclear, and more research is needed to determine the bioaccessibility of nutrients in subsoil horizons. [Copyright &y& Elsevier]

Lowland rice paddy soils may accumulate significant amounts of organic matter. Our aim was to investigate the role of prolonged paddy management on the nitrogen ( N) status of the soils, and to elucidate the contribution of bacteria and fungi to long-term N accumulation processes. For this purpose, we sampled a chronosequence of 0-2000 years of rice cropping with adjacent non-paddy systems in the Bay of Hangzhou, China. The samples were analyzed for bulk density, total, mineral and microbial N ( Nmic), and amino sugars as markers for microbial residues. The results showed that during the first 100 years of land embankment, both paddy and non-paddy soils accumulated N at a rate of up to 61 and 77 kg ha−1 per annum, reaching steady-state conditions after 110-172 years, respectively. Final N stocks in paddy fields exceeded those of the non-paddies by a factor of 1.3. The contribution of amino sugars to total N increased to a maximum of 34 g N kg−1 N in both land-use systems, highlighting a significant accumulation of N in microbial residues of the surface soils. Correspondingly, the ratio of Nmic to microbial residue- N decreased to a constant value. In the paddy subsoils, we found that bacterial residues particularly contributed to the pool of microbial residue- N. Nevertheless, the absolute contents of amino sugars in paddy subsoils decreased during the last 1700 years of the chronosequence. We conclude that under paddy cultivation, soil microorganisms may accumulate parts of this N in their residues despite low overall N availability. However, this N accumulation is limited to initial stages of paddy soil development and restricted to the surface horizons, thus challenging its sustainability with future land-use changes. [ABSTRACT FROM AUTHOR]

Black carbon (BC) is the product of incomplete burning processes and a significant component of the passive soil organic carbon (SOC) pool. The role of BC in the global carbon cycle is still unclear. This study aimed to quantify and characterize BC in major grassland ecosystems of the world. Twenty-eight representative soil profiles (mainly Mollisols) were sampled in the Russian Steppe, the U.S. Great Plains, the Argentinian Pampa, the Manchurian Plains in China, and the Chemozem region in central Germany. Black carbon contents were estimated using benzene polycarboxylic acids (BPCA) as a molecular marker, and indications about the origin of the BC were derived from bulk and compound-specific δ13C analyses and radiocarbon dating of bulk soil organic matter (SOM). Our findings suggest that between 5% and 30% of SOC stocks consist of BC. Maximum BC contributions to SOC frequently were found at deeper parts of the A horizon with 14C ages younger than 7000 years BP; that is, incorporation of C as charred particles accompanied ecosystem development since the mid-Holocene. Most of this BC formed from local vegetation, as indicated by a 13C isotope signature similar to that of bulk SOM. At some sites, also nonlocal sources contributed to soil BC, e.g., fossil fuel BC inputs at the German sites. Black carbon stocks were highest in Chernozems and lowest in Kastanozems. The Russian Steppe and Chinese Manchurian sites stored about 3-4 times more BC (around 3 kg m-2) than did the other sites because of thicker A horizons that were rich in BC. On a global scale, we estimate that steppe ecosystems contain between 4 and 17 Pg BC. [ABSTRACT FROM AUTHOR]

Abstract: Paddy soils make up the largest anthropogenic wetlands on earth. They may originate from any type of soil in pedological terms, but are highly modified by anthropogenic activities. The formation of these Anthrosols is induced by tilling the wet soil (puddling), and the flooding and drainage regime associated with the development of a plough pan and specific redoximorphic features. Redox potential oscillations due to paddy management control microbial community structure and function and thus short-term biogeochemical processes. After flooding, microbial reduction processes sequentially use NO3 −, Mn4+, Fe3+, SO4 2 − as electron acceptors, accompanied by the emission of the trace gases N2O, N2, H2S, CH4 and — due to reduction-induced increasing pH — NH3. This results in N losses and low N fertilizer use efficiency. However, transport of atmospheric O2 to the roots via the rice plant''s aerenchyma modifies conditions in the rhizosphere, leading to nitrification and methane oxidation, and precipitation of Mn and Fe oxides. High concentrations and fluxes of dissolved organic matter (DOM) in paddy soils from plant debris trigger microbial activity and thus the emission of greenhouse gases. Retention of DOM by soil minerals and its subsequent stabilisation against microbial decay depend on the redox state (e.g. DOM precipitation by Fe2+ under anaerobic conditions). Oscillation in redox conditions may enhance retention and stabilisation of DOM by Fe oxyhydroxides. Induced by the periodic short-term redox cycles, paddy management over long periods has strong effects on long-term biogeochemical processes. Frequent irrigation intensifies mineral weathering and leaching processes. High concentrations of DOM during flooding seasons enhance the changes and the release of structural iron in clay minerals, and support the formation of ferrihydrite. Repeated redox alternations lead to a translocation of iron in various directions, and particularly increase the crystallinity of iron oxides. This results also in higher total iron oxide contents in paddy compared to non-paddy soils. The large accumulation of soil organic matter (SOM) observed in some, but not all paddy soils, is considered to be due to high input of plant residues and charred material associated with retarded decomposition under anaerobic conditions. There is also evidence of SOM stabilisation via occlusion into aggregates and phytoliths as well as interactions with clay minerals and iron oxides. SOM accumulation in paddy subsoils can be explained by downward movement of DOM and its stabilisation by interaction with iron oxides. A specific feature of paddy soils is the coupling of organic matter turnover with mineral transformations and fluxes, which seem to be intensified by the alternating redox conditions with increasing age of paddy soil development. Bioavailability of soil organic N is strongly coupled to SOM cycling and is a crucial parameter determining crop yield. Anaerobic conditions inhibit N mineralization, with a high risk of gaseous N losses. In paddy soils the management-induced, microbially mediated redox processes control the dynamics of soil minerals and soil organic matter, which are strongly related to the microbial accessibility of C and N, but also of Fe. [Copyright &y& Elsevier]

Abstract: It is well known that termites enrich organic matter in their nests and alter its properties, but their influence on phosphorus forms is less clear. The aim of our study was to quantify and systemize phosphorus forms in different nest parts (outer wall, inner wall, and central part) of termites of varying feeding guilds and soils in seven dominant Brazilian ecosystems. Organic (Po) and inorganic P (Pi) were analyzed after sequential extraction of six labile and stable P fractions in samples from the Amazon region (Terra firme, and Várzea and Igapó floodplains), the Pantanal, the Atlantic rain forest (Mata Atlântica), the Cerrado and the Caatinga. In addition, selected alkaline extracts were characterized by means of liquid-state 31P NMR spectroscopy. The central part of termite nests contained between 275 and 1871mgkg−1 total P, dominated by Pi in nests of soil/wood interface-feeder termites, but with balanced Po and Pi contents in nests of wood-feeding termites. All Po fractions, and, in most nests, also labile Pi forms were enriched in termite mounds in comparison to soils. The P enrichment was most pronounced in nests of the Terra firme (enrichment factors for labile Pi forms and for Po fractions were 1.7–36.5 and 1.1–16.8, respectively), and the Igapó (labile Pi: 4.1–34.0; Po: 1.1–25.7) where the total P content of the soil was the lowest with 141 and 171mgkg−1. Nests of wood-feeding termites mainly accumulated highly labile resin-Pi (enrichment factor 10.5–36.5) and partially the stable HClconc-Pi fractions, apart from Po, which can be all attributed to the organic food source and food digestion. The soil/wood interface-feeder termites had more variations in their nests, but in general had higher portions of P bonded to minerals. Hence, we conclude that (i) termite activity results in a gross enrichment of labile P forms in the nests, (ii) the P composition in termite nests reflects the major feeding guild, and (iii) the degree of this P enrichment is partly driven by the P stock in the soil. [Copyright &y& Elsevier]

Tracing back the N use efficiency of long-term fertilizer trials is important for future management recommendations. Here we tested the changes in natural N-isotope composition as an indicator for N- management within a long-term fertilization lysimeter experiment in a low mountain range pasture ecosystem at Rengen (Eifel Mountains), Germany. Cattle slurry (δ15N = 8.9 ± 0.5‰) and mineral fertilizers (calcium ammonium nitrate; δ15N = −1.0 ± 0.2‰) were applied at a rate between 0 and 480 kg N ha−1 yr−1 throughout 20 years from 1985 onwards. In 2006, samples were taken from different grass species, coarse and fine particulate soil organic matter, bulk soil and leachates. Total soil N content hardly changed during fertilization experiment. As also N leaching has been small within the stagnant water regime, most N was lost through the gaseous phase beside plant uptake and cutting. Unlike N uptake by plants, the process of N volatilization resulted in strong discrimination against the 15N isotope. As a consequence, the δ15N values of top soil samples increased from 1.8 ± 0.4‰ to 6.0 ± 0.4‰ and that of the plants from −1.2 ± 1.3‰ to 4.8 ± 1.2‰ with increasing N fertilizer rate. Samples receiving organic fertilizer were most enriched in δ15N. The results suggest that parts of the fertilizer N signal was preserved in soils and even discovered in soil organic matter pools with slow N turnover. However, a 15N/14N isotope fractionation of up to 1.5‰ added to the δ15N values recovered in soils and plants, rendering the increase in δ15N value a powerful indicator to long-term inefficient N usage and past N management in the terrestrial environment. [ABSTRACT FROM AUTHOR]

Studies the persistent organic pollutants (PO) in native grassland soils along a climosequence in North America. Role of soil organic matter (SOM) and climate in controlling the fate of hazardous POP; Description of study sites and soils; Polycyclic aromatic hydrocarbon (PAH) and polychlorinated biphenyl (PCB) concentrations and patterns.

• Organic matter accumulation depends on pedogenic soil differentiation. • Pedogenic processes lead to soil-group-specific organic matter accrual in subsoil. • Reference Soil Groups have unique process combinations of organic matter accrual. Soil organic matter (SOM) accumulation is different in certain soil groups with differences in parent material, degree of weathering and mineral composition. These differences are modulated by climatic factors, but also by pedogenesis, in particular by the formation of reactive mineral surfaces, by soil aggregation, as well as by translocation processes such as eluviation and illuviation and different types of turbation. However, there is still a lack of conceptualization of how such processes and thus important Reference Soil Groups influence the composition and properties of OM. Here we summarize the basic processes of OM storage as they differ from soil group to soil group, in order to present a first overview of the processes of OM formation in the different terrestrial soils of the world. We distinguish between soils of different climatic zones, i.e. Cryosols in permafrost regions, soils of limited development (Cambisols), Podzols, Phaeozems, Chernozems, Kastanozems, and Luvisols in temperate climate zones, as well as Acrisols, Ferralsols, Plinthosols and Nitisols in the subtropics and tropics. We also include soils derived from a specific parent material (Andosols, Vertisols), as well as Anthrosols (paddy soils, Terra Preta, plaggen soils) as examples for human-made SOM accumulations. The compilation of the literature shows that research on OM is clearly focused on specific Reference Soil Groups in temperate climate zones and some man-made soils, while other soils such as Nitisols and Acrisols are clearly underrepresented. The contribution of the different soil groups to global organic carbon (OC) stocks varies, with large amounts of OC found for the first metre in Cryosols, Cambisols, and Podzols, due to the large land area they cover, followed by Acrisols and Ferralsols. In part, these differences can be attributed to differences in the formation of SOM, which we ascribe to three main mechanisms. We emphasize that in all major Reference Soil Groups, both the mechanism of sorptive conservation as well as the protection within the aggregates contribute to the storage of OM. However, the reactant partners and aggregate forming agents and therewith the intensity of these stabilisation processes vary among the Reference Soil Groups. As a result, there are differences in the SOM composition in the topsoil. Within the entire soil profile, however, pedogenic processes lead as third mechanism to soil-group-specific accrual of SOM in the subsoil, e.g. by means of illuviation, by cryo-/bio-, and peloturbation, as well as by management. We conclude that the specific pedogenic environment must be considered in the assessment of global SOM storage potentials and thus also in future global C models. [ABSTRACT FROM AUTHOR]

Abstract: Prolonged arable cropping of subtropical sandy grassland soils resulted in substantial losses of water-stable aggregates. The objective of this study was to evaluate how rapidly and to which degree soil aggregates may be restored when degraded cropland soils are converted to secondary pasture lands. Hence, chronosequences of degraded cropland soils (more than 20years of arable cropping) were sampled that had been converted to secondary pastures between 1 and 52years ago in three agro-ecosystems of the South African Highveld. Primary grasslands as well as sites under long-term cropping served as references. The surface soils (Plinthusthalfs; 0–10cm) were fractionated to aggregates of different sizes by wet sieving (8000–2800μm, 2800–2000μm, 2000–500μm, 500–250μm, 250–53μm and<53μm). All fractions were analyzed for their respective content of soil organic carbon and total nitrogen and corrected for sand content. The results showed that reconversion of cropland into secondary pasture restored the soil structure after 9.5–18years of pasture management almost completely: the amount of sand-corrected large macroaggregates (>2000μm) approached 85–94% of the levels of the primary grasslands. The stocks of organic matter bound to large macroaggregates recovered more slowly and reached only 50% of that of the primary grassland, which, however, exhibited also a slightly elevated clay content. The carbon concentration in the aggregates did not change significantly. Thus, the increase in C stocks in the secondary pasture soils was mainly due to a rebuilding of large macroaggregates, which contained more C than the smaller-sized aggregate classes, but not more C than the respective aggregates in the degraded cropland. We conclude that only the amount of large macroaggregates was restored upon land conversion, while their protective capacity was obviously not restored and thus not strong enough to account for a full sequestration of soil organic matter. [Copyright &y& Elsevier]

Abstract: Converting grassland soils to cropland changes the turnover of aggregates and their protective effects on soil organic matter (SOM). Our aim was to elucidate how rapid soil aggregation and associated SOM contents change with increased duration of long-term cropping. The study was conducted in the South African Highveld in three agro-ecosystems with different cropping durations (0–98years) after conversion of the native grassland. Soil samples (0–20cm; Plinthustalfs) were analyzed for aggregate size distribution (six water-stable aggregate fractions) and the C and N contents in each single fraction. Additionally, sieved and ground bulk soil was oxidized with 0.33M KMnO4. With an increase of cultivation time the aggregate size distribution changed remarkably: the large macroaggregates (2000–2800μm) and peds (>2800μm) broke down into finer size classes (exponential rate constant k =0.20year−1), accompanied especially by an increase of the contents of small macroaggregates (250–2000μm) and not of the microaggregates as previously reported for other soil types. The C and N concentrations decreased in all aggregates (k =0.05 to 0.52year−1) and the rate of decline was fastest in the peds. Most C was lost from the 250–2000μm and 53–250μm sized fractions (about 60%). At the primary grassland, the peds (>2800μm) stored 47% of total C, but less than 10% after 90years of cropping. Parts of this C were recovered in smaller aggregate fractions. Especially the C stored in the small macroaggregates (250–2000μm) increased from 27±1% to 61±1% of total C. The total loss of SOM and large aggregates went along with an accelerated loss of its C resistant to oxidation. While 40% of soil C was not oxidizable with KMnO4 from the grassland soil, the contents of this C resistant to chemical oxidation were reduced by up to 50% as cultivation proceeded. Grinding the soil hardly changed this amount. We conclude that the loss of soil structure made also resistant SOM available for decay, which might then be difficult to replace when a restoration of the sites is anticipated. [Copyright &y& Elsevier]

Abstract: Organic matter is generally enriched in termite nests. Since lignin resists degradation by most termite species, it could be a prominent tracer of the organic matter incorporated into termite nests and released into nest surroundings. Our objective was to quantify the content and alteration degree of lignin in termite nests and the surrounding soils, and to analyze the development of the lignin status from young to older termite nests. We sampled Cornitermes silvestrii earth mounds and the surrounding soil in an extensive grid scheme in a Cerrado savanna in Central Brazil. Older mounds were secondarily inhabited by Nasutitermes kemneri and other inquilines; those mounds and reference soil sites were also sampled. Lignin contents were estimated by the sum of lignin-derived phenols and additionally by mid-infrared spectroscopy followed by a partial least squares regression. Nests inhabited by primary termites had a lignin content of 1952mgkg−1, which was 15 times more than at 10cm depth of the reference soils. In comparison, organic carbon was only enriched by a factor of 2 in primarily inhabited nests. Our accompanying microcosm experiment showed that the lignin composition was not changed during incorporation into termite nests. Elevated lignin contents could still be found at 60cm distance from the nest border as well as up to 60cm soil depth beneath the nests. The lignin content in older nests was only half that of younger nests, and the influence on the nest surroundings was less prominent. Higher acid-to-aldehyde ratios in those older nests indicated that the remaining lignin was oxidized and thus partly degraded during nest aging. We conclude that the savanna termite C. silvestrii enriches lignin in its earth mounds, but that only a minor part enters the soil-protected lignin fraction when nests decay, as most of the lignin is lost during nest aging. [Copyright &y& Elsevier]

• Soil microaggregates revealed highly heterogenous compound composition. • Organic C accumulated at several sites in aggregates and at surfaces. • Spatial co-abundance of plant and microbial matter correlated with C accrual. • No spatial correlation between organics and minerals was found. The physical arrangement of compounds in soil microaggregates is controlling many ecosystem functions such as soil stability and C sequestration. However, little is known about the spatial arrangement of organic and inorganic compounds in soil microaggregates, due to the lack of in-situ analyses of undisturbed material. We hypothesized that microaggregates are spatially organized due to interactions between organic matter and mineral phases. To test this, we separated the water stable, occluded, large and small microaggregate fractions (250–53 and 53–20 µm, 60 J ml−1 dispersion energy) from Ap horizons of a toposequence of sandy to loamy Luvisols (Germany) with increasing clay contents (19 to 35% clay), and subjected in total 60 individual aggregates to elemental mapping by electron probe micro analysis (EPMA), which recorded C, N, P, Al, Fe, Ca, K, Cl, and Si contents at 1 × 1 µm resolution. Stoichiometric element ratios characteristic for organic and inorganic matter were used to define discrete components of the aggregates and analyze their spatial arrangement. We found a pronounced heterogeneity in content and arrangement of discrete aggregate components, which was not reproducible for different specimens from the same soil microaggregate fraction, and thus largely independent of clay content in soil. However, nearest neighbor analyses revealed close spatial correlations between plant detritus (C:N app. 100:10) and microbial organic matter (C:N app. 10:1) indicating a spatial relationship between source and consumer. There was no systematic relationship between soil minerals and organic matter, suggesting that well-established macroscale correlations between contents of pedogenic oxides and clay minerals with soil organic matter storage do not apply to soil microaggregates. [ABSTRACT FROM AUTHOR]

Drained peatlands are hotspots of carbon dioxide (CO 2) emissions from agricultural soils. As a consequence of both drainage-induced mineralisation and anthropogenic mixing with mineral soils, large areas of former peatlands under agricultural use underwent a secondary transformation of the peat (e.g. formation of aggregates). These soils show contents of soil organic carbon (SOC) at the boundary between mineral and organic soils. However, the carbon (C) dynamics of such soils have rarely been studied so far. The aim of the present study was to evaluate the vulnerability of soil organic matter (SOM) to decomposition over the whole range of peat-derived soils under agriculture including very carbon rich mineral soils (76–526 g kg−1 SOC). A total of 62 soil samples covering a broad range of soil and site characteristics were selected from the sample set of the German Agricultural Soil Inventory. Potential CO 2 production was measured by aerobic incubation. Fen and bog peat samples were grouped into disturbance classes according to their soil properties. Specific basal respiration rates (SBR), i.e. CO 2 fluxes per unit SOC, showed the highest values for the most disturbed samples for both fen peat (13.9 ± 6.0 μg CO 2 -C g SOC−1 h−1) and bog peat (10.9 ± 4.7 μg CO 2 -C g SOC−1 h−1). Respiration rates of bog peat increased more strongly with an increasing degree of disturbance than those of fen peat. Perhaps counterintuitively, SOM vulnerability to decomposition thus increased with an increasing degree of disturbance and a decreasing SOC content, indicating positive feedback mechanisms as soon as peat soils are disturbed by drainage. Furthermore, the variability of the SBR increased drastically with increasing degree of disturbance. The turnover of SOM in less disturbed peat samples tended to be higher in samples with higher nitrogen (N) content, higher pH value and lower C:N ratio, while plant-available phosphorus was important for the mineralisation of more severely disturbed peat. However, clear correlations between a single soil property and SBR could not be identified. The high potential of CO 2 emissions from organic soils with a low SOC content implies that mixing organic soil with mineral soil does not seem to be a promising option for mitigating greenhouse emissions. • Specific basal respiration (SBR) determined by aerobic incubation as a measure of vulnerability. • Higher and more variable SBR rates for stronger degraded organic soils. • Bog peat seems to be more vulnerable than fen peat. • High plant-available phosphorus contents increase SBR rates. • SBR rates of heavily disturbed samples remain unpredictable. [ABSTRACT FROM AUTHOR]

The Atacama Desert is the oldest and driest desert on earth, nevertheless traces of life have been observed in several places, accumulating residues of organic matter (OM) on the desert surface, i.e., in its soil. We asked if these OM traces of life preferentially accumulate in topsoil, or whether also enrichment of OM in deeper subsoil layers appears. We also evaluated to which degree spatial and vertical heterogeneity in the distribution of soil organic carbon (SOC) stocks depends on increasing aridity in the desert ecosystem. We sampled an east-west directed aridity transect with several replicates spanning the Atacama Desert from north to south, i.e. Codpa (18.4°S), Rio Tana (19.2°S), Quebrada Aroma (19.4°S), Yungay (24.6°S) and Paposo (25.1°S). With a nested sampling design (3≥n≤18) we addressed topsoil heterogeneity at each sampling site (n=433). For 13 of these sites soil profiles were dug to 0.8 to 3.9 m depth depending on soil thickness. This amounted to a total of 637 samples. Bulk density was taken for each site and depth of 15 cm. SOC concentrations were taken for each sample by thermally resolved dry combustion with a Soli TOC (Elementar Analysensysteme, Hanau, Germany DIN 19539GS). We found that with decreasing precipitation (~7 to <0.1 mm yr-1), SOC stocks decreased from 3.42±4.08 to 0.09±0.01 kg SOC m-2 in the 15 cm of topsoil. Hence, traces of life were ubiquitously preserved both in the arid and hyper-arid desert areas. Intriguingly, relatively large amounts of soil organic matter were also found at depth below 15cm, adding an extra of 52.9 to 95.9 % of subsoil C to the topsoil SOC stock. Furthermore, the subsoil SOC concentrations peaked between 40 and 150 cm depth in the soil, irrespectively of the presence or absence of visible vegetation. Biomarker analyses are currently performed to reveal the origin of SOC found in the various depths. We conclude that C accrual in this arid to hyper-arid system shifts from the preferential C enrichment in topsoil to that in the subsoil, therewith providing the potential for deep(er) biosphere foodwebs and demonstrating the likelyhood to need to dig into soil for discovering traces of (active) life in comparable environments, including other planets such as Mars. [ABSTRACT FROM AUTHOR]

The mean residence time of organic matter in soil ranges from days to millenia. Although chemical and physical processes are known to control the storage of organic matter in soil, the role of spatial arrangements in soil remains largely unknown due to the lack of in-situ techniques operating on the micro-scale. We asked whether deterministic patterns in element arrangement occur in large and small soil microaggregates (250-53 and 53-20 µm), and how these contribute to the storage of organic matter. To do so, we studied surface soils with increasing clay contents (sandy to loamy Luvisols, Germany) and subjected 60 individual aggregates to elemental mapping by electron probe micro analysis (EPMA), which recorded C, N, P, Al, Fe, Ca, K, Cl, and Si contents at micrometer scale resolution. We further developed the first laser-ablation isotope ratio mass spectrometry technique in soil science detecting d13C composition on the micro-scale (LA-IRMS) and employed this to trace micro-gradients in undisturbed soil and soil microaggregate samples. We found a pronounced heterogeneity in aggregate structure and composition, which was not reproducible for different microaggregates from the same soil fraction, and which was largely independent from clay content in soil. However, neighborhood analyses revealed close spatial correlations between organic matter debris (C:N app. 100:10) and microbial organic matter (C:N app. 10:1), indicating a spatial relationship between source and consumer. There was no systematic relationship between soil minerals and organic matter, suggesting that well-established macroscale correlations between pedogenic oxides and clay with soil organic matter storage do not apply to soil microaggregates. From first applications of LA-IRMS to soil we found also a high d13C variability of up to 9 per mill along spatial gradients of less than 300 µm, suggesting the appearance of very small hotspots of isotope enrichment and organic matter turnover by metabolic processes. [ABSTRACT FROM AUTHOR]