Grassland Topsoil Research Articles

Vegetation and Precipitation Patterns Define Annual Dynamics of CO2 Efflux from Soil and Its Components

Respiration of soil heterotrophs—mainly of bacteria and fungi—is a substantial part of carbon balance in terrestrial ecosystems, which tie up organic matter decomposition with the rise of atmospheric CO2 concentration. Deep understanding and prediction of seasonal and interannual variation of heterotrophic and autotrophic components of CO2 efflux from soil is limited by the lack of long-term, full-year measurements. To better understand the impact of current climate changes on CO2 emissions from soils in the mixed forest and mowed grassland, we measured CO2 efflux every week for 2 years. Heterotrophic (SOM-derived + leaf litter) and root-associated (root with rhizosphere microorganisms) components were partitioned by the root exclusion method. The total CO2 efflux from soil was averaged 500 g C m−2 yr−1 in the forest and 650 g C m−2 yr−1 in the grassland, with shares of the no-growing cold season (Nov–Mar) of 22% and 14%, respectively. The heterotrophic component of CO2 efflux from the soil averaged 62% in the forest and 28% in the grassland, and it was generally stable across seasons. The redistribution of the annual precipitation amounts as well as their deficit (droughts) reduced soil respiration by 33–81% and heterotrophic respiration by 24–57% during dry periods. This effect was more pronounced in the grassland (with an average decline of 56% compared to 39% in the forest), which is related to lower soil moisture content in the grassland topsoil during dry periods.

Humidity controls soil organic carbon accrual in grassland on the Qinghai–Tibet Plateau

The huge soil organic C (SOC) storage (around 34 Pg in the top 0.7 m) in Qinghai–Tibet Plateau (QTP) grasslands is commonly explained by slow decomposition of litter under cold climate therein, but this view may not be reliable as humidity also affects microbial activity. We sampled the 20 cm topsoil of grasslands along an altitudinal gradient from 1286 m on the western Loess Plateau (LP) to 4200 m above sea level on the northeastern QTP. The light-fraction SOC (LFOC), composition of non-cellulosic neutral carbohydrates, and amino sugars were used as biomarkers to investigate the intensity of microbial action on SOC as a function of climate along this altitudinal gradient. From the lowest-to the highest-humidity site with rising altitude, the root biomass tripled and the SOC content increased approximately sevenfold (from 13.5 g kg−1 to 93.3 g kg−1). The non-cellulosic neutral carbohydrate, microbial biomass C (MBC), and microbial necromass C (MNC) contents increased, but the LFOC content decreased. The contribution of MNC to the SOC and ratios between microbially- and plant-derived sugars in the non-cellulosic carbohydrate pool increased, but the proportion of LFOC in the SOC dropped. Consequently, besides the increased root biomass, the selective preservation of microbial compounds at colder and more humid sites contributed to SOC accruals in grasslands. The higher MBC in cold and humid grasslands perfectly explained the increased selective preservation of microbial derived C at the expense of plant C in higher-relative to lower-altitude areas. Importantly, the above humidity-controlled accumulations of microbial substances and SOC in grasslands were confirmed by results synthesized from published data across the LP and QTP. The higher SOC contents in cold and humid QTP grasslands relative to warm and dry regions were ascribed to the increased accumulation of microbial residues because of the increased humidity in QTP grasslands.

Environment and microbiome drive different microbial traits and functions in the macroscale soil organic carbon cycle.

Soil microbial traits and functions play a central role in soil organic carbon (SOC) dynamics. However, at the macroscale (regional to global) it is still unresolved whether (i) specific environmental attributes (e.g., climate, geology, soil types) or (ii) microbial community composition drive key microbial traits and functions directly. To address this knowledge gap, we used 33 grassland topsoils (0-10 cm) from a geoclimatic gradient in Chile. First, we incubated the soils for 1 week in favorable standardized conditions and quantified a wide range of soil microbial traits and functions such as microbial biomass carbon (MBC), enzyme kinetics, microbial respiration, growth rates as well as carbon use efficiency (CUE). Second, we characterized climatic and physicochemical properties as well as bacterial and fungal community composition of the soils. We then applied regression analysis to investigate how strongly the measured microbial traits and functions were linked with the environmental setting versus microbial community composition. We show that environmental attributes (predominantly the amount of soil organic matter) determined patterns of MBC along the gradient, which in turn explained microbial respiration and growth rates. However, respiration and growth normalized for MBC (i.e., specific respiration and growth) were more linked to microbial community composition than environmental attributes. Notably, both specific respiration and growth followed distinct trends and were related to different parts of the microbial community, which in turn resulted in strong effects on microbial CUE. We conclude that even at the macroscale, CUE is the result of physiologically decoupled aspects of microbial metabolism, which in turn is partially determined by microbial community composition. The environmental setting and microbial community composition affect different microbial traits and functions, and therefore both factors need to be considered in the context of macroscale SOC dynamics.

The Role of Climate, Mineralogy and Stable Aggregates for Soil Organic Carbon Dynamics Along a Geoclimatic Gradient

AbstractOrganic matter accumulation in soil is understood as the result of the dynamics between mineral‐associated (more decomposed, microbial derived) organic matter and free particulate (less decomposed, plant derived) organic matter. However, from regional to global scales, patterns and drivers behind main soil organic carbon (SOC) fractions are not well understood and remain poorly linked to the pedogenetic variation across soil types. Here, we separated SOC associated with silt‐ and clay‐sized particles (S + C), stable aggregates (>63 μm, SA) and particulate organic matter (POM) from a diverse range of grassland topsoils sampled along a geoclimatic gradient. The relative contribution of the two mineral‐associated fractions (S + C & SA) to SOC differed significantly across the gradient, while POM was never the dominant SOC fraction. Stable aggregates (>63 μm) emerged as the major SOC fraction in carbon‐rich soils. The degree of decomposition of carbon in stable aggregates (>63 μm) was consistently between that of the S + C and POM fractions and did not change along the investigated gradient. In contrast, carbon associated with the S + C fraction was less microbially decomposed in carbon‐rich soils than in carbon‐poor soils. The amount of SOC in the S + C fraction was positively correlated to pedogenic oxide contents and texture, whereas the amount of SOC associated with stable aggregates (>63 μm) was positively correlated to pedogenic oxide contents and negatively to temperature. We present a conceptual summary of our findings, which integrates the role of stable aggregates (>63 μm) with other major SOC fractions and illustrates their changing importance across (soil‐)environmental gradients.

Tibetan Plateau grasslands might increase sequestration of microbial necromass carbon under future warming

Microbial necromass carbon (MNC) can reflect soil carbon (C) sequestration capacity. However, changes in the reserves of MNC in response to warming in alpine grasslands across the Tibetan Plateau are currently unclear. Based on large-scale sampling and published observations, we divided eco-clusters based on dominant phylotypes, calculated their relative abundance, and found that their averaged importance to MNC was higher than most other environmental variables. With a deep learning model based on stacked autoencoder, we proved that using eco-cluster relative abundance as the input variable of the model can accurately predict the overall distribution of MNC under current and warming conditions. It implied that warming could lead to an overall increase in the MNC in grassland topsoil across the Tibetan Plateau, with an average increase of 7.49 mg/g, a 68.3% increase. Collectively, this study concludes that alpine grassland has the tendency to increase soil C sequestration capacity on the Tibetan Plateau under future warming.

Divergent controlling factors of freeze–thaw-induced changes in dissolved organic carbon and microbial biomass carbon between topsoil and subsoil of cold alpine grasslands

Freeze-thaw (FT) processes in cold regions significantly influence the mineralization and sequestration of soil organic carbon (SOC), particularly in its active pools, such as dissolved organic carbon (DOC) and microbial biomass carbon (MBC). However, manipulation experiments often amplify the effects of natural FT processes and overlook soils below 20 cm depth. To investigate carbon responses to seasonal FT events across soil depths (0–80 cm) and identify controlling factors, field sampling was conducted before freezing and after thawing in alpine grassland soils on the Qinghai-Tibet Plateau. Results are as follows: 1) After soil thawing, the entire soil profile showed insignificant changes in SOC (−4.46%) and MBC (3.21%), but DOC (45.27%) and DOC/SOC (160.08%) increased significantly, and strong associations were observed in DOC changes between adjacent soil layers. 2) FT indicators (number and temperature amplitude of daily FT cycles, frozen temperature, and frozen days) influenced SOC changes differently in the topsoil (0–10 cm) and subsoil (20–30 cm). Contrary to the impact of frozen temperature, other FT indicators largely facilitated the production of DOC0–10 and DOC20–30. The effects of FT indicators on MBC were nonlinear. 3) Structural equation models indicated that average soil water governed FT-induced changes in both DOC0–10 and DOC20–30. The varying effects of porosity and pH from topsoil to subsoil, along with the weakening relationship between changes in DOC and MBC, favored a stronger increase in DOC20–30. Increases in soil water and DOC after thawing stimulated the increment of MBC0–10, while porosity (positively) and sand content (negatively) regulated the MBC20–30 change. It indicates that, influenced by soil structure and biogeochemical environment, DOC and MBC in grassland soils are more sensitive to natural FT processes than SOC. Nevertheless, changes in FT patterns due to long-term climate change may cumulatively affect SOC.

Rapid loss of organic carbon and soil structure in mountainous grassland topsoils induced by simulated climate change

Mountainous grassland soils are considered one of the most unique biological hotspots, rich in organic carbon (OC). At the same time, they are exposed to great threats, as climate warming is more pronounced in mountainous regions than in lowland areas. In this study, we assessed the effect of simulated warming (+1K, +2K, and + 3 K) on OC stocks and soil structure in grassland soils of the Northern Limestone Alps in Germany by translocating plant-soil mesocosms from high- (1260 m a.s.l., Rendzic Phaeozem) and mid- (860 m a. s. l., Haplic Cambisol) to low-elevation (600 m a.s.l). Plant-soil mesocosms were exposed to both extensive and intensive grassland management practices. Four years after translocation, we observed a rapid decrease of topsoil SOC stocks under intensive (−1.0 t C ha yr−1) and extensive management (-2.2 t C ha yr−1), under the highest temperature increase. Intensive management with about 1 t C ha−1 yr−1 higher manure C return than extensive management (1.6 vs. 0.8 t C ha−1 yr−1 intensive and extensive, respectively) may explain the difference in SOC losses between different management treatments. Under both management practices, the loss of SOC was mainly associated with a decrease of large macroaggregates, at both management practices. In addition, different aggregate specific OC loss rates resulted in an altered distribution of OC among the aggregate size classes. Our study provides evidence that simulated climate change induced a rapid and substantial decline of SOC in mountainous, OC-rich grassland soils, which may be attributed to decreased physical OC protection within large macroaggregates. Optimized grassland management in form of increased application of organic fertilizers could only partially offset the SOC loss by improved formation of small macroaggregates.

Trace and rare earth elements as the source and transport indicators of different topsoil end-members in the desert peripheral regions of China

Solis in grassland regions, located downwind of and adjacent to deserts, display grain-size sorting and weathering features that cause the mineral and chemical elements of their particulate matter to have some characteristics different from those in the source area. This makes it an issue to be explored whether source tracers previously obtained from deserts can be well applied in grassland areas. We collected 40 topsoil samples in the western Inner Mongolia grassland near the Badain Jaran Desert and the Mongolian Gobi and then studied the three grain-size end-members of their <300 μm components. End-member 1 is matched with samples' <30 μm grain-size fractions, end-member 2 and end-member 3 are matched with some samples' 30–300 μm grain-size fractions. By obtaining the trace element and rare earth element characteristics of the two different grain-size fractions, that of the three end- members are obtained. Therefore, the impact of sorting and weathering on different element ratio proxies is discussed from the perspective of end-members. Then element ratio proxies that are more applicable to grasslands on the periphery of deserts are established, where Sm/Gd, Tb/Dy, Nb/Ta, Er/Tm, La/Ce, Sc/V, Yb/Lu, and Ce/Ce* are least affected by sorting and weathering, and they effectively distinguish and identify the sources of each grain-size end-member. Using these proxies, it can be inferred that end-member 1 (mode grain size is 5 μm) in the grassland topsoil comes from a vast area of deserts and Gobi in Mongolian Plateau. End-member 2 (70 μm) and end-member 3 (120 μm) have stronger source affinity with the Badain Jaran Desert and Altay Mountains in the western Mongolian Plateau; and end-member 2 also contains a small amount of material from the local Yinshan Mountains. This result can be further applied to the sediment sequences of the grassland region in the future, which will help to explain the sources of their end-members and the environmental change processes at different time scales implied by end-members.

Can the soil seed bank of Rumex obtusifolius in productive grasslands be explained by management and soil properties?

Rumex obtusifolius is a problematic weed in temperate grasslands worldwide as it decreases yield and nutritional value of forage. Because the species can recruit from the seed bank, we determined the effect of management and soil properties on the soil seed bank of R. obtusifolius in intensively managed, permanent grasslands in Switzerland (CH), Slovenia (SI), and United Kingdom (UK). Following a paired case-control design, soil cores were taken from the topsoil of grassland with a high density of R. obtusifolius plants (cases) and from nearby parcels with very low R. obtusifolius density (controls). Data on grassland management, soil nutrients, pH, soil texture, and density of R. obtusifolius plants were also collected. Seeds in the soil were germinated under optimal conditions in a glasshouse. The number of germinated seeds of R. obtusifolius in case parcels was 866 ±152 m-2 (CH, mean ±SE), 628 ±183 m-2 (SI), and 752 ±183 m-2 (UK), with no significant difference among countries. Densities in individual case parcels ranged from 0 up to approximately 3000 seeds m-2 (each country). Control parcels had significantly fewer seeds, with a mean of 51 ±18, 75 ±52, and 98 ±52 seeds m-2 in CH, SI, and UK, respectively, and a range between 0 and up to 1000 seeds m-2. Across countries, variables explaining variation in the soil seed bank of R. obtusifolius in case parcels were soil pH (negative relation), silt content (negative), land-use intensity (negative), and aboveground R. obtusifolius plant density (positive). Because a large soil seed bank can sustain grassland infestation with R. obtusifolius, management strategies to control the species should target the reduction in the density of mature plants, prevention of the species' seed production and dispersal, as well as the regulation of the soil pH to a range optimal for forage production.

Mineral type and land-use intensity control composition and functions of microorganisms colonizing pristine minerals in grassland soils

Mineral surfaces in soil are an important interface for organic matter (OM) and nutrient cycling, with associated microorganisms contributing to the formation and turnover of mineral-associated OM (MAOM). However, the relevance of intrinsic (mineral type) versus extrinsic (land-use intensity) factors on the co-development of MAOM and microorganisms under natural conditions remains poorly understood. Mineral containers filled with mixtures of quartz-sand and pristine secondary minerals (goethite or illite) were exposed to 50 grassland topsoils of the Schwäbische Alb (Germany) along a land-use intensity gradient for five years. Mineral samples and soils were analyzed for organic carbon (OC) and nutrients (N and P), abundance and composition of major microbial groups based on phospholipid fatty acid profiles, as well as enzyme activities (β-glucosidase, β-xylosidase, N-acetylglucosaminidase, and acid phosphatase). Microorganisms colonized both mineral samples to the same extent, with goethite samples exhibiting greater MAOM accumulation and higher enzyme activities than illite samples. Both mineral samples differed from the overlying soils with greater relative abundances of fungi and Gram-negative bacteria and greater microbial acquisition of nutrients (N and P) relative to C as indicated by the stoichiometry of enzyme activities. Increasing land-use intensity was associated with decreasing C:N ratios and microbial abundances for goethite samples and increasing β-glucosidase activity for illite samples while the proportion of fungi was reduced in both mineral samples. We conclude that in the studied temperate grasslands the association of OM and microorganisms with secondary minerals is driven more by mineral type and reactivity than by differences in land-use intensity. The different minerals apparently formed distinct microhabitats with unique characteristics that differed in MAOM accumulation and microbial access to OC and nutrients, thus affecting microbial colonization and functionality.

Structure turnover times of grassland soils under different moisture regimes

Soil structure is a dynamic property of soils, which refers to temporal changes in the spatial arrangement of pores, organic matter, and minerals. Its turnover, i.e. the irreversible redistribution of soil constituents, determines essential soil functions including carbon storage. Structure turnover times and its response to biotic versus abiotic drivers have never been quantified directly under natural conditions. We used a novel combination of structure labelling with inert garnet particles and X-ray Computed Tomography to determine structure turnover times of two grassland topsoils with either access or exclusion of roots and fauna > 30 µm. Both, abiotic and biotic factors developed soil structure at a site-specific rate towards a dynamic equilibrium, at which bulk properties remain constant because creation and destruction of structural properties are in balance. Its turnover, however, was mainly determined by macrofaunal activity which varied with environmental conditions. Under dry conditions less favorable for bioturbation, the extrapolated structure turnover time was 33 ± 3 years, while being 16 ± 1 years under moist conditions. Previous studies on organic matter turnover determined in the vicinity of the experimental site showed similar turnover times for particulate organic matter. The similar turnover times suggest that the accessibility of particulate organic matter to decomposers is closely linked to structure turnover, thus highlighting the intimate nexus between structure evolution and carbon persistence in soil.

Relative importance of influencing factor‐driven soil enzyme activity during the early plantation stage in the hilly‐gully Loess regions

AbstractWidespread and rapid land‐use changes have changed the ecosystem structure and threatened soil functions. Soil enzymes are the important driving force and sensitive indicators for soil ecological function. However, the effect of afforestation on soil enzyme activity and the relative importance of the influencing factors (plants, soil, structure, and nutrition) are still inconclusive, especially in the early stage of afforestation. Here, the enzymatic activities and influencing factors of five types of plantations (18 years old) and natural grassland topsoil (0–20 cm) in the hilly‐gully region of the Loess Plateau were studied. The results showed that the soil enzyme activity of the plantations was significantly lower than that of the natural grassland, and the activities of invertase, urease, and alkaline phosphatase were decreased by 32%, 26%, and 10% on average, respectively. Partial correlation analysis and multiple regression analysis showed that the influence of soil and nutrient factors on invertase and alkaline phosphatase was more important than that of plant and structural factors, while urease was more influenced by plant and nutrition factors. Our results suggest that soil and nutrient characteristics were important factors affecting soil enzyme activity in plantations, but the disturbance of vegetation may be the main reason for the decrease in enzyme activity in the early stage of restoration.

The potential to increase grassland soil C stocks by extending reseeding intervals is dependent on soil texture and depth

Grasslands account for ∼30% of global terrestrial carbon (C), of which most is stored in soils and provide important ecosystem services including livestock and forage production. Reseeding of temporary grasslands on a 5-year cycle is a common management practice to rejuvenate sward productivity and reduce soil compaction, but is physically disruptive and may reduce soil organic carbon (SOC) stocks. However, research to date is limited, which impacts on the ability to optimise grassland management for climate change mitigation. To determine whether extending the time interval up to 20 years between grassland reseeding can increase stable SOC stocks, a soil survey was conducted across three UK grassland chrono-sequences comprising 24 fields on contrasting soil types. We found that grassland SOC stocks (39.8–114.8 Mg C ha−1) were higher than co-located fields in arable rotations (29.3–83.2 Mg C ha−1) and the relationship with grassland age followed a curvilinear relationship with rapid SOC stock accumulation in the year following reseeding (2.69–18.3 Mg C ha−1 yr−1) followed by progressively slower SOC accumulation up to 20 years. Contrary to expectation, all grasslands had similar soil bulk densities and sward composition questioning the need for traditional 5-year reseeding cycles. Fractionation of soils into stable mineral associated fractions revealed that coarse textured grassland topsoils (0–15 cm) were near-saturated in C irrespective of grassland age whilst loam soils reached saturation ∼10 years after reseeding. Fine-textured topsoils and subsoils (15–30 cm) of all textures were under saturated and thus appear to hold the most potential to accrue additional stable C. However, the lack of a relationship between C saturation deficit and grassland age in subsoils suggests that more innovative management to promote SOC redistribution to depth, such as a switch to diverse leys or full inversion tillage may be required to maximise subsoil SOC stocks. Taken together our findings suggest that extending the time between grassland reseeding could temporarily increase SOC stocks without compromising sward composition or soil structure. However, detailed monitoring of the trade-offs with grassland productivity are required. Fine textured soils and subsoils (15–30 cm) have the greatest potential to accrue additional stable C due to under saturation of fine mineral pools.

Factors controlling shallow subsurface dissolved reactive phosphorus concentration and loss kinetics from poorly drained saturated grassland soils.

Shallow subsurface pathways dominate dissolved reactive phosphorus (DRP) losses in grassland soils that are: poorly drained, shallow, or have a perched water table in wetter months causing saturation-excess runoff. Saturated conditions can lead to anoxia, which can accelerate phosphorus (P) loss. Two scales of investigation were utilized in this study. First, at the field scale, soil cores were extracted to 2.5m, subdivided and samples extracted using water extractable P (WEP) and sodium-bicarbonate-dithionite extractable P (NaBD-P). Second, at the laboratory scale, detailed incubation studies using field-moist grassland topsoils from sites in Ireland and New Zealand examined the kinetics of WEP under anoxic (WEPanox ) and oxic (WEPox ) conditions with imposed temperature and soil P fertilizer input treatments. Results from soil-core samples showed that redox-sensitive NaBD-P concentrations were depleted where artificial drainage lines were installed (100cm deep), but WEP concentrations available to shallow flow were enriched in topsoil. The laboratory scale incubation experiment investigated the influence of temperature (3vs. 18 °C), anoxia (designed to simulate saturation following a rainfall event), and superphosphate fertilizer (10 to 60kg P ha-1 yr-1 ) on WEP concentrations over 24h in three grassland topsoils (clay, silt, and sandy loam textures). Concentrations increased with fertilizer rate, temperature, and-in two soils-anoxic conditions. This was commensurate with nitrate (NO3 - ) depletion and the reductive dissolution of iron and manganese. The release of P during anoxia was complete within 24h. The results highlighted late winter to spring as the riskiest period for topsoil P losses in shallow subsurface flow due to wet soil conditions, increasing temperatures, and low soil NO3 - concentrations. This knowledge highlights the necessity to consider and refine tests used to assess topsoil P loss risk, where in the landscape P losses are likely, and what strategies can be used to mitigate losses.

An improved estimate of soil carbon pool and carbon fluxes in the Qinghai-Tibetan grasslands using data assimilation with an ecosystem biogeochemical model

The accurate estimation of soil carbon (C) pool and fluxes is a prerequisite to better understand the terrestrial C feedback to climate change. However, recent studies showed considerable uncertainties in soil C estimates. To provide a reliable C estimate in the grasslands of the Qinghai-Tibet Plateau (QTP), we calibrated key parameters in a process-based ecosystem model (the CENTURY model) through data assimilation based on 570 soil samples and 21 sites of eddy covariance measurements. Two assimilating strategies (Opt1 – assimilating C pool observations; Opt2 –assimilating both C pool and C flux) were examined. Compared to default parameterization, our results showed both Opt1 and Opt2 improved the soil organic carbon density (SOCD) estimation, with R2 increasing from 0.59 to 0.75 and 0.73, respectively. Opt2 was superior to Opt1 in constraint of parameters dominating aboveground processes and yield a better estimation of net ecosystem production (NEP). Based on different parameterization, the spatial variability of SOCD and NEP across the QTP grassland were generated. Both Opt1 and Opt2 ameliorated the overestimation of SOCD by the default model, estimating a total soil C of 6.63 Pg and 6.48 Pg C for the topsoil (0–30 cm) of the QTP grasslands, respectively. Opt2 showed lower uncertainties in the NEP estimation and predicted a net sink of 14.33 Tg C annually. Compared with existing datasets, our study provided a more reliable estimation of carbon storage and fluxes in the QTP grassland with the calibrated ecosystem model. The results highlight that data assimilation with multiple observational data sets is promising to constrain process-based ecosystem models and increase the robustness of model predictions for terrestrial C cycle feedback to future climate change.

Soil organic matter changes under experimental pedoclimatic modifications in mountain grasslands of the French Alps

Mountain grasslands contain large stocks of soil organic carbon (SOC), of which a good part is in labile particulate form. This labile SOC may be protected by cold climate that limits microbial activity. Strong climate change in mountain regions threatens to destabilize these SOC stocks. However, so far the climate response of SOC stocks in mountain grasslands remains highly uncertain, under either warming or cooling conditions. To overcome this knowledge gap, we studied the effect of pedoclimatic regime changes on topsoil (0–10 cm) SOC in two complementary experiments: 3 °C of warming or cooling by reciprocal transplanting to an alpine (2450 m a.s.l.) and a subalpine (1950 m a.s.l.) grassland and 1 °C of warming by open-top chambers in the same grasslands.Topsoil SOC stocks were higher at the alpine site than at the subalpine site, and the biogeochemical signature of the soil organic matter (SOM) also differed between the two study sites. SOM was O-enriched, H-depleted, and more thermally stable at the warmer subalpine site. After three years, abrupt warming by transplanting tended to decrease topsoil SOC content. The remaining SOC was characterized by a more thermostable signature. This result suggests the preferential depletion of labile SOC upon experimental topsoil warming. Cooling did not modify overall SOC content but uphill transplanted topsoils showed a more thermolabile biogeochemical signature. In contrast, open-top chamber warming of alpine and subalpine topsoils caused limited changes to SOC stocks and SOM biogeochemical signature, possibly because the induced pedoclimatic change was more limited and more gradual compared to the warming by transplantating which reduced the annual snow cover period by around 60 days and increased cumulative degree days by a factor of ten as compared to the OTC-induced warming. Gradual temperature changes may take longer to become effective than a shock transplant treatment. We conclude that SOC in mountain grassland topsoils can be highly reactive to climate shocks.

Genomic evidence for two pathways of formaldehyde oxidation and denitrification capabilities of the species Paracoccus methylovorus sp. nov.

Three strains (H4-D09T, S2-D11 and S9-F39) of a member of the genus Paracoccus attributed to a novel species were isolated from topsoil of temperate grasslands. The genome sequence of the type strain H4-D09T exhibited a complete set of genes required for denitrification as well as methylotrophy. The genome of H4-D09T included genes for two alternative pathways of formaldehyde oxidation. Besides the genes for the canonical glutathione (GSH)-dependent formaldehyde oxidation pathway, all genes for the tetrahydrofolate-formaldehyde oxidation pathway were identified. The strain has the potential to utilize methanol and/or methylamine as a single carbon source as evidenced by the presence of methanol dehydrogenase (mxaFI) and methylamine dehydrogenase (mau) genes. Apart from dissimilatory denitrification genes (narA, nirS, norBC and nosZ), genes for assimilatory nitrate (nasA) and nitrite reductases (nirBD) were also identified. The results of phylogenetic analysis based on 16S rRNA genes coupled with riboprinting revealed that all three strains represented the same species of genus Paracoccus. Core genome phylogeny of the type strain H4-D09T indicated that Paracoccus thiocyanatus and Paracoccus denitrificans are the closest phylogenetic neighbours. The average nucleotide index (ANI) and digital DNA-DNA hybridization (dDDH) with the closest phylogenetic neighbours revealed genetic differences at the species level, which were further substantiated by differences in several physiological characteristics. The major respiratory quinone is Q-10, and the predominant cellular fatty acids are C18 : 1ω7c, C19 : 0cyclo ω7c, and C16 : 0, which correspond to those detected in other members of the genus. The polar lipid profile consists of a diphosphatidylglycerol (DPG), phosphatidylethanolamine (PE), phosphatidylglycerol (PG), phosphatidylcholine (PC), aminolipid (AL), glycolipid (GL) and an unidentified lipid (L).On the basis of our results, we concluded that the investigated isolates represent a novel species of the genus Paracoccus, for which the name Paracoccus methylovorus sp. nov. (type strain H4-D09T=LMG 31941T= DSM 111585T) is proposed.

Enzyme kinetics inform about mechanistic changes in tea litter decomposition across gradients in land-use intensity in Central German grasslands.

Grassland ecosystems provide important ecosystem services such as nutrient cycling and primary production that are affected by land-use intensity. To assess the effects of land-use intensity, operational and sensitive ecological indicators that integrate effects of grassland management on ecosystem processes such as organic matter turnover are needed. Here, we investigated the suitability of measuring the mass loss of standardized tea litter together with extracellular enzyme kinetics as a proxy of litter decomposition in the topsoil of grasslands along a well-defined land-use intensity gradient (fertilization, mowing, grazing) in Central Germany. Tea bags containing either green tea (high-quality litter) or rooibos tea (low-quality litter) were buried in 5 cm soil depth. Litter mass loss was measured after three (early-stage decomposition) and 12 months (mid-stage decomposition). Based on the fluorescence measurement of the reaction product 4-methylumbelliferone, Michaelis-Menten enzyme kinetics (Vmax: potential maximum rate of activity; Km: substrate affinity) of five hydrolases involved in the carbon (C)-, nitrogen (N)- and phosphorus (P)-cycle (β-glucosidase (BG), cellobiohydrolase (CBH), cellotriohydrolase (CTH), 1,4-β-N-acetylglucosaminidase (NAG), and phosphatase (PH)) were determined in tea litter bags and in the surrounding soil. The land-use intensity index (LUI), summarizing fertilization, mowing, grazing, and in particular the frequency of mowing were identified as important drivers of early-stage tea litter decomposition. Mid-stage decomposition was influenced by grazing intensity. The higher the potential activity of all measured C-, N- and P-targeting enzymes, the higher was the decomposition of both tea litters in the early-phase. During mid-stage decomposition, individual enzyme parameters (Vmax of CTH and PH, Km of CBH) became more important. The tea bag method proved to be a suitable indicator which allows an easy and cost-effective assessment of land-use intensity effects on decay processes in manged grasslands. In combination with enzyme kinetics it is an appealing approach to identify mechanisms driving litter break down.

Overgrazing, not haying, decreases grassland topsoil organic carbon by decreasing plant species richness along an aridity gradient in Northern China

Climate and land-use change are some of the most profound threats to the biodiversity and functioning of the Earth’s ecosystems. However, potential synergistic effects of these drivers through biodiversity change on ecosystem functioning remain unclear. Here we examined how aridity and land-use (overgrazing and haying) affect above-ground biomass and soil organic carbon (SOC) through changes in plant species richness across 701 grassland sites in China. We found that aridity and grazing reduced SOC through decreasing plant species richness, but did not significantly affect above-ground biomass. Notably, we observed strong negative synergistic effects of aridity and grazing, suggesting that soil carbon content was particularly threatened by grazing in arid environments. By contrast, haying reduced above-ground biomass and had no significant effect on SOC, although it increased plant species richness. Plant species richness had greater positive effects on SOC than on above-ground biomass, and its effects became stronger in more arid regions. Together, the results demonstrate that aridity and overgrazing threaten soil carbon content via their detrimental effects on plant diversity, and that detrimental overgrazing effects are particularly strong under arid conditions. However, the study also indicates that certain management types like haying or less intensive grazing can maintain or enhance plant diversity and soil carbon content, and that the beneficial effects of plant diversity are particularly important in arid environments.

Soil and land use factors control organic carbon status and accumulation in agricultural soils of Lower Austria

Policies for restoring soil health and mitigating climate change require information on soil organic carbon (SOC) stocks and their spatial and temporal variation, and related sequestration potentials. Using the province of Lower Austria as environmentally diverse model region, we present a detailed analysis of SOC stocks, saturation potentials and deficits along with SOC monitoring data for the past three decades.Using the provincés soil database (sampling 1991) we estimated the saturation potential for mineral-protected C (Csat) by boundary line regression to the particle fraction <20 µm (f<20µm). Compared to published work, parameterization by Lower Austrian data yielded considerably larger Csat values which agree well with independent maximum organic C load estimates based on specific surface area. Csat is particularly small in topsoils of regions with non-calcareous igneous rock, but large on fine-textured quaternary and tertiary sediments, and weathering residuals of carbonate rock. During the past three decades, the medians of SOC in grassland topsoils (0–20 cm) increased significantly (p < 0.05) by 29.7% from 39.4 to 51.1 g kg−1, corresponding to annual accumulation rates of 0.87 Mg C ha−1. SOC in cultivated soils increased by 17.1% from 12.7 to 14.8 g kg−1 (0.20 Mg C ha−1 y−1). Because of the large initial C gap, the observed SOC accumulation is not related to Cdef but likely reflects improved soil management.