Edaphic rather than climatic controls over 13 C enrichment between soil and vegetation in alpine grasslands on the Tibetan Plateau

Effects of simulated litter inputs on plant-microbe carbon pool trade-offs in degraded alpine meadows

Introduction

Litter, as a major carbon source in alpine meadow ecosystems, seriously affect the variation of plant-microbe carbon pools in alpine meadows. In order to study the response of plant-microbial biomass carbon pool trade-offs in degraded alpine meadow to litter inputs.

Methods

We investigated the effects of different levels of litter inputs on the carbon pools of alpine meadows plant aboveground communities, the carbon pools of the root, and the total carbon pools of the plant communities and the soil microbial biomass carbon pools, and clarified the variable factors that affect the balance of the plant-microbial biomass carbon pools and the process of influencing the trade-offs.

Result

(1) Litter inputs had a positive effect on plant carbon pools, and the aboveground community carbon pools, root carbon pools, total plant community carbon pools and soil microbial biomass carbon pools of alpine meadows were all maximized in the T3 treatment. (2) The trade-off analyses showed that the trade-off relationships of ungrazed alpine meadows TPCPMBCP in the following order under different levels of litter treatments: T1(0.0414) > T2 (0.0269) > T0 (0.0086) > T3 (0.0012), the trade-off relationship of TPCP-MBCP in lightly grazed alpine meadows was in the order of T2 (0.0494) > T3 (0.0140) > T0 (0.0097) > T1 (0.002), and the tradeoff relationship of TPCP-MBCP in moderately grazed alpine meadows was in the order of T3 (0.0383) > T1 (0.0307) > T2 (0.0196) > T0 (0.0005). (3) Propensity analysis showed that the TPCP-MBCP trade-offs tended to favor MBCP under ungrazed, lightly grazed and moderately grazed meadows under the T1 treatment. (4) Structural equation modeling showed that RB and APC were positively correlation, RCP was significantly negatively correlated with the TPCPMBCP trade-off in lightly grazed grassland (P<0.05), and MBC was significantly positively correlated with the TPCP-MBCP trade-off in moderately grazed grassland (P<0.05).

Discussion

There was no uniform pattern in TPCP-MBCP trade-off and propensities in ungrazed, lightly grazed, and moderately grazed alpine meadows under different levels of litter inputs. This study can help to optimize the grazing management measures, predict the changes of carbon pools in alpine meadows and clarify the transfer and storage of carbon pools between plants and microorganisms, so as to provide a theoretical basis for the study of carbon pools in degraded alpine meadows.

Modeling the carbon dynamics of ecosystem in a typical permafrost area

Climate change poses mounting threats to fragile alpine ecosystem worldwide. Quantifying changes in carbon stocks in response to the shifting climate was important for developing climate change mitigation and adaptation strategies. This study utilized a process-based land model (Community Land Model 5.0) to analyze spatiotemporal variations in vegetation carbon stock (VCS) and soil organic carbon stock (SOCS) across a typical permafrost area - Qinghai Province, China, from 2000 to 2018. Multiple potential factors influencing carbon stocks dynamics were analyzed, including climate, vegetation, soil hydrothermal status, and soil properties. The results indicated that provincial vegetation carbon storage was 0.22 PgC (0.32 kg/m2) and soil organic carbon pool was 9.12 PgC (13.03 kg/m2). VCS showed a mild increase while SOCS exhibited fluctuating uptrends during this period. Higher carbon stocks were observed in forest (21.74 kg/m2) and alpine meadow (18.08 kg/m2) compared to alpine steppes (9.63 kg/m2). Over 90 % of the carbon was stored in the 0-30 cm topsoil layer. The contribution rates of soil carbon in the 30-60 cm and 60-100 cm soil layers were significantly small, despite increasing stocks across all depths. Solar radiation, temperature, and NDVI emerged as primary influential factors for overall carbon stocks, exhibiting noticeable spatial variability. For SOCS at different depths, the normalized differential vegetation index (NDVI) was the foremost predictor of landscape-level carbon distributions, which explained 52.8 % of SOCS variability in shallow layers (0-30 cm) but dropped to just 12.97 % at the depth of 30-60 cm. However, the dominance of NDVI diminished along the soil depth gradients, superseded by radiation and precipitation. Additionally, with an increase in soil depth, the influence of inherent soil properties also increased. This simulation provided crucial insights for landscape-scale carbon responses to climate change, and offered valuable reference for other climate change-sensitive areas in terms of ecosystem carbon management.

An overlooked soil carbon pool in vegetated coastal ecosystems: National-scale assessment of soil organic carbon stocks in coastal shelter forests of China

Protection and restoration of vegetated coastal ecosystems provide opportunities to mitigate climate change. Coastal shelter forests as one of vegetated coastal ecosystems play vital role on sandy coasts protection, but less attention is paid on their soil organic carbon (OC) sequestration potential. Here, we provide the first national-scale assessment of the soil OC stocks, fractions, sources and accumulation rates from 48 sites of shelter forests and 74 sites of sandy beaches across 22° of latitude in China. We find that, compared with sandy beaches, shelter forest plantation achieves an average soil desalination rate of 92.0 % and reduces the soil pH by 1.3 units. The improved soil quality can facilitate OC sequestration leading to an increase of soil OC stock of 11.8 (0.60-64.2) MgC ha^-1 in shelter forests. Particulate OC (POC) is a dominant OC fraction in both sandy beaches and shelter forests, but most sites are >80 % in shelter forests. The low δ¹³C values and higher C:N ratios, which are more regulated by climate and tree species, together with high POC proportions suggest a substantial contribution of plant-derived OC. Bayesian mixing model indicates that 71.8 (33.5-91.6)% of the soil OC is derived from local plant biomass. We estimate that soil OC stocks in Chinese shelter forests are 20.5 (7.44-79.7) MgC ha^-1 and 4.53 ± 0.71 TgC in the top meter, with an accumulation rate of 45.0 (6.90 to 194.1) gC m^-2 year^-1 and 99.5 ± 44.9 GgC year^-1. According to coastal shelter forest afforestation plan, additional 1.72 ± 0.27 TgC with a rate of 37.9 ± 17.1 GgC year^-1 can be sequestrated in the future. Our findings suggest that construction of coastal shelter forests can be an effective solution to sequester more soil carbon in coastal ecosystems.

Leaf functional traits have more contributions than climate to the variations of leaf stable carbon isotope of different plant functional types on the eastern Qinghai-Tibetan Plateau

Elucidating the mechanisms that control the leaf stable carbon isotope values (δ¹³C_leaf) is the prerequisite for the widespread application of δ¹³C_leaf. However, the competing effects of physiological and environmental factors on δ¹³C_leaf variations of the different plant functional types (PFTs) have not been disentangled, and the corresponding mechanisms remain unclear. Based on large-scale δ¹³C_leaf measurements on the eastern Qinghai-Tibetan Plateau, the relative contributions and regulatory pathways of leaf functional traits (LFTs) and climatic factors to δ¹³C_leaf variations of the different PFTs were investigated. We found that δ¹³C_leaf of the different PFTs was correlated with annual mean precipitation negatively, but not a simple linear relationship with annual mean temperature and varied by PFTs. Leaf nitrogen content per unit area and leaf mass per area (correlated with δ¹³C_leaf positively) had more substantial effects on the δ¹³C_leaf variations of the different PFTs than other LFTs. The relative contributions of LFTs to the δ¹³C_leaf variations were greater than that of climatic factors, and the direct and indirect effects of climatic factors on δ¹³C_leaf variations varied by PFTs. Our findings provide new insights into understanding key drivers of δ¹³C_leaf variations at the PFT level on a regional scale.

Altitude patterns of seed C, N, and P concentrations and their stoichiometry in an alpine meadow on the eastern Tibetan Plateau

Understanding the altitudinal patterns of plant stoichiometry in seeds is critical for characterizing important germination and dormancy strategies, soil seed bank composition, seed predation probability, efficiency of seed dispersal and seedling performance, and to predict how biodiversity might be influenced by climate change. However, our understanding of the altitudinal patterns of seed stoichiometry is extremely limited. In this study, we measured the concentrations of carbon (C), nitrogen (N) and phosphorus (P) in the seeds of 253 herbaceous species along an altitudinal transect (2,000–4,200 m) on the eastern Tibetan Plateau, China, and further to characterize seed C:N:P stoichiometry. The geometric means of C, N, and P concentrations were 569.75 mg/g, 34.76 mg/g, and 5.03 mg/g, respectively. The C:N, C:P, and N:P ratios were 16.39, 113.31, and 6.91, respectively. The seed C, N, and P concentrations and C:N:P ratios varied widely among major plant groups and showed significant altitudinal trends. In general, C, N, and P concentrations increased, whereas seed C:N:P ratios decreased with elevation. These results inform our understanding of the altitudinal patterns of seed stoichiometry and how to model ecosystem nutrient cycling.

Elevational shifts in foliar-soil δ15 N in the Hengduan Mountains and different potential mechanisms

The natural abundance of stable nitrogen isotopes (δ¹⁵ N) provides insights into the N dynamics of terrestrial ecosystems, the determination of which is considered an effective approach for gaining a better understanding ecosystem N cycling. However, there is currently little information available regarding the patterns and mechanisms underlying the variation in foliar-soil δ¹⁵ N among mountain ecosystems. In this study, we examined the determinants of foliar-soil δ¹⁵ N in association with N transportation rates along an elevational gradient in the Hengduan Mountains. Despite the relatively high levels of available N produced from high N fixation and mineralization, we detected the lowest levels of foliar δ¹⁵ N at 3500 m a.s.l., reflecting the stronger vegetation N limitation at medium high elevations. The enhanced vegetation N limitation was driven by the combined effects of higher microbial immobilization and inherent plant dynamic (the shifts of δ¹⁵ N in vegetation preference, including vegetation community) with changing climate along the elevational gradient. Unexpectedly, we established that soil δ¹⁵ N was characterized by an undulating rise and uncoupled correlation with foliar δ¹⁵ N with increasing elevation, thereby indicating that litter input might not be a prominent driver of soil δ¹⁵ N. Conversely, soil nitrification and denitrification were found to make a more pronounced contribution to the pattern of soil δ¹⁵ N along the elevational gradient. Collectively, our results serve to highlight the importance of microbial immobilization in soil N dynamics and provide novel insights that will contribute to enhancing our understanding of N cycling as indicated by foliar-soil δ¹⁵ N along elevational gradients.

Spatial distribution of soil δ13C in the central Brazilian savanna

Stable carbon isotope ratios (δ¹³C) of soil record information regarding C₃ and C₄ plants at the landscape scale that can be used to document vegetation distribution patterns. The Central Brazilian savanna (locally called the Cerrado) has a substantial potential to develop studies of patterns of dynamics and distribution of soil δ¹³C, due to its environmental diversity. The purpose of this work was to develop a spatial model of soil δ¹³C (soil δ¹³C isoscape) to the Cerrado, based on multiple linear regression analysis, and compare the results with the existing model to obtain greater detail of the soil δ¹³C distribution. The model used 219 soil samples (0-20 cm depth) and a set of climatic, pedological, topographic, and vegetation correlations. The soil δ¹³C isoscape model presented amplitude between -29‰ and -13‰, with the highest estimated values in the southern and the lowest values in the northern of the Cerrado. Results indicate that soil δ¹³C, by reflecting the relative contribution of C₃ and C₄ species to plant community productivity, served as a proxy indicator of the vegetation history at the landscape scale for the Central Brazilian savanna. Despite the large sampling effort, there are still regions with some gaps that the model could not estimate. However, the soil δ¹³C isoscape model filled most the existing gaps and provided greater detail of some unique local aspects of the Cerrado.

Carbon isotopic measurements from coastal zone protected forests in northern China: Soil carbon decomposition assessment and its influencing factors

Panting protected forests to increase soil carbon sequestration is an effective means of reducing carbon emissions. Soil organic carbon (SOC) decomposition is one of the main indicators of soil carbon sequestration. However, SOC decomposition and its influencing factors in protected forests have not been fully characterized, especially in coastal zones. In this paper, coastal zone protected forest stands composed of Quercus acutissima Carruth (QAC), Pinus thunbergii Parl (PTP) and mixed PTP and QAC (MF) were selected as the research objects. The trends of the SOC decomposition rate were characterized by the beta (β) value, and the influencing factors were further explored with structural equation models. The results were as follows: The SOC content decreased from leaf to litter and then to the soil profile at all sites, while the δ¹³C value increased. The β value ranged from -3.12 to -5.76, with an average of -3.81. The β value was positively correlated with the diversity and richness of soil bacteria, supporting the hypothesis that the increase in δ¹³C with depth was mainly caused by isotope fractionation in the process of microbial SOC decomposition. The structural equation model showed that nitrogen and the availability of nitrogen have a strong ability to explain the value of β, which indicates that nitrogen-based edaphic variables play an important role in affecting SOC decomposition. The SOC decomposition rate in PTP was higher than that in QAC and MF. The results of this study indicate that the prediction of SOC decomposition based on the β value is suitable for coastal zone protected forests. The incorporation of edaphic variables into global carbon cycle models may enhance the predictions of SOC dynamics in coastal zone protected forests.

Soil Moisture and Soluble Salt Content Dominate Changes in Foliar δ13C and δ15N of Desert Communities in the Qaidam Basin, Qinghai-Tibetan Plateau

Changing precipitation and temperature are principal drivers for nutrient cycling dynamics in drylands. Foliar isotopic carbon (C) and nitrogen (N) composition (δ¹³C and δ¹⁵N) are often used to describe the plant's water use efficiency and nitrogen use strategy in plant ecology research. However, the drivers and mechanisms under differential foliar δ¹³C and δ¹⁵N among plant species and communities are largely unknown for arid high-elevation regions. This study collected 462 leaf samples of ten top-dominant plant species (two or three replicates per species) across 16 sites in 2005 and 2010 to measure the community-weighted means (CWMs) of foliar δ¹³C and δ¹⁵N, northeastern Qaidam Basin, Qinghai-Tibetan Plateau. Our results showed that the CWM of foliar δ¹⁵N was higher in 2005 than in 2010 and was lower in the warm-dry season (July and August) than the cool-wet one (June and September) in 2010. Similarly, the CWM of foliar δ¹³C was higher in 2005 than in 2010, but no difference between warm-dry and cool-wet seasons in 2010. C₄ plants have higher δ¹³C and generally grow faster than C₃ species under warm-wet weathers. This might be why the CWM of foliar δ¹³C was high, while the CWM of foliar δ¹⁵N was low in the wet sampling year (2010). The general linear mixed models revealed that soil moisture was the most critical driver for the CWM of foliar δ¹⁵N, which explained 42.1% of the variance alone. However, the total soluble salt content was the crucial factor for the CWM of foliar δ¹³C, being responsible for 29.7% of the variance. Growing season temperature (GST) was the second most vital factor and explained 28.0% and 21.9% of the variance in the CWMs of foliar δ¹⁵N and δ¹³C. Meanwhile, remarkable differences in the CWMs of foliar δ¹⁵N and δ¹³C were also found at the species level. Specifically, Kalidium gracile and Salsola abrotanoides have higher foliar δ¹⁵N, while Ephedra sinica and Tamarix chinensis have lower foliar δ¹⁵N than other species. The foliar δ¹³C of Calligonum Kozlov and H. ammodendron was the highest among the ten species. Except for the foliar δ¹³C of E. sinica was higher than Ceratoide latens between the two sampling years or between the cool-wet and warm-dry seasons, no significant difference in foliar δ¹³C was found for other species. Overall, the CWMs of foliar δ¹⁵N and δ¹³C dynamics were affected by soil properties, wet-dry climate change, and species identity in high-elevation deserts on the Qinghai Tibetan Plateau.

The effect of climatic and edaphic factors on soil organic carbon turnover in hummocks based on δ13C on the Qinghai-Tibet Plateau

Hummocks (thúfur, pounus) are peculiar landforms usually formed by repeated freeze-thaw processes and differential frost heave, and are common in frost soil regions, especially in the Qinghai-Tibet Plateau. However, little is known about the response of δ¹³C in soil organic carbon (δ¹³C_SOC) to soil and climate properties in hummocks. The β value indicates the decomposition rate of soil organic carbon (SOC) in soil, and was obtained from the slope of the regression between the log10-transformed SOC concentration and δ¹³C_SOC in soil depth profiles. In this study, we investigated δ¹³C_SOC and SOC contents along a soil profile (0-60 cm), together with edaphic and climatic properties, both in hummocks and control plots (alpine grasslands) on the northeastern Qinghai-Tibet Plateau. Then, the variations in δ¹³C_SOC and β values, and the main factors affecting them, were analyzed. The results show that δ¹³C_SOC increases with soil depth, while SOC decreases both in the hummocks and control plots. However, β values in the hummocks were significantly (P < 0.05) higher than in the control plots while δ¹³C_SOC showed no difference between hummock and control. Redundancy analysis showed that altitude is the main control factor for δ¹³C_SOC and β in the hummocks. Climate type was the main factor affecting δ¹³C_SOC in the control plots, while mean annual precipitation and soil fractal dimension were the main factors controlling β. Overall, climate, rather than soil, is the key factor that affects the carbon turnover rate in the hummock in the northeastern QTP. The findings of this study will expand our understanding of the soil carbon cycle and δ¹³C_SOC changes, especially in the case of hummocks.

The driving factors of mercury storage in the Tibetan grassland soils underlain by permafrost

Soils, especially permafrost in the Arctic and the Tibetan Plateau, are one of the largest reservoirs of mercury (Hg) in the global environment. The Hg concentration in the grassland soils over the Tibetan Plateau and its driving factors have been less studied. This study analyzes soil total mercury (STHg) concentrations and its vertical distribution in grassland soil samples collected from the Tibetan Plateau. We adopt a nested-grid high-resolution GEOS-Chem model to simulate atmospheric Hg deposition. The relationship between STHg and soil organic carbon (SOC), as well as atmospheric deposition, are explored. Our results show that the STHg concentrations in the Tibetan Plateau are 19.8 ± 12.2 ng/g. The concentrations are higher in the south and lower in the north in the Tibetan Plateau, consistent with the previous results. Our model shows that the average deposition flux of Hg is 3.3 μg m^-2 yr^-1, with 57% contributed by dry deposition of elemental mercury (Hg⁰), followed by dry (19%) and wet (24%) deposition of divalent mercury. We calculate the Hg to carbon ratio (R_Hg:C) as 5.6 ± 6.5 μg Hg/g C, and the estimated STHg is 86.6 ± 101.2 Gg in alpine grasslands in the Tibetan Plateau. We find a positive relationship between STHg and SOC in the Tibetan Plateau (r² = 0.36) and a similar positive relationship between STHg and atmospheric total Hg deposition (r² = 0.24). A multiple linear regression involving both variables better model the observed STHg (r² = 0.42). We conclude that SOC and atmospheric deposition influence STHg simultaneously in this region. The data provides information to quantify the size of the soil Hg pool in the Tibetan Plateau further, which has important implications for the Hg cycles in the permafrost regions as well as on the global scale.

Shift in functional plant groups under flooding impacted ecosystem C and N dynamics across riparian zones in the Three Gorges of China

Large water conservancy project can strongly alter the plant community composition, however, how these changes can potentially affect ecosystem carbon (C) and nitrogen (N) dynamics is not fully understood. Here, we investigated natural ¹³C and ¹⁵N abundance of C₃ and C₄ plants and soil in different fractions [labile C (LC) and N (LN), recalcitrant C (RC) and N (RN)]from 6 sites with two elevations (flooding zone, 145-175 m, area with revegetation due to flooding, N = 6); and unflooding zone, >175 m with original plant as a control, N = 3) in riparian zones of the Three Gorges Reservoir, China. The dominant species were the C₄ plants in the upstream including Changshou (CS), Fuling (FL) and Zhongxian (ZX) and the C₃ plants in the downstream in unflooding zone including Wanzhou (WZ) Badong (BD), and Zigui (ZG). C₄ plant in flooding zone was significant decreased by mean 25% compared with unflooding zone in the upstream but significantly increased the by mean 59% in the downstream. The ¹³C isotopic differences between soil and plant (Δδ¹³C) was lower than zero in both flooding and unflooding in the upstream, but was only lower than zero in flooding zone in the downstream. The proportion of C₃-derived C in soil organic carbon pool (average 74.64%) was lower for the flooding zone compared to the unflooding zone (average 87.26%) in most sites, while the proportion of C₃-derived C in LC (average 44.38%) was decreased in the flooding zone compared to the unflooding zone (69.52%) in the downstream. Additionally, the δ¹⁵N values of soil were higher than plant community in most sites, and were strongly associated with soil C and N pool content, as well as soil pH. Overall, our results revealed that soil C accumulation was primarily determined by C₃ plant in situ and new C input by existing dominant C₄ plant, whereas soil N dynamics was predictably dependent on soil relative C and N availability in response to flooding at regional scale.

Magnitude and Drivers of Potential Methane Oxidation and Production across the Tibetan Alpine Permafrost Region

Methane (CH₄) dynamics across permafrost regions is critical in determining the magnitude and direction of permafrost carbon (C)-climate feedback. However, current studies are mainly derived from the Arctic area, with limited evidence from other permafrost regions. By combining large-scale laboratory incubation across 51 sampling sites with machine learning techniques and bootstrap analysis, here, we determined regional patterns and dominant drivers of CH₄ oxidation potential in alpine steppe and meadow (CH₄ sink areas) and CH₄ production potential in swamp meadow (CH₄ source areas) across the Tibetan alpine permafrost region. Our results showed that both CH₄ oxidation potential (in alpine steppe and meadow) and CH₄ production potential (in swamp meadow) exhibited large variability across various sampling sites, with the median value being 8.7, 9.6, and 11.5 ng g^-1 dry soil h^-1, respectively. Our results also revealed that methanotroph abundance and soil moisture were two dominant factors regulating CH₄ oxidation potential, whereas CH₄ production potential was mainly affected by methanogen abundance and the soil organic carbon content, with functional gene abundance acting as the best explaining variable. These results highlight the crucial role of microbes in regulating CH₄ dynamics, which should be considered when predicting the permafrost C cycle under future climate scenarios.

Carbon and nitrogen isotope variability in the seeds of two African millet species: Pennisetum glaucum and Eleusine coracana

RATIONALE

A range of important small seeded C₄ crops were domesticated in Africa, but little is known about their carbon and nitrogen isotope ratios (δ¹³ C and δ¹⁵ N values). Understanding natural isotopic variability within and among millets has the potential to help us to understand the conditions under which ancient cereals were grown and has significant implications for the interpretation of ancient diets based on stable isotope signatures.

METHODS

We conducted carbon and nitrogen isotope analyses of modern and historical pearl millet (Pennisetum glaucum, n = 108) and finger millet (Eleusine coracana, n = 17) seed samples sourced from the United States Department of Agriculture as well as the Harlan Collection curated at the Crop Evolution Laboratory Herbarium at the University of Illinois.

RESULTS

The millet species have significantly different mean carbon and nitrogen isotope ratios over broad temporal and spatial scales. We also found substantial isotopic variation within species (range of 1.9‰ and 8.5‰ in δ¹³ C and δ¹⁵ N values, respectively). Both water availability and growing season temperature significantly affected the P. glaucum δ¹³ C and δ¹⁵ N values; cumulative annual precipitation was positively correlated with both seed δ¹³ C and δ¹⁵ N values, while temperature was positively correlated with δ¹⁵ N values but negatively correlated with seed δ¹³ C values.

CONCLUSIONS

The importance of both temperature and precipitation as predictors of δ¹³ C and δ¹⁵ N values in millets suggests that C₄ plants may be more sensitive to environmental parameters than previously appreciated. Given the high degree of carbon and nitrogen isotope variability among accessions of these species, it is imperative that site-relevant plant isotope ratios are used for making isotope-based paleo-dietary predictions.

Increased precipitation accelerates soil organic matter turnover associated with microbial community composition in topsoil of alpine grassland on the eastern Tibetan Plateau

Large quantities of carbon are stored in alpine grassland of the Tibetan Plateau, which is extremely sensitive to climate change. However, it remains unclear whether soil organic matter (SOM) in different layers responds to climate change analogously, and whether microbial communities play vital roles in SOM turnover of topsoil. In this study we measured and collected SOM turnover by the ¹⁴C method in alpine grassland to test climatic effects on SOM turnover in soil profiles. Edaphic properties and microbial communities in the northwestern Qinghai Lake were investigated to explore microbial influence on SOM turnover. SOM turnover in surface soil (0-10 cm) was more sensitive to precipitation than that in subsurface layers (10-40 cm). Precipitation also imposed stronger effects on the composition of microbial communities in the surface layer than that in deeper soil. At the 5-10 cm depth, the SOM turnover rate was positively associated with the bacteria/fungi biomass ratio and the relative abundance of Acidobacteria, both of which are related to precipitation. Partial correlation analysis suggested that increased precipitation could accelerate the SOM turnover rate in topsoil by structuring soil microbial communities. Conversely, carbon stored in deep soil would be barely affected by climate change. Our results provide valuable insights into the dynamics and storage of SOM in alpine grasslands under future climate scenarios.

Afforestation impacts microbial biomass and its natural (13)C and (15)N abundance in soil aggregates in central China

We investigated soil microbial biomass and its natural abundance of δ(13)C and δ(15)N in aggregates (>2000μm, 250-2000μm, 53-250μm and <53μm) of afforested (implementing woodland and shrubland plantations) soils, adjacent croplands and open area (i.e., control) in the Danjiangkou Reservoir area of central China. The afforested soils averaged higher microbial biomass carbon (MBC) and nitrogen (MBN) levels in all aggregates than in open area and cropland, with higher microbial biomass in micro-aggregates (<250μm) than in macro-aggregates (>2000μm). The δ(13)C of soil microbial biomass was more enriched in woodland soils than in other land use types, while δ(15)N of soil microbial biomass was more enriched compared with that of organic soil in all land use types. The δ(13)C and δ(15)N of microbial biomass were positively correlated with the δ(13)C and δ(15)N of organic soil across aggregates and land use types, whereas the (13)C and (15)N enrichment of microbial biomass exhibited linear decreases with the corresponding C:N ratio of organic soil. Our results suggest that shifts in the natural (13)C and (15)N abundance of microbial biomass reflect changes in the stabilization and turnover of soil organic matter (SOM) and thereby imply that afforestation can greatly impact SOM accumulation over the long-term.

The permafrost carbon inventory on the Tibetan Plateau: a new evaluation using deep sediment cores

The permafrost organic carbon (OC) stock is of global significance because of its large pool size and the potential positive feedback to climate warming. However, due to the lack of systematic field observations and appropriate upscaling methodologies, substantial uncertainties exist in the permafrost OC budget, which limits our understanding of the fate of frozen carbon in a warming world. In particular, the lack of comprehensive estimates of OC stocks across alpine permafrost means that current knowledge on this issue remains incomplete. Here, we evaluated the pool size and spatial variations of permafrost OC stock to 3 m depth on the Tibetan Plateau by combining systematic measurements from a substantial number of pedons (i.e. 342 three-metre-deep cores and 177 50-cm-deep pits) with a machine learning technique (i.e. support vector machine, SVM). We also quantified uncertainties in permafrost carbon budget by conducting Monte Carlo simulations. Our results revealed that the combination of systematic measurements with the SVM model allowed spatially explicit estimates to be made. The OC density (OC amount per unit area, OCD) exhibited a decreasing trend from the south-eastern to the north-western plateau, with the exception that OCD in the swamp meadow was substantially higher than that in surrounding regions. Our results also demonstrated that Tibetan permafrost stored a large amount of OC in the top 3 m, with the median OC pool size being 15.31 Pg C (interquartile range: 13.03-17.77 Pg C). 44% of OC occurred in deep layers (i.e. 100-300 cm), close to the proportion observed across the northern circumpolar permafrost region. The large carbon pool size together with significant permafrost thawing suggests a risk of carbon emissions and positive climate feedback across the Tibetan alpine permafrost region.