Grasslands store approximately one third of the global terrestrial carbon stocks and can act as an important soil carbon sink. Recent studies show that plant diversity increases soil organic carbon (SOC) storage by elevating carbon inputs to belowground biomass and promoting microbial necromass contribution to SOC storage. Climate change affects grassland SOC storage by modifying the processes of plant carbon inputs and microbial catabolism and anabolism. Improved grazing management and biodiversity restoration can provide low-cost and/or high-carbon-gain options for natural climate solutions in global grasslands. The achievable SOC sequestration potential in global grasslands is 2.3 to 7.3 billion tons of carbon dioxide equivalents per year (COe year) for biodiversity restoration, 148 to 699 megatons of COe year for improved grazing management, and 147 megatons of COe year for sown legumes in pasturelands.

The sign and magnitude of permafrost carbon (C)-climate feedback are highly uncertain due to the limited understanding of the decomposability of thawing permafrost and relevant mechanistic controls over C release. Here, by combining aerobic incubation with biomarker analysis and a three-pool model, we reveal that C quality (represented by a higher amount of fast cycling C but a lower amount of recalcitrant C compounds) and normalized CO2-C release in permafrost deposits were similar or even higher than those in the active layer, demonstrating a high vulnerability of C in Tibetan upland permafrost. We also illustrate that C quality exerts the most control over CO2-C release from the active layer, whereas soil microbial abundance is more directly associated with CO2-C release after permafrost thaw. Taken together, our findings highlight the importance of incorporating microbial properties into Earth System Models when predicting permafrost C dynamics under a changing environment.

Studies on the amount and spatial distribution of soil organic carbon/nitrogen in Inner Mongolia and their relationship to main climate factors showed that the content of soil organic carbon and nitrogen was 3.24～43.24 kg·m^-3 and 269.56～3085.60 g·m^-3,respectively, and the C/N ratio was about 4.46～17.13. The correlation between soil organic carbon/nitrogen and temperature was negative, and R was 0.557 and 0.460,respectively.Soil organic carbon/nitrogen had a weak positive correlation to precipitation, and Rwas 0.285 and 0.203. Soil organic carbon and nitrogen appeared a reducing trend with increasing temperature and decreasing precipitation from northeast to southwest.

采用全国策二次土壤普查中内蒙古自治区的典型土种剖面资料，在剖面深度的基础上，用地统计学和地理信息系统(GIS)方法，分别按土壤类型和土地覆被类型计算了土壤有机碳、氮密度，分析了内蒙古自治区土壤有机碳、氮蓄积量的空间分布特征，探讨了土壤有机碳、氮蓄积量与主要气候要素的关系．结果表明，内蒙古自治区土壤有机碳密度处于3．24—43．24kg·m^-3之间，土壤有机氮密度处于269．56—3085．60g·m^-3之间，土壤碳、氮比(C／N)大致在4．46—17．13之间．土壤有机碳、氮密度与温度呈负相关，相关系数分别为0．557和0．460(n＝245)；与年均降水量呈正相关，但相关性不是很强，相关系数分别为0．285和0．203．从内蒙古自治区东北地区到西南地区，土壤有机碳、氮蓄积量随着温度递升和降水量递减呈现降低的趋势。

Doetterl, Sebastian; Boeckx, Pascal Univ Ghent, Isotope Biosci Lab, B-9000 Ghent, Belgium. Doetterl, Sebastian Univ Augsburg, Dept Geog, D-86159 Augsburg, Germany. Stevens, Antoine; Van Oost, Kristof Catholic Univ Louvain, George Lemaitre Ctr Earth & Climate Res, Earth & Life Inst, B-1348 Louvain, Belgium. Stevens, Antoine; Six, Johan ETH, Dept Environm Syst Sci, CH-8092 Zurich, Switzerland. Merckx, Roel Katholieke Univ Leuven, Dept Earth & Environm Sci, B-3001 Heverlee, Belgium. Casanova Pinto, Manuel Univ Chile, Dept Ingn & Suelos, Santiago 8820808, Chile. Casanova-Katny, Angelica Univ Santiago Chile, Fac Quim & Farm, Santiago 8820808, Chile. Munoz, Cristina; Zagal Venegas, Erick Univ Concepcion, Dept Ciencias Suelo & Recursos Nat, Chillan 3812120, Chile. Boudin, Mathieu Royal Inst Cultural Heritage, B-1000 Brussels, Belgium.

Adequate understanding of the controlling factors of soil carbon (C) stock is crucial for improving the predictability of Earth System Models in exploring terrestrial C-climate feedback. Current studies, however, mainly focus on climatic and edaphic variables and rarely explore the effects of mineral protection in regulating soil organic carbon (SOC) stock over broad geographic scale. Particularly, the relative importance of mineral protection compared with other factors is unclear. Based on large-scale soil inventory, here we filled this knowledge gap by exploring the effects of Al/Fe-(hydr) oxides on SOC and three C fractions across Tibetan alpine grasslands via linear regression, partial correlation, and variance partitioning analyses, and also by comparing the degree of mineral protection in alpine grasslands with other ecosystems. Our results showed that SOC and C fractions across Tibetan alpine grasslands were regulated by Al/Fe-(hydr) oxides, with the incorporation of mineral variables increasing the explained variations by 10.1% for SOC content, 13.4% for coarse particulate organic matter, 12.6% for microaggregate associated C, and 21.9% for silt and clay associated C. Moreover, the contribution of mineral effects exceeded that of climatic and edaphic factors, particularly in the silt and clay associated C fraction. In addition, about 15.812.0% of SOC pools were associated with Fe, which was equal to or higher than those in temperate and tropical-subtropical ecosystems. Taken together, these results demonstrate the significant role of Al/Fe minerals in the stabilization of SOC across Tibetan alpine grasslands, highlighting the importance of incorporating C-mineral interactions into ESMs for better understanding the terrestrial C-climate feedback.

. Accurate assessment of anthropogenic carbon dioxide (CO2) emissions and\ntheir redistribution among the atmosphere, ocean, and terrestrial biosphere\nin a changing climate – the “global carbon budget” – is important to\nbetter understand the global carbon cycle, support the development of\nclimate policies, and project future climate change. Here we describe and\nsynthesize data sets and methodology to quantify the five major components\nof the global carbon budget and their uncertainties. Fossil CO2\nemissions (EFOS) are based on energy statistics and cement production\ndata, while emissions from land-use change (ELUC), mainly\ndeforestation, are based on land use and land-use change data and\nbookkeeping models. Atmospheric CO2 concentration is measured directly\nand its growth rate (GATM) is computed from the annual changes in\nconcentration. The ocean CO2 sink (SOCEAN) and terrestrial\nCO2 sink (SLAND) are estimated with global process models\nconstrained by observations. The resulting carbon budget imbalance\n(BIM), the difference between the estimated total emissions and the\nestimated changes in the atmosphere, ocean, and terrestrial biosphere, is a\nmeasure of imperfect data and understanding of the contemporary carbon\ncycle. All uncertainties are reported as ±1σ. For the last\ndecade available (2010–2019), EFOS was 9.6 ± 0.5 GtC yr−1 excluding the cement carbonation sink (9.4 ± 0.5 GtC yr−1 when the cement carbonation sink is included), and\nELUC was 1.6 ± 0.7 GtC yr−1. For the same decade, GATM was 5.1 ± 0.02 GtC yr−1 (2.4 ± 0.01 ppm yr−1), SOCEAN 2.5 ±  0.6 GtC yr−1, and SLAND 3.4 ± 0.9 GtC yr−1, with a budget\nimbalance BIM of −0.1 GtC yr−1 indicating a near balance between\nestimated sources and sinks over the last decade. For the year 2019 alone, the\ngrowth in EFOS was only about 0.1 % with fossil emissions increasing\nto 9.9 ± 0.5 GtC yr−1 excluding the cement carbonation sink (9.7 ± 0.5 GtC yr−1 when cement carbonation sink is included), and ELUC was 1.8 ± 0.7 GtC yr−1, for total anthropogenic CO2 emissions of 11.5 ± 0.9 GtC yr−1 (42.2 ± 3.3 GtCO2). Also for 2019, GATM was\n5.4 ± 0.2 GtC yr−1 (2.5 ± 0.1 ppm yr−1), SOCEAN\nwas 2.6 ± 0.6 GtC yr−1, and SLAND was 3.1 ± 1.2 GtC yr−1, with a BIM of 0.3 GtC. The global atmospheric CO2\nconcentration reached 409.85 ± 0.1 ppm averaged over 2019. Preliminary\ndata for 2020, accounting for the COVID-19-induced changes in emissions,\nsuggest a decrease in EFOS relative to 2019 of about −7 % (median\nestimate) based on individual estimates from four studies of −6 %, −7 %,\n−7 % (−3 % to −11 %), and −13 %. Overall, the mean and trend in the\ncomponents of the global carbon budget are consistently estimated over the\nperiod 1959–2019, but discrepancies of up to 1 GtC yr−1 persist for the\nrepresentation of semi-decadal variability in CO2 fluxes. Comparison of\nestimates from diverse approaches and observations shows (1) no consensus\nin the mean and trend in land-use change emissions over the last decade, (2)\na persistent low agreement between the different methods on the magnitude of\nthe land CO2 flux in the northern extra-tropics, and (3) an apparent\ndiscrepancy between the different methods for the ocean sink outside the\ntropics, particularly in the Southern Ocean. This living data update\ndocuments changes in the methods and data sets used in this new global\ncarbon budget and the progress in understanding of the global carbon cycle\ncompared with previous publications of this data set (Friedlingstein et al.,\n2019; Le Quéré et al., 2018b, a, 2016, 2015b, a, 2014,\n2013). The data presented in this work are available at https://doi.org/10.18160/gcp-2020 (Friedlingstein et al., 2020).\n

. In permafrost soils, the temperature regime and the resulting cryogenic processes are important determinants of the storage of organic carbon (OC) and its small-scale spatial variability. For cryoturbated soils, there is a lack of research assessing pedon-scale heterogeneity in OC stocks and the transformation of functionally different organic matter (OM) fractions, such as particulate and mineral-associated OM. Therefore, pedons of 28 Turbels were sampled in 5 m wide soil trenches across the Siberian Arctic to calculate OC and total nitrogen (TN) stocks based on digital profile mapping. Density fractionation of soil samples was performed to distinguish between particulate OM (light fraction, LF, < 1.6 g cm−3), mineral associated OM (heavy fraction, HF, > 1.6 g cm−3), and a mobilizable dissolved pool (mobilizable fraction, MoF). Across all investigated soil profiles, the total OC storage was 20.2 ± 8.0 kg m−2 (mean ± SD) to 100 cm soil depth. Fifty-four percent of this OC was located in the horizons of the active layer (annual summer thawing layer), showing evidence of cryoturbation, and another 35 % was present in the upper permafrost. The HF-OC dominated the overall OC stocks (55 %), followed by LF-OC (19 % in mineral and 13 % in organic horizons). During fractionation, approximately 13 % of the OC was released as MoF, which likely represents a readily bioavailable OM pool. Cryogenic activity in combination with cold and wet conditions was the principle mechanism through which large OC stocks were sequestered in the subsoil (16.4 ± 8.1 kg m−2; all mineral B, C, and permafrost horizons). Approximately 22 % of the subsoil OC stock can be attributed to LF material subducted by cryoturbation, whereas migration of soluble OM along freezing gradients appeared to be the principle source of the dominant HF (63 %) in the subsoil. Despite the unfavourable abiotic conditions, low C / N ratios and high δ13C values indicated substantial microbial OM transformation in the subsoil, but this was not reflected in altered LF and HF pool sizes. Partial least-squares regression analyses suggest that OC accumulates in the HF fraction due to co-precipitation with multivalent cations (Al, Fe) and association with poorly crystalline iron oxides and clay minerals. Our data show that, across all permafrost pedons, the mineral-associated OM represents the dominant OM fraction, suggesting that the HF-OC is the OM pool in permafrost soils on which changing soil conditions will have the largest impact.\n

Managing soil organic matter (SOM) stocks to address global change challenges requires well-substantiated knowledge of SOM behavior that can be clearly communicated between scientists, management practitioners, and policy makers. However, SOM is incredibly complex and requires separation into multiple components with contrasting behavior in order to study and predict its dynamics. Numerous diverse SOM separation schemes are currently used, making cross-study comparisons difficult and hindering broad-scale generalizations. Here, we recommend separating SOM into particulate (POM) and mineral-associated (MAOM) forms, two SOM components that are fundamentally different in terms of their formation, persistence, and functioning. We provide evidence of their highly contrasting physical and chemical properties, mean residence times in soil, and responses to land use change, plant litter inputs, warming, CO enrichment, and N fertilization. Conceptualizing SOM into POM versus MAOM is a feasible, well-supported, and useful framework that will allow scientists to move beyond studies of bulk SOM, but also use a consistent separation scheme across studies. Ultimately, we propose the POM versus MAOM framework as the best way forward to understand and predict broad-scale SOM dynamics in the context of global change challenges and provide necessary recommendations to managers and policy makers.© 2019 John Wiley & Sons Ltd.

Land-use change is a major driver of biodiversity loss worldwide. Although biodiversity often shows a delayed response to land-use change, previous studies have typically focused on a narrow range of current landscape factors and have largely ignored the role of land-use history in shaping plant and animal communities and their functional characteristics. Here, we used a unique database of 220,000 land-use records to investigate how 20-y of land-use changes have affected functional diversity across multiple trophic groups (primary producers, mutualists, herbivores, invertebrate predators, and vertebrate predators) in 75 grassland fields with a broad range of land-use histories. The effects of land-use history on multitrophic trait diversity were as strong as other drivers known to impact biodiversity, e.g., grassland management and current landscape composition. The diversity of animal mobility and resource-acquisition traits was lower in landscapes where much of the land had been historically converted from grassland to crop. In contrast, functional biodiversity was higher in landscapes containing old permanent grasslands, most likely because they offer a stable and high-quality habitat refuge for species with low mobility and specialized feeding niches. Our study shows that grassland-to-crop conversion has long-lasting impacts on the functional biodiversity of agricultural ecosystems. Accordingly, land-use legacy effects must be considered in conservation programs aiming to protect agricultural biodiversity. In particular, the retention of permanent grassland sanctuaries within intensive landscapes may offset ecological debts.

Global warming has greatly altered winter snowfall patterns, and there is a trend towards increasing winter snow in semi-arid regions in China. Winter snowfall is an important source of water during early spring in these water-limited ecosystems, and it can also affect nutrient supply. However, we know little about how changes in winter snowfall will affect ecosystem productivity and plant community structure during the growing season. Here, we conducted a 5-year winter snow manipulation experiment in a temperate grassland in Inner Mongolia. We measured ecosystem carbon flux from 2014 to 2018 and plant biomass and species composition from 2015 to 2018. We found that soil moisture increased under deepened winter snow in early growing season, particularly in deeper soil layers. Deepened snow increased the net ecosystem exchange of CO (NEE) and reduced intra- and inter-annual variation in NEE. Deepened snow did not affect aboveground plant biomass (AGB) but significantly increased root biomass. This suggested that the enhanced NEE was allocated to the belowground, which improved water acquisition and thus contributed to greater stability in NEE in deep-snow plots. Interestingly, the AGB of grasses in the control plots declined over time, resulting in a shift towards a forb-dominated system. Similar declines in grass AGB were also observed at three other locations in the region over the same time frame and are attributed to 4 years of below-average precipitation during the growing season. By contrast, grass AGB was stabilized under deepened winter snow and plant community composition remained unchanged. Hence, our study demonstrates that increased winter snowfall may stabilize arid grassland systems by reducing resource competition, promoting coexistence between plant functional groups, which ultimately mitigates the impacts of chronic drought during the growing season.© 2020 John Wiley & Sons Ltd.

Large-scale evidence reveals that mineral and microbial properties mediate permafrost C release and its temperature response.

. Soil carbon (C) is the largest C pool in the terrestrial biosphere and includes both inorganic and organic components. Studying patterns and controls of soil C help us to understand and estimate potential responses of soil C to global change in the future. Here we analyzed topsoil data of 81 sites obtained from a regional survey across grasslands in the Inner Mongolia and on the Tibetan Plateau during 2006–2007, attempting to find the patterns and controls of soil inorganic carbon (SIC) and soil organic carbon (SOC). The averages of inorganic and organic carbon in the topsoil (0–20 cm) across the study region were 0.38% and 3.63%, ranging between 0.00–2.92% and 0.32–26.17% respectively. Both SIC and SOC in the Tibetan grasslands (0.51% and 5.24% respectively) were higher than those in the Inner Mongolian grasslands (0.21% and 1.61%). Regression tree analyses showed that the spatial pattern of SIC and SOC were controlled by different factors. Chemical and physical processes of soil formation drive the spatial pattern of SIC, while biotic processes drive the spatial pattern of SOC. SIC was controlled by soil acidification and other processes depending on soil pH. Vegetation type is the most important variable driving the spatial pattern of SOC. According to our models, given the acidification rate in Chinese grassland soils in the future is the same as that in Chinese cropland soils during the past two decades: 0.27 and 0.48 units per 20 yr in the Inner Mongolian grasslands and the Tibetan grasslands respectively, it will lead to a 30% and 53% decrease in SIC in the Inner Mongolian grasslands and the Tibetan grasslands respectively. However, negative relationship between soil pH and SOC suggests that acidification will inhibit decomposition of SOC, thus will not lead to a significant general loss of carbon from soils in these regions.\n

. Soil is currently thought to be a sink for carbon; however, the response of this sink to increasing levels of atmospheric carbon dioxide and climate change is uncertain. In this study, we analyzed soil organic carbon (SOC) changes from 11 Earth system models (ESMs) contributing simulations to the Coupled Model Intercomparison Project Phase 5 (CMIP5). We used a reduced complexity model based on temperature and moisture sensitivities to analyze the drivers of SOC change for the historical and high radiative forcing (RCP 8.5) scenarios between 1850 and 2100. ESM estimates of SOC changed over the 21st century (2090–2099 minus 1997–2006) ranging from a loss of 72 Pg C to a gain of 253 Pg C with a multi-model mean gain of 65 Pg C. Many ESMs simulated large changes in high-latitude SOC that ranged from losses of 37 Pg C to gains of 146 Pg C with a multi-model mean gain of 39 Pg C across tundra and boreal biomes. All ESMs showed cumulative increases in global NPP (11 to 59%) and decreases in SOC turnover times (15 to 28%) over the 21st century. Most of the model-to-model variation in SOC change was explained by initial SOC stocks combined with the relative changes in soil inputs and decomposition rates (R2 = 0.89, p < 0.01). Between models, increases in decomposition rate were well explained by a combination of initial decomposition rate, ESM-specific Q10-factors, and changes in soil temperature (R2 = 0.80, p < 0.01). All SOC changes depended on sustained increases in NPP with global change (primarily driven by increasing CO2). Many ESMs simulated large accumulations of SOC in high-latitude biomes that are not consistent with empirical studies. Most ESMs poorly represented permafrost dynamics and omitted potential constraints on SOC storage, such as priming effects, nutrient availability, mineral surface stabilization, and aggregate formation. Future models that represent these constraints are likely to estimate smaller increases in SOC storage over the 21st century.\n

<p><b><i>Aims </i></b>Soil aggregate is an important component of soil structure, playing an important role in the physical and biological protection mechanism of soil organic carbon (SOC) through isolating SOC from microorganisms. As far as we know, there are few studies, however, on exploring the spatial distribution of soil aggregate at the regional scale. Our objective was to investigate the mass allocation and stability of soil aggregate in different types of Nei Mongol grasslands.<br> <b><i>Methods </i></b>We have established 78 sites with a size of 10 m × 10 m across the transect of Nei Mongol grasslands and collected soil samples from different soil depth up to 1 m. We used wet sieving method to separate different sizes of aggregate partition and used mean mass diameter (<i>MMD</i>) and geometric mean diameter (<i>GMD</i>) in order to evaluate the stability of soil aggregate. The two-way ANOVA was used to test the difference of mass percentage and stability of soil aggregate in different grassland types and soil depths. In addition, a linear regression analysis was used to analyze the correlations of mass percentage and stability of soil aggregate with both mean annual precipitation (<i>MAP</i>) and mean annual temperature (<i>MAT</i>).<br> <b><i>Important findings</i></b> The results showed that the mass percentages of soil aggregate were highest in meadow steppe, while almost equal in typical steppe and desert steppe. However, no significant patterns were found along the soil depth. The mass percentage of soil aggregate fractions were positively correlated with MAP in all soil layers, but negatively correlated with <i>MAT</i> except the layer of 70-100 cm. For the stability of soil aggregate, at 0-10 and 10-20 cm, <i>MMD</i> and <i>GMD</i> of meadow steppe were significantly greater than those of typical and desert steppes, whereas no significant differences among three grassland types were found for other soil layers. Besides, <i>MMD</i> and<i> GMD</i> in meadow steppe and typical steppe gradually decreased along the soil depth.</p>

土壤团聚体是土壤结构的重要组成部分, 是土壤保护其有机碳的一种重要物理与生物机制, 但迄今为止对其空间格局分布的研究较少。该文研究了我国内蒙古3种草原类型(草甸草原、典型草原、荒漠草原)不同土层深度的土壤团聚体质量百分比及其稳定性的分布规律。结果显示: 土壤团聚体的质量百分比在3种草原类型各个土层深度的分布均呈现草甸草原>典型草原=荒漠草原的趋势, 而沿土层深度3种草原类型的土壤团聚体的质量百分比含量并未呈现显著规律。各层的土壤团聚体质量百分比均与年降水量呈显著正相关关系; 除70-100 cm土层外, 其与年平均气温均呈负相关关系。对土壤团聚体稳定性而言, 在0-10 cm和10-20 cm两个土层深度, 草甸草原土壤团聚体的平均质量直径与几何平均直径显著大于典型草原和荒漠草原, 而在其他土层, 3种草原类型间无显著差异。随着土层深度的增加, 草甸草原和典型草原土壤团聚体的平均质量直径与几何平均直径均呈现逐渐降低的趋势。该文对于理解内蒙古不同类型草原土壤有机碳的稳定性和保护机制具有重要意义。

利用内蒙古呼伦贝尔草原额尔古纳右旗牧业气象试验站1994-2009年牧草生长季逐月实测资料,对CENTURY模型进行检验,模拟了呼伦贝尔草甸草原1961-2010年间地上净初级生产力(ANPP)动态,并与26个气象因子进行相关性分析。模型检验结果显示,生长季内逐月地上生物量模拟值与观测值之间的相关系数为R²=0.53,斜率b=0.94,误差平方根值为72.07 g/m²,平均绝对百分比误差为38.02%。检验结果表明,CENTURY模型能够成功地模拟这类草原的季节动态和年际变化。在过去的50年中,呼伦贝尔草甸草原温度增加,降水略增,ANPP增加。相关分析表明,ANPP与生长季降水量(r=0.372)呈极显著正相关;与年平均最低气温、年平均地面气温、年平均气温、7月降水量呈显著正相关;与年平均风速(r=-0.382)呈极显著负相关;与其他气象因子无显著的相关关系。应用区域气候模式系统PRECIS输出的2021-2050年气候情景数据分析得出,在SRES B2、A2和A1B情景下,未来呼伦贝尔草甸草原平均最高气温和最低气温都将呈显著升高趋势,降水量略增,ANPP虽然在年际间存在波动,但总体呈明显增加态势,分别较基准时段增加了67.14%,69.65%和76.58%,增加速率分别为16.51,17.34和16.42 g/(m²·10 a)。在3种情景下,未来气候变化均会对呼伦贝尔草甸草原群落生产力产生显著的正面影响。