Amount, distribution and controls of the soil organic carbon storage loss in the degraded China's grasslands

More than 80 % of China's grasslands are classified as degraded, and the loss of soil carbon storage due to degradation has a significant impact on China's terrestrial carbon sinks as well as carbon neutrality targets. The loss of soil carbon storage in degraded grasslands can serve as a benchmark for quantifying the carbon sequestration capacity of restored grasslands in the future. Here, above- and below-ground biomass, soil organic carbon (SOC) content at various depths (0-100 cm) and soil bulk density were collected from 226 degradation sequences around China. The above information was integrated and statistically analyzed to quantify the difference of SOC storage between the degraded and natural grassland at national scale. The result showed that grassland degradation led to a significant reduction in SOC storage across different depths. SOC (0-100 cm) of degraded grassland decreased by 39 % compared to that of natural grassland, ranging from 21 % in the lightly degraded sites to 59 % of the extremely degraded sites. 15 potential predictors were used to estimate the national amount of these differences of 0-20 cm depth SOC storage as 5.29 ± 1.59 Pg C. This considerable carbon storage gap implies the necessity of China's grassland restoration project in achieving carbon neutrality goals in the future.

Temperate grassland vegetation restoration influenced by grazing exclusion and climate change

Grasslands are one of the most important terrestrial biomes, supporting a wide range of ecological functions and services. Grassland degradation due to overgrazing is a severe issue worldwide, especially in developing regions. However, observations from multiple sources have shown that temperate grasslands in China have significantly increased during the past two decades. It remains controversial what factors have driven the vegetation restoration in this region. In this study, we combined remote-sensing images and field survey datasets to quantify the contributions of different factors to vegetation restoration in six temperate grasslands in northern China. Across the six grasslands, the Normalized Difference Vegetation Index (NDVI) increased by 0.003-0.0319 year-1. The average contributions of grazing exclusion and climate change to the NDVI increase were 49.23 % and 50.77 %, respectively. Precipitation change was the primary climate factor driving vegetation restoration, contributing 50.76 % to the NDVI variance. By contrast, climate warming tended to slow vegetation restoration, and atmospheric CO2 concentration change contributed little to the NDVI increase in the temperate grasslands. These results emphasize the significant contributions of both climate change and human management to grassland vegetation restoration.

Unknown bacterial species lead to soil CO2 emission reduction by promoting lactic fermentation in alpine meadow on the Qinghai-Tibetan Plateau

Highly variable soil microbial respiration among grasslands has been identified as a major cause of uncertainty in regional carbon (C) budget estimation in the Qinghai-Tibetan Plateau; microbial metabolism mechanisms might explain this variation, but remain elusive. Therefore, we investigated soil CO2 production in incubated soils and detected the associated functional genes at four sampling sites from two major alpine grasslands on the Qinghai-Tibetan Plateau. The results showed that the cumulative CO2 emissions from alpine meadow soils were 71 %-83 % lower than those from alpine steppe soils. Both the enriched genes abundance encoding fermentation and glycolysis (Embden-Meyerhof pathway (EMP)) and the diminished genes encoding tricarboxylic acid cycle (TCA) and phosphate pentose pathway (PPP) explained the CO2 emission reduction in the alpine meadow soils. The EMP: PPP and fermentation: TCA cycle ratios in alpine meadow soils were 1.45- and 1.50-fold higher than those in alpine steppe soils, respectively. Such shifts in metabolic pathways were primarily caused by the increasing dominance of an unknown species of Desulfobacteraceae with high glycolytic potential, carrying a higher abundance of ldh genes during fermentation. These unknown species were promoted by warmer temperatures and higher precipitation in the alpine meadows. Further studies on the unknown species would enhance our understanding and predictability of C cycling in alpine grasslands.

Divergences of soil carbon turnover and regulation in alpine steppes and meadows on the Tibetan Plateau

The grasslands of the Tibetan Plateau store approximately 2.5% of global soil organic carbon (SOC) and considerable soil inorganic carbon (SIC) and have the potential to become a vast carbon source or sink as climate change progresses. However, the soil carbon (C) sequestration mechanisms that occur across large-scale natural gradients remain unclear. Here, humic substances (HS) were utilized to trace soil C turnover at 0-20 cm, and we compared divergences among three main grassland types (alpine meadow, alpine steppe, and artificial plantation) using structural equation modeling (SEM). The results showed that the alpine meadows sequestered the most soil C (63.99 ± 4.41 g kg^-1 SOC and 4.11 ± 0.63 g kg^-1 SIC), sequestering 2-3 times more than the alpine steppe ecosystems (19.78 ± 1.98 g kg^-1 SOC and 9.21 ± 0.66 g kg^-1 SIC). The alpine steppe and artificial plantation regions have strong C sink potential due to their low C/N ratios (P < 0.05). Importantly, SIC played an important role in the alpine steppes, accounting for nearly 26-37% of soil C. The ratios of recalcitrant HS to SOC were estimated as 46.50%, 65.09%, and 78.17% in the alpine meadow, alpine steppe, and artificial plantation ecosystems, respectively, indicating that SOC in the alpine meadow was the most sensitive to climate change. Fulvic acid (FA) accounted for 50.86% of SOC in the 0-20-cm interval, contributing most to the formation of SOC in all vegetation types. In addition, in contrast to climatic controls on soil C turnover in the alpine meadow, climate conditions rarely controlled C turnover in the alpine steppe. Moreover, sand and silt were the main soil minerals involved in C turnover in alpine meadow and alpine steppe ecosystems, respectively. Our study improves understanding of the mechanism by which soil C sinks form on the Tibetan Plateau under warming and wetting conditions.

Assessment of Human-Related Driving Forces for Reduced Carbon Uptake Using Neighborhood Analysis and Geographically Weighted Regression: A Case Study in the Grassland of Inner Mongolia, China

The ever-rising concentration of atmospheric carbon is viewed as the primary cause for global warming. To discontinue this trend, it is of urgent importance to either cut down human carbon emissions or remove more carbon from the atmosphere. Grassland ecosystems occupy the largest part of the global land area but maintain a relatively low carbon sequestration flux. While numerous studies have confirmed the impacts on grassland vegetation growth from climate changes and human activities, little work has been done to understand the driving forces for a reduced carbon uptake (RCU)—a loss in vegetation carbon sequestration because of inappropriate grassland management. This work focused on assessing RCU in the grassland of Inner Mongolia and understanding the influential patterns of the selected variables (including grazing intensity, road network, population, and vegetation productivity) related to RCU. Neighborhood analysis was proposed to locate optimized grassland management practices from historical data and to map RCU. Ordinary least squares (OLS) and geographically weighted regression (GWR) models were applied to explore the driving forces for RCU. The results indicated that the human-related factors, including stock grazing intensity, population density, and road network were likely to present a spatially varied impact on RCU, which accounted for more than 1/4 of the total carbon sequestration.

Stabilization of carbon sequestration in a Chinese desert steppe benefits from increased temperatures and from precipitation outside the growing season

The carbon (C) dynamics of desert steppes play an important role in the C budget of temperate steppes. Using the Terrestrial Ecosystem Regional model (TECO-R) model for desert steppes, we examined the dynamics and potential driving mechanisms for C stocks at different temporal and spatial scales from 2000 to 2017 in northern China. The ecosystem C density averaged 2.73 kg C m^-2 and soil organic C accounted for 91.6%. The grassland biome stored 2.85 kg C m^-2, which is higher than the shrub biome (2.19 kg C m^-2). The ecosystem storage increased by an average of 27.75 g C m^-2 yr^-1, with the fastest increase in the southeastern part of the study area. The grassland biome storage increased by an average of 33.54 g C m^-2 yr^-1, versus 25.74 g C m^-2 yr^-1 for the shrub biome. The desert steppe C stock totaled 288.29 Tg C, and increased at 3.09 Tg C yr^-1. An average of >45% of the aboveground biomass was browsed by livestock. The growing season precipitation was significantly positively correlated with changes in the C stock. Increasing temperature was negatively correlated with the C stock, especially for soil carbon. Precipitation was an important driving factor, but warming interacted with precipitation to affect C sequestration during the growing season. Outside the growing season, the increased precipitation and temperature stabilized C sequestration in the desert steppe. This improved understanding of feedbacks between the desert steppe's C cycle and climate will improve predictions of C dynamics in terrestrial ecosystems and of the impacts of climate change.