As components of terrestrial carbon sinks, vegetation and soil carbon pools are important for offsetting CO2 emissions. However, differences in their carbon sequestration capacities and their responses to global change in the future are poorly understood. This study assessed the changes in vegetation and soil carbon and their ratios and drivers under the SSP126 scenario from 2015 to 2060, using Coupled Model Intercomparison Project phase 6 (CMIP6) simulations in China, a major carbon sink region in global terrestrial ecosystems. The content of vegetation carbon (29±1 PgC) was observed to be lower than that of soil carbon (113±23 PgC), and the ratio of vegetation to soil carbon was the highest in the subtropical-tropical monsoon climatic region (0.55±0.12). Moreover, the total stock of vegetation and soil carbon increased by 10±1 PgC during the study period, and the increase in vegetation carbon was 4.31 times that of soil carbon, because the responses of vegetation carbon stocks to increased forest coverage and atmospheric CO2 were greater than that of soil carbon stocks, especially in the subtropical-tropical and temperate monsoonal climatic regions. However, bare land encroachment on grasslands reduced their increments in the temperate monsoonal and high-cold Tibetan Plateau climatic regions. Furthermore, compared with SSP245 and SSP585 scenarios, vegetation and soil carbon sinks can offset a greater amount of carbon emissions in 2060 under the SSP126 scenario, accounting for 53% of all carbon emissions, offsetting 60%–79% of carbon emissions from China under its policy of increasing forest coverage. The study revealed the important role of afforestation in increasing ecosystem carbon stocks, additionally, grassland conservation and deep reductions in carbon emissions cannot be ignored in the future. This study provides a basis for determining the response of vegetation and soil carbon to environmental factors and the realization of net-zero emissions globally.
Continued climate warming and precipitation fluctuations are expected to further affect aboveground net primary productivity (ANPP) across alpine grasslands, with associated implications for ecosystem functions. The spatial and temporal variability of ANPP in Tibetan Plateau (TP) grasslands and its response to temperature and precipitation were investigated in this study, based on the ANPP estimated by means of ensemble analysis of multiple machine learning models. First, the response of the spatial distribution of ANPP to variations in the precipitation gradient was nonlinear and showed an S-shaped growth pattern, which could be divided into four stages (stationary, rapid growth, slower growth, and saturation) corresponding to arid (mean annual precipitation (MAP) < 250 mm), semiarid (250 mm < MAP < 450 mm), semihumid (450 mm < MAP < 650 mm) and humid (MAP > 650 mm) precipitation regimes, respectively. Second, regional precipitation regimes affected the sensitivity of ANPP’s interannual variability to temperature and precipitation; ANPP is more sensitive to temperature in wetter areas than in dryer areas of the TP region. The results of our study suggest that in the region of Tibetan Plateau, the response of grassland ANPP variation to climate was constrained by the background precipitation regimes.
The high uncertainty associated with the response of terrestrial carbon (C) cycle to climate is dominated by ecosystem C turnover time (τ eco ). Although the relationship between τ eco and climate has been extensively studied, significant knowledge gaps remain regarding the differential climate sensitivities of turnover time in major biomass (τ veg ) and soil (τ soil ) pools, and their effects on vegetation and soil C sequestration under climate change are poorly understood. Here, we collected multiple time series observations on soil and vegetation C from permanent plots in 10 Chinese forests and used model-data fusion to retrieve key C cycle process parameters that regulate τ soil and τ veg . Our analysis showed that τ veg and τ soil both decreased with increasing temperature and precipitation, and τ soil was more than twice as sensitive (1.27 years/°C, 1.70 years/100 mm) than τ veg (0.53 years/°C, 0.40 years/100 mm). The higher climate sensitivity of τ soil caused a more rapid decrease in τ soil than in τ veg with increasing temperature and precipitation, thereby significantly reducing the difference between τ soil and τ veg (τ diff ) under warm and humid conditions. τ diff , an indicator of the balance between the soil C input and exit rate, was strongly responsible for the variation (more than 50%) in soil C sequestration. Therefore, a smaller τ diff under warm and humid conditions suggests a relatively lower contribution from soil C sequestration. This information has strong implications for understanding forest C-climate feedback, predicting forest C sink distributions in soil and vegetation under climate change, and implementing C mitigation policies in forest plantations or soil conservation. Plain Language SummaryCarbon turnover time is the average time that a carbon atom stays in an ecosystem from entrance to exit. Together, ecosystem carbon input via photosynthesis (i.e., productivity) and carbon turnover time determine ecosystem carbon sequestration. However, in contrast to the well-studied ecosystem productivity, carbon turnover time was found to dominate the uncertainty in terrestrial carbon sequestration and its response to climate. However, the climate sensitivities of carbon turnover times in various plant and soil pools and their effects on carbon storage have not been well-studied. Here, we quantified that carbon turnover time in soil (τ soil ) was more sensitive to climate than that of vegetation (τ veg ). This finding indicated the difference between τ veg and τ soil (τ diff ) being shortened in warm and humid regions. We further found that τ diff , as an indicator of the balance between soil carbon input and the carbon exit rate, is closely associated with the capacity for soil carbon sequestration. Therefore, a decreasing τ diff with increasing temperature/precipitation indicates a smaller proportion of carbon sequestered by soil than vegetation. Our findings facilitate understanding of carbon-climate feedback and the prediction of carbon sink distributions GE ET AL.
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