Optimal methods for incorporating soil microbial mechanisms of carbon (C) cycling into Earth system models (ESMs) are still under debate. Specifically, whether soil microbial physiology parameters and residual materials are important to soil organic C (SOC) content is still unclear. Here, we explored the effects of biotic and abiotic factors on SOC content based on a survey of soils from 16 locations along a ~4000 km forest transect in eastern China, spanning a wide range of climate, soil conditions, and microbial communities. We found that SOC was highly correlated with soil microbial biomass C (MBC) and amino sugar (AS) concentration, an index of microbial necromass. Microbial C use efficiency (CUE) was significantly related to the variations in SOC along this national‐scale transect. Furthermore, the effect of climatic and edaphic factors on SOC was mainly via their regulation on microbial physiological properties (CUE and MBC). We also found that regression models on explanation of SOC variations with microbial physiological parameters and AS performed better than the models without them. Our results provide the empirical linkages among climate, microbial characteristics, and SOC content at large scale and confirm the necessity of incorporating microbial biomass and necromass pools in ESMs under global change scenarios.

While most studies account for the effect of total annual precipitation on aboveground net primary production (ANPP), few studies have demonstrated how decreased precipitation within specific periods of growing season affects ANPP. Using a precipitation manipulation experiment, we assessed the response patterns and underlying mechanisms of ANPP to decreased precipitation in the early (DEP) and late (DLP) growing seasons on a temperate steppe in northern China. The results showed that both DEP and DLP decreased ANPP by averages of 28.3 and 39.4 g m−2, respectively. The decline of ANPP in DEP was primarily attributed to a 20.7 g m−2 decrease in grass and 4.9 g m−2 decrease in sub‐shrubs, whereas the decrease in ANPP in DLP was due to a 36.6 g m−2 decrease in perennial forbs and an 8.7 g m−2 decrease in sub‐shrubs. The reduction in grass ANPP under DEP was likely due to decreased soil moisture and increased specific leaf area of grass during the early growing season, whereas the reduction in perennial forbs ANPP under DLP was mainly caused by decreased soil moisture during late growing season. The decline of sub‐shrubs ANPP under DEP and DLP could be explained by shallow root length of sub‐shrubs in DEP and by high soil temperature in DLP. Our findings demonstrated how droughts timing affects ANPP at both community and functional group levels. Different responses of functional groups to drought timing indicate that future studies should not neglect the role of precipitation timing in regulating ecosystem productivity.

Global changes and human disturbances can strongly affect the quantity of aboveground litter entering soils, which could result in substantial cascading effects on soil biogeochemical processes in forests. Despite extensive reports, it is unclear how the variations in litter depth affect soil carbon and nitrogen cycling. The responses of soil carbon and nitrogen to the variability of litter inputs were examined in a coniferous–broadleaf mixed forest of Central China. The litter input manipulation included five treatments: no litter input, natural litter, double litter, triple litter, and quadruple litter. Multifold litter additions decreased soil temperature but did not affect soil moisture after 2.5 years. Reductions in soil pH under litter additions were larger than increases under no litter input. Litter quantity did not affect soil total organic carbon, whereas litter addition stimulated soil dissolved organic carbon more strongly than no litter input suppressed it. The triggering priming effect of litter manipulation on soil respiration requires a substantial litter quantity, and the impacts of a slight litter change on soil respiration are negligible. Litter quantity did not impact soil total nitrogen, and only strong litter fluctuations changed the content of soil available nitrogen (nitrate nitrogen and ammonium nitrogen). Litter addition enhanced soil microbial biomass carbon and nitrogen more strongly than no litter input. Our results imply that the impacts of multifold litter inputs on soil carbon and nitrogen are different with a single litter treatment. These findings suggest that variability in aboveground litter inputs resulting from environmental change and human disturbances have great potential to change soil carbon and nitrogen in forest ecosystems. The variability of aboveground litter inputs needs to be taken into account to predict the responses of terrestrial soil carbon and nitrogen cycling to environmental changes and forest management.

Terrestrial gross primary productivity (GPP) is an important flux that drives the global carbon cycle. However, quantifying the trend and the control factor of GPP from the pixel level to the regional level is still a challenge. We generated monthly GPP dataset using the vegetation photosynthesis model and calculated the interannual linear trend for China during 2000-2016. The Breaks For Additive Seasonal and Trend method was applied to detect the timing of breakpoint and trends shift of monthly GPP, while boosted regression tree analysis was used to identify the most important factor and its relative influence on GPP based on gridded leaf area index (LAI), aerosol optical thickness, and NCEP-DOE Reanalysis II meteorological data. The results show that annual mean GPP was significantly (P<0.001, R 2 =0.78) increased, especially in the Loess Plateau and South China, from 2000 to 2016. The change rate of annual mean GPP declined from 18.82 g C m −2 yr −1 in 2000-2008 to 3.48 g C m −2 yr −1 in 2008-2016. About 55.4% of the breakpoints occur between 2009 and 2011 and was mainly distributed in Qinghai-Tibet Plateau, Central China, Southwestern China, and South China, and negative oriented GPP trends variation type still accounts for about 28.76%. LAI and temperature related factors generally had the highest relative influence on GPP in the north part and south part of China, respectively. Our study indicates that the ecological restoration projects and rapid urbanization have respectively induced the most obvious increase and decrease trends of GPP in China. Land cover change and climate change are the main reasons for GPP dynamics in the north and south part of China, respectively.

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