Understanding the temperature sensitivity (Q ) of soil organic C (SOC) decomposition is critical to quantifying the climate-carbon cycle feedback and predicting the response of ecosystems to climate change. However, the driving factors of the spatial variation in Q at a continental scale are fully unidentified. In this study, we conducted a novel incubation experiment with periodically varying temperature based on the mean annual temperature of the soil origin sites. A total of 140 soil samples were collected from 22 sites along a 3,800 km long north-south transect of forests in China, and the Q of soil microbial respiration and corresponding environmental variables were measured. Results showed that changes in the Q values were nonlinear with latitude, particularly showing low Q values in subtropical forests and high Q values in temperate forests. The soil C:N ratio was positively related to the Q values, and coniferous forest soils with low SOC quality had higher Q values than broadleaved forest soils with high SOC quality, which supported the "C quality temperature" hypothesis. Out of the spatial variations in Q across all ecosystems, gram-negative bacteria exhibited the most importance in regulating the variation in Q and contributed 25.1%, followed by the C:N ratio (C quality), fungi, and the fungi:bacteria ratio. However, the dominant factors that regulate the regional variations in Q differed among the tropical, subtropical, and temperate forest ecosystems. Overall, our findings highlight the importance of C quality and microbial controls over Q value in China's forest ecosystems. Meanwhile, C dynamics in temperate forests under a global warming scenario can be robustly predicted through the incorporation of substrate quality and microbial property into models.

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SummaryWorld-wide conversion of natural forests to other land uses has profound effects on soil microbial communities. However, how soil microbial b-diversity responds to land-use change and its driving mechanisms remains poorly understood. In this study, therefore, we examined the effect of forest conversion from native broad-leaved forest to coniferous plantation on soil microbial b-diversity and its underlying mechanisms in both summer and winter in subtropical China. Microbial communities increasingly differed in structure as geographical distance between them increased, and the slope of the relationship among distances and community similarity differed among forest covers. In general, as with microbial b-diversity, slopes also shifted across seasons. Finally, null deviations of bacterial and fungal communities were lower in coniferous plantation and presented opposing seasonal variations with greater influences of deterministic processes in summer for soil fungi and in winter for soil bacteria. Integrating previous frameworks with our b-null model results, we propose a conceptual model to link microbial secondary succession to stochastic/deterministic shifts in forest ecosystems. Overall, forest conversion induced significant increases in stochastic processes in both bacterial and fungal community assemblies. Therefore, our results highlight the importance of spatiotemporal scales to assess the influence of land-use change on microbial b-diversity.