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.

The response of soil organic carbon (SOC) decomposition to global warming is a potentially major source of uncertainty in climate prediction. However, the magnitude and direction of SOC cycle feedbacks under climate warming remain uncertain because of the knowledge gap about the global‐scale spatial pattern and temperature sensitivity (Q10) mechanism of SOC decomposition. Here, we collected data of Q10 and corresponding soil variables from 81 peer‐reviewed papers using laboratory incubation to explore how Q10 varied among different ecosystems at the global scale and whether labile and recalcitrant SOC pools had equal Q10 values. Q10 with a global average of 2.41 substantially varied among different ecosystems, ranging from the highest in cropland soils (2.76) and the lowest in wetland soils (1.84). Hump‐shaped correlations of Q10 values with the maximum at SOC = 190 g/kg and the minimum at clay = 37% were observed. However, the main influencing factors of Q10 differed among various ecosystems. Q10 values showed a clear decrease with increasing incubation temperature but no significant decrease above 25°C. In general, labile SOC was less sensitive than recalcitrant SOC to warming. Structural equation model analyses showed that total N and SOC accounted for 53% and 46%, respectively, of the variation in Q10 of labile SOC and recalcitrant SOC. This finding suggested that Q10 values of labile and recalcitrant SOC pools had different controlling factors. Our findings highlighted the importance of Q10’s variations in ecosystem types and the response of recalcitrant SOC to warming in predicting the soil C cycling and its feedback to climate change. Therefore, ecosystem type and difference in Q10 of labile and recalcitrant SOC should be considered to precisely predict the soil C dynamics under global warming. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13256/suppinfo is available for this article.

Aim: Soil organic carbon (SOC) stabilization has become an important topic in recent years in the context of global climate change. Microbial residues represent a significant component of stabilized SOC pools. However, spatial variations in the contributions of bacterial and fungal residues to SOC and their determinants at a continental scale remain poorly understood. We aimed to evaluate the spatial variations and controls of the contributions of microbial residues to SOC in forest topsoil.Location: North-south transect in eastern China. Time period: 2014. Major taxa studied: Forest ecosystems. Methods: A total of 195 surface (0-10 cm) soils were sampled from 28 forest sites across tropical and boreal forests in eastern China from July to August to assess how biotic and abiotic factors govern the geographic patterns of the contributions of soil microbial residues (indicated by amino sugars) to SOC.Results: Fungal residues (30.0%) had a greater average contribution to SOC than bacterial residues (15.5%). The contributions of bacterial (CBR) and total microbial residues (CMR) to SOC showed negative latitudinal patterns and were positively correlated with mean annual temperature (MAT). In contrast, the contribution of fungal residues to SOC (CFR) showed no clear geographic or climatic patterns. On average, the CBR (9.7%), CFR (21.1%) and CMR (30.8%) were lower in boreal forests than in other biome forests. SOC concentration negatively mediated CBR, CFR and CMR.The piecewise structural equation model results showed that MAT was the primary driver of the geographic pattern of CBR, whereas SOC and soil carbon : nitrogen ratio were more directly associated with the CFR. Additionally, plant factors and microbial properties (i.e., microbial biomass and composition) played relatively little roles in regulating CBR, CFR and CMR.Main conclusions: These findings advance the current knowledge of the different geographic patterns of CBR and CFR regulated by different potential mechanisms in forest ecosystems. This highlights that the dynamics of microbial residues could potentially have unexpected consequences for topsoil SOC stocks under climate change.

The temperature sensitivity (Q10) of soil organic carbon (SOC) decomposition is an important parameter for those seeking accurate projections of SOC dynamics and its feedback on climate change in terrestrial ecosystems. However, how Q10 responds to N deposition across environmental gradients and the underlying mechanism remain largely unresolved. We conducted a novel incubation experiment with periodically varying temperature based on the of soil origin sites to elucidate the responses of Q10 to N addition across China. Our results demonstrated that N addition effects (NAEs) on Q10 were negatively related to latitude and were strongly site dependent. Bioclimatic, edaphic, and microbial variables together explained 50.1% of the total variation in NAEs on Q10, but bioclimate (16.0%) had the greater explanation than edaphic (11.8%) and microbial properties (6.3%). The response of soil exchangeable Ca2+ to N addition was a predictive power for NAEs on Q10, contributing 7.2% relative importance in regulating this variation. Furthermore, arbuscular mycorrhizal fungi indicated by Glomeromycota were the best microbial predictor and contributed 10.9% relative importance in the variation regulating NAEs on Q10. Overall, our results suggest that increasing N addition will increase the sensitivity of SOC decomposition to global warming and highlight the importance of bioclimate, exchangeable Ca2+, and arbuscular mycorrhizal fungi in predicting the response of Q10 to N deposition in natural terrestrial ecosystems. The biogeographic variation in response of Q10 to N deposition should be considered in carbon‐climate models to decrease the prediction uncertainties of SOC dynamics and its feedback to global warming.

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