Soil Organic Matter Quality, Rather than Quantity, Drives Spatial Variation of Soil Microbial Basal Respiration in Boreal Forests

Soil microbial basal respiration (BR), a key process for quantifying the soil-atmospheric CO 2 fluxes, has been extensively studied at individual sites. However, the controlling factors of the spatial variation in BR at larger spatial scales characterized by the same vegetation types are still poorly identified. Here, we examined the influence of soil organic matter (SOM) quantity, quality, soil physical, and microbial properties on BR along a 650 km north-south boreal forest transect in eastern China. The chemical structures of soil organic carbon (SOC) were characterized by 13 C-nuclear magnetic resonance spectroscopy, and soil microbial compositions were determined by high-throughput sequencing. Results showed that the variation of BR was independent of latitude both in organic and mineral layers. The BR in the O layer was significantly ( P < 0.001) higher than that in the M layer. Multi-model averaging and partial regression suggested that SOM chemical compositions exhibited the most importance controlling the variation in BR and contributed 23.0%, followed by the C/N ratio in organic layers. While in mineral layers, only soil C/N ratio was the most important predictor for changes in the BR. In addition, soil physical properties (pH, clay, silt, and sand) indirectly affected BR via regulating soil microbial composition (especially bacteria). Overall, we argue that the importance of C quality (C/N and SOC chemical structures) should be considered when exploring the BR in China’s forest ecosystems. These findings may contribute to improving the substrate quality modules in Earth system models for better prediction of soil CO 2 flux and the feedbacks to climate change.