The contribution of biotic factors in explaining the global distribution of inorganic carbon in surface soils

Soil inorganic carbon (SIC) plays a crucial role in regulating global carbon (C) cycling by linking the long-term geological and short-term biological C cycles. Soil inorganic carbon stocks are thought to be mainly driven by abiotic factors. However, despite the well-known influence of vegetation and soil microbes on terrestrial C pools, the relative contribution of biotic and abiotic factors in explaining the global distribution of SIC remains virtually unknown. Here, we conducted a global field survey including information on SIC of 398 composite topsoil samples from 134 locations to investigate the contribution of biotic drivers in explaining the global distribution of SIC in surface soils compared with climate and abiotic factors. Overall, SIC content peaked in arid and temperate ecosystems with warmer and drier conditions, particularly shrublands. We further revealed that although soil properties (e.g., Ca and C/N ratio) explained the highest variance in SIC globally, biotic factors, associated with vegetation and soil microbes, explained a considerable proportion of the global variation in SIC. In particular, plant richness, plant cover, and fungal biomass were significantly and positively associated with SIC, suggesting that biotic control could play an important role in explaining the global distribution of topsoil SIC. We propose that changes in the biotic factors, such as alterations in vegetation and soil microbes resulting from global changes, may have important direct and indirect consequences for global SIC dynamics and terrestrial C-climate feedback.