Climate and substrate stoichiometry co-regulate free-living nitrogen fixation at regional and global scales

The traditional view holds that the global distribution of biological nitrogen fixation (BNF) in terrestrial ecosystems is primarily driven by climate, associating certain warm, wet low-latitude regions with higher BNF rates. However, this view fails to explain the low free-living BNF rates observed in these regions. Here, we conducted two field experiments and a global synthesis to assess the regulatory patterns of free-living BNF at both regional and global scales. The field experiments showed that the regional distributions of litter free-living BNF did not necessarily peak at warm and wet sites, due to co-regulation by climate and substrate stoichiometry (especially C : [N : P]). Subsequently, the global synthesis demonstrated that these phenomena and co-regulatory patterns persisted in free-living BNF in litter and other substrates (e.g., soil) at global scale. Our findings highlight the important role of substrate stoichiometry in regulating the spatial distribution of free-living BNF.