Rainfall Shapes the Diversity of Soil Nitrogen-Fixing Microorganisms Worldwide

Soil nitrogen-fixing microorganisms naturally fertilize terrestrial ecosystems, but the primary driver of their diversity across the globe and the underlying mechanisms remain unclear. We analyzed the nifH gene in 1257 (1137 publicly available + 120 self-generated) soil metagenomes from 318 terrestrial ecosystems globally. Mean annual precipitation was identified as the key factor influencing the relative abundance, richness, and composition of the potential nitrogen-fixers. Precipitation was directly associated with nitrogen-fixers (e.g., water availability) rather than indirectly via other soil variables (e.g., pH). Lower precipitation increased the contribution of deterministic processes (e.g., interspecific competition) in driving their community assembly and selected species with larger genomes, while higher precipitation increased the contribution of stochastic processes (e.g., random birth/death) and favored smaller-genome species. A multifactorial experiment further demonstrated that precipitation increase had a larger regulatory effect on the stochastic processes than other factors (e.g., climate warming). eXtreme Gradient Boosting (XGBoost) projections under future global change scenarios indicate a general increase in their relative abundance across most regions worldwide, with declines only in specific areas. These findings reveal distinct patterns and mechanisms governing the global biodiversity and biogeography of soil nitrogen-fixers, providing valuable insights for developing region-specific management strategies aimed at maintaining ecosystem productivity.