Symbiotic dinitrogen (N 2)-fixing trees fulfill a critical function in tropical forests by bringing in new nitrogen, yet it remains unclear how they overcome constraints by highly weathered, nutrient-poor tropical soils. We advance forest biogeochemistry and microbial ecology with the discovery from field trials in Panama that fast-growing N 2-fixing trees in tropical forests exhibit accelerated mineral weathering and distinctive soil metagenomes that improve their access to inorganic nutrients in nutrient-poor soils. Furthermore, we show that N 2-fixing trees exert similar effects on non-N 2–fixing trees nearby thus having previously overlooked community-wide effects on tropical forest nutrient cycling. These results offer insights into the role of N 2-fixing trees and their associated microbiomes in safeguarding the function of tropical forests within the global biosphere.
Legume trees form an abundant and functionally important component of tropical forests worldwide with N 2-fixing symbioses linked to enhanced growth and recruitment in early secondary succession. However, it remains unclear how N 2-fixers meet the high demands for inorganic nutrients imposed by rapid biomass accumulation on nutrient-poor tropical soils. Here, we show that N 2-fixing trees in secondary Neotropical forests triggered twofold higher in situ weathering of fresh primary silicates compared to non-N 2–fixing trees and induced locally enhanced nutrient cycling by the soil microbiome community. Shotgun metagenomic data from weathered minerals support the role of enhanced nitrogen and carbon cycling in increasing acidity and weathering. Metagenomic and marker gene analyses further revealed increased microbial potential beneath N 2-fixers for anaerobic iron reduction, a process regulating the pool of phosphorus bound to iron-bearing soil minerals. We find that the Fe(III)-reducing gene pool in soil is dominated by acidophilic Acidobacteria, including a highly abundant genus of previously undescribed bacteria, Candidatus Acidoferrum, genus novus. The resulting dependence of the Fe-cycling gene pool to pH determines the high iron-reducing potential encoded in the metagenome of the more acidic soils of N 2-fixers and their nonfixing neighbors. We infer that by promoting the activities of a specialized local microbiome through changes in soil pH and C:N ratios, N 2-fixing trees can influence the wider biogeochemical functioning of tropical forest ecosystems in a manner that enhances their ability to assimilate and store atmospheric carbon.