Of all the plant functional groups and traits in tropical forests, nitrogen-fixing trees and the trait of nitrogen fixation have the potential to impact ecosystem function in distinct and significant ways. Recent findings indicate that fixers have a powerful impact on ecosystems by bringing in large quantities of nitrogen and supporting carbon sequestration in tropical forests. But nitrogen fixers belong to the most abundant and biodiverse family of tropical tree species – the Fabaceae, which raises the question of whether nitrogen-fixing tree species function in similar ways across tropical rainforest ecosystems, or if taxonomic differences in fixers across the legume phylogeny lead to distinct functions. I will examine this question by considering a series of collaborative studies that look at species differences and the functional biodiversity of fixers in tropical biogeochemical cycles. These studies include an out-planting experiment, a large-scale ecosystem experiment and an evaluation of a large-scale dataset that captures the dynamics and patterns of fixers and non-fixers in Neotropical forests (RAINFOR).
My findings indicate that nitrogen-fixing taxa function in distinct ways within tropical forest ecosystems. Across the fixer phylogeny, nitrogen-fixing tree taxa have different maximum fixation rates, responses of fixation to soil phosphorus, and abundances in forests across gradients in soil nutrients and disturbance. Further analyses suggest that the ecosystem function of fixers (nitrogen fixation activity and their contribution to carbon accumulation) cannot be determined by the abundance of fixers in a forest, and that we need to understand the species identity and the fixation activity of individual trees in order to determine their effect on ecosystems. Even though fixers are often considered one functional group and have the potential to collectively contribute a major input of new nitrogen to tropical ecosystems, I conclude that fixer species have evolved to partition distinct biogeochemical niches. Thus, the biodiversity of nitrogen fixers creates redundancy and resilience across tropical forests, and contributes to ecosystem functioning via the insurance that, in forests with biodiverse fixers, at least some taxa would be able to fix nitrogen when needed and respond to climate change.