Do N-fixing legumes promote neighbouring diversity in the tropics?

1. Although nitrogen-fixing plants play a crucial role in maintaining high ecosystem productivity, their effects on forest diversity are greatly debated. Legumes can reduce local diversity because of the fertilization effect; however, they can also facilitate species coexistence through complementary resource utilization. 2. In natural forests, nitrogen requirements and biological nitrogen fixation (BNF) vary widely among legumes. We used leaf nitrogen isotopic composition to differentiate the level of BNF activity among seven legumes across a steep gradient of soil available nitrogen within a 60-ha stem-mapping plot in a montane tropical rainforest in Hainan Island, China, and we evaluated their spatial distribution and neighbourhood diversity. 3. Results show that the levels of BNF activity are tightly correlated with legume association to available soil nitrogen, where legumes associated with nitrogen-rich habitats exhibit surprisingly greater BNF activity and a more diverse neighbourhood. 4. Synthesis. These findings indicate that legumes satisfy their nitrogen-demanding metabolism through a synergy of habitat preference and BNF activity. High BNF activity drives local diversity by promoting complementary resource utilization rather than intensifying above-ground competition. This may be achieved by improving litter quality and stimulating mycorrhizal and microbial diversity. The abundance of nitrogen fixers in nitrogen-rich habitats also explains how legumes contribute to maintaining high levels of soil nitrogen in tropical forests.