Symbiotic nitrogen fixation in a tropical rainforest:: 15N natural abundance measurements supported by experimental isotopic enrichment

Leguminous trees are very common in the tropical rainforests of Guyana. Here, species-specific differences in N-2 fixation capability among nodulating legumes growing on different soils and a possible limitation of N-2 fixation by a relatively high nitrogen (N) and low phosphorus (P) availability in the forest were investigated. Leaves of 17 nodulating species and 17 non-nodulating reference trees were sampled and their delta N-15 values measured. Estimates of N-2 fixation rates were calculated using the N-15 natural abundance method. Pot experiments were conducted on the effect of N and P availability on N-2 fixation using the N-15-enriched isotope dilution method. Nine species showed estimates of > 33% leaf N derived from N-2 fixation, while the others had low or undetectable N-2 fixation rates. High N and low P availability reduced N-2 fixation substantially. The results suggest that a high N and low P availability in the forest limit N-2 fixation. At the forest ecosystem level, N-2 fixation was estimated at c. 6% of total N uptake by the tree community. We conclude that symbiotic N2 fixation plays an important role in maintaining high amounts of soil available N in undisturbed forest.