Variations in biological nitrogen (N) fixation strongly structure the N cycle of ecosystems with impacts on nutrient losses, community and successional dynamics, and responses to global change. Despite extensive research into the mechanistic controls on N fixation in agricultural ecosystems, we still know little about the ecological controls on N fixation in natural terrestrial ecosystems. This is especially true for diverse lowland tropical forests, which harbor an abundance of putative N fixing organisms, even in mature forests. In this study, we evaluate the relationship between fixation rates for adult legume trees in the genus Inga and concentrations and fluxes of different soil nutrients across four typical forest types in a lowland tropical forest.

Results/Conclusions

Nodulation and fixation in our sites was strongly to correlated to soil N concentration, increasing rapidly with decreasing levels of soil nitrate. Contrary to expectation, N fixation was not limited in soils with low phosphorus (P), but rather appeared to decrease with increasing P - most likely due to parallel changes in N and P with site fertility. No other soil parameters appeared to be closely correlated with fixation. Overall, our results suggest that adult legume trees suppress nodule formation when soil nitrate is abundant and that this is a key determinant of N fixation rates of canopy trees in the field.