Stoichiometry of multiple soil nutrients controls leguminous nitrogen fixation in a tropical forest

Background/Question/Methods

Biological N₂ fixation is the main source of nitrogen inputs for tropical forest ecosystems and may constrain the ability of these systems to respond to elevated CO₂. In spite of the importance of this process, we have a limited understanding of how soil nutrients control symbiotic N₂ fixation in nature. Abundant soil nitrogen should suppress biological N₂ fixation by leguminous trees, while phosphorus and molybdenum, key elements in both functioning of the nitrogenase enzyme and the growth of plants, may constrain the ability of legumes to support the metabolic processes of symbiotic N₂ fixation. Here we explore the role of N, P and Mo in controlling symbiotic N₂ fixation by tropical leguminous trees. Across a series of highly diverse lowland forests of Panama that vary widely in soil N and P availability, we sampled 100 canopy-height trees across six species groups of leguminous N₂-fixers. From each tree we sampled nodule biomass, N₂ fixation rates (acetylene reduction activity) and soil chemistry. In a complementary greenhouse fertilization study, we explored the role of P and Mo as limiting elements of N₂ fixation in seedlings of Inga punctata.

Results/Conclusions

From our field analysis, roughly half of the sampled leguminous trees possessed N₂ fixing nodules. Fixation rates (both nodulation levels and acetylene reduction activities) varied widely across tree species groups and individuals. The most prolific N₂-fixers were Abarema barbouriana, Enterolobium schomburgkii, Inga spp. (Mimosoideae) and Tachigali versicolor (Caesalpinioideae). Most individuals of Platypodium elegens and Lonchocarpus latifolius (Papilionoideae) supported little or no N₂ fixation. Across all tree species, the highest rates of symbiotic N₂ fixation were consistently observed in soils with low extractable N:P ratios. Our field study suggests that N and P interact to control symbiotic N₂ fixation. While in general, N acts to suppress and P acts to promote fixation, the stoichiometry of these two elements was the best predictor of N₂ fixation rates by legumes in tropical forests. In our greenhouse fertilization experiment, relative investment of N₂ fixation by Inga seedlings responded positively to P availability. However, seedling growth and total N₂ fixation were co-limited by both P and Mo. The results of our field and greenhouse studies indicate that symbiotic N₂ fixation in tropical forests is tightly regulated by the availability and interaction of multiple soil nutrients.