Nitrogen fixation in dry tropical forests

Background/Question/Methods

Nitrogen (N) availability is one of the most important constraints on net primary productivity in tropical forests. But most biogeochemical research in the tropics has been carried out in wet forests, leaving dry tropical forests (DTFs) comparatively understudied. Because DTFs make up more than two-fifths of tropical forests worldwide, understanding how N fixation operates in these ecosystems is essential for efforts to model the N cycle. Over the last several years, we have conducted a linked set of field surveys and shade house experiments in Costa Rica and Panama to determine the variability of N fixation across this biome and to understand which environmental factors ultimately constrain this process. First, we assessed how legume species adjust N fixation in response to changes in resource availability and the composition of microbial symbiont communities. Second, we measured the magnitude of symbiotic N fixation – relative to other inputs of N – to DTF ecosystems in diverse secondary forests. 

Results/Conclusions

Legume seedlings grown in shade houses fixed more N under increased light and/or phosphorus (P), while adding extra N from fertilizer decreased fixation rates. However, legume species regulated their N fixation in response to the availability of environmental resources to varying degrees, with strategies ranging from obligate to facultative. When we manipulated bacterial symbiont identity, seedlings differed in both total biomass and nodule mass, but plant species effects accounted for more variation among treatments. In diverse secondary DTFs, N fixation by legumes was 73 % higher than inputs from both free-living fixation and atmospheric N deposition. The degree of nodulation in mature legume trees was not related to soil N or P but did exhibit strong seasonal variation. Because nodule mass reached its minimum during the dry season, we suggest that water availability is the dominant control on N fixation and could act to mediate its response to other environmental factors. These results highlight the possibility that, should precipitation decrease across Mesoamerica, as is predicted, declining N inputs from fixation could impose new limits on the productivity of dry tropical forests across the region.