High rates of total nitrogen fixation in a tropical dry forest

Tropical legumes are usually the dominant family in forests of the Neotropics and Africa, and thus represent the greatest potential terrestrial cover of N-fixing species globally. Tropical dry forests, where legumes are particularly abundant, may have the greatest rates of biological N-fixation of any ecosystem type. I determined the number of legume species that fix N along an early successional gradient (10, 20, and 30 years) and quantified heterotrophic N-fixation by free-living fixers in litter and soil. 21 out of 23 legume species were nodulated and calculations of percent N from fixation, based on N isotopic composition, ranged from 1-78 percent for individual species. At the stand level, estimates of symbiotic N-fixation were 13.9, 23.1, and 27.8 kg N/ha/yr in 10, 20, and 30 year old forest, respectively. Total heterotrophic N-fixation, based on acetylne reduction incubations of leaf litter, fine woody debris and surface soil was 7.5 kg N/ha/yr during the dry season and 17.1 kg N/ha/yr during the wet season. Total ecosystem estimates were 38.5, 47.7, and 52.4 kg N/ha/yr in 10, 20, and 30 year old forest, respectively. Overall, heterotrophic N-fixation was a greater proportion of total biological N-fixation than has been reported in other tropical dry forests and savanas, but total biological N-fixation values were similar. The unique finding of relatively high rates of heterotrophic N-fixation appears to be related to high litter quality. When litter quality is high, N supply can limit rates of decomposition and there can be an appreciable benefit to heterotrophic N-fixation.