Friday, August 10, 2007 - 8:20 AM

OOS 49-2: High rates of total nitrogen fixation in a tropical dry forest

Louis Santiago, University of California, Riverside

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.

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