PARENT SESSION
Thursday, August 10, 1:30-5:00 pm
COS 92 - Nitrogen cycling II
Sultana, Mezzanine Level, Cook Convention Center
Presiders: J Ely and S Baer
Biogeochemical and microbial community controls over free-living nitrogen fixation in a lowland tropical rain forest.
Reed, Sasha*,1, Nemergut, Diana1, Cleveland, Cory1, Townsend, Alan1, 1 University of Colorado, Boulder, Boulder, CO
ABSTRACT- A growing body of evidence suggests that free-living nitrogen (N) fixation is a significant source of N to tropical rain forest ecosystems. However, the biotic and abiotic causes of high N fixation in such systems remain debated, especially in light of the prevalence of P-poor and N-rich soils found within the biome. Our prior work in forests in southwest Costa Rica documented high rates of free-living N fixation and showed that fertilization with phosphorus (+P) stimulated even higher rates. However, significant seasonal and spatial variations in N fixation rates imply a complex set of controls. In addition, other data from these sites show the potential for large, rapid shifts in microbial communities following changes in resource availability. Thus, we hypothesized that P fertilization might not only enhance N fixation by alleviating a key nutrient limitation, but also by stimulating a change in the community composition of N-fixing microorganisms. In the fall of 2005, we used the acetylene reduction assay (ARA) to show that P fertilization enhanced litter N fixation by nearly fourfold. Litter samples used in the ARA analyses were then returned to Colorado where we extracted bulk DNA, performed nested PCR of the nifH (N fixation) gene, and created clone libraries. Subsequent comparisons of these libraries suggested that +P litter contained a significantly different N-fixer community than that found in control samples, perhaps implying that the higher N fixation rates may be at least partly a consequence of a P-induced shift in the microbial community composition.
Key words: nitrogen fixation, lowland tropical rain forest, nitrogen fixing microorganism