IBL Tropical Forest Research Program Nitrogen Fixation A substantial amount of data from multiple, direct manipulations of nutrient availability in temperate and high latitude ecosystems suggest that in the absence of human intervention, nitrogen (N) limits plant growth and organic matter storage. In contrast, N appears relatively abundant in many tropical rain forests, and nutrient constraints on processes in tropical ecosystems frequently arise from shortages in rock-derived elements, such as base cations (Ca, K, Mg) or phosphorus (P). Nonetheless, rates of N fixation in tropical rain forests are among the highest measured on an ecosystem basis. In a system where N is often so abundant,why are rates of N fixation so high? Led by Sasha Reed, our group is studying this paradox by investigating the effect of soil and litter resource availability on N fixation rates, and how small scale changes in microbial community composition regulate the process.