Symbiotic N2 fixation in tropical forests: Scaling from individuals to ecosystems

Abstract

Symbiotic N2 fixation is a critical ecosystem process that provides new nitrogen to ecosystems and may support a tropical forest CO2 sink, yet it is still unclear when fixation occurs across tropical forests, and what controls where and how much is present. Nitrogen and phosphorus may be particularly important for fixation, both on the individual plant and whole ecosystem scale. In this dissertation, I test specific mechanisms of how fixers adjust their strategy of fixation to soil nitrogen and phosphorus levels and what that means for the ecosystem by using experimental, theoretical and empirical approaches. Fixation is strongly controlled by soil nitrogen availability, but, although phosphorus can constrain fixers growing in a greenhouse, it has little influence on fixer abundance across tropical forests. Fixation by a diverse suite of species is especially critical for the CO2 sink in young regrowing forests, and supplies >50% of the nitrogen needed to support the accumulation of over 50 tons of carbon per hectare in the first decade of recovery. Fixation rates are low in mature forests, although some fixation occurs by species that take advantage of localized disturbance. These patterns suggest that plants up-regulate fixation when there is a mis-match in plant nitrogen demand and soil nitrogen supply. The findings of this dissertation indicate that ecosystem-scale patterns of fixation emerge from the behavior of individual plants and their interactions with the localized environment, and point to control by plant-soil nitrogen imbalance for regulating fixation.