Biological nitrogen fixation (BNF) is the conversion of atmospheric nitrogen gas to a form that is available for uptake by plants. This process, which is carried out by a type of microorganism known as diazatrophs, is responsible for 97% of new N inputs into unmanaged terrestrial ecosystems. Most current research focuses on BNF of microbes that live in symbiotic association with plant roots. However, free-living N fixing diazatrophs are highly abundant in many soils, and their contribution to ecosystem N cycling is not well understood. While inputs of readily available N such as fertilizer are known to inhibit BNF, very little is known about the relationship between BNF and recalcitrant N availability, or availability of N that is tightly bound in the soil. This is a major knowledge gap, because diazatrophs are very common in environments where the vast majority of N exists in recalcitrant pools. This project will 1) screen diazatroph species to determine the proportion that can access recalcitrant N pools, 2) study the effects of recalcitrant N on diazatroph BNF, and 3) quantify the relationships between BNF and degradation of recalcitrant N. Because N availability is a major limitation for plant growth in most terrestrial ecosystems, this project has significant implications for managing fertilization and agricultural productivity. The project will involve undergraduate and high school students, and the results will be integrated into an inquiry-based learning module for teachers.
The role of recalcitrant N in regulating BNF by free-living diazatrophs is entirely unknown. This project proposes and tests a Labile-Atmospheric-Recalcitrant (LAR) N-acquisition strategy, a novel framework for how diazotrophs obtain cellular nitrogen based on cost-benefit principles, with large implications at the ecosystem scale. The project will measure N acquisition of diazatrophs in pure culture under conditions of low labile N but high recalcitrant N availability. These experiments will be followed by pilot incubations in field soils to investigate links between exoenzyme activity and BNF in natural ecosystems. The experiments will contribute to a process-level understanding of BNF by directly characterizing the role of N fixation in the overall N acquisition strategies of diazotrophic microbes. The results will ultimately contribute to microbially-explicit predictive models of BNF in terrestrial ecosystems.