Diazotrophic Nitrogen Fixation in the Rhizosphere and Endosphere

Abstract

A wide variety of nitrogen (N)-fixing Bacteria and Archaea inhabit plant rhizospheres and endospheres. This diazotrophic community is often dominated by α-Proteobacteria, although many other taxa are also present. Diazotrophic community composition is influenced by physicochemical factors, including land use, geography, climate, and soil type. Similarly, soil N fixation rates are predicted by short-term soil physicochemical conditions (e.g., moisture, temperature, nutrient concentrations). Microbial community composition may also influence N fixation rates, but it is unclear if it does so through taxon-specific N fixation abilities, or if the physicochemical template influences the microbial community and N fixation rates similarly. In agricultural ecosystems, rhizosphere and endosphere N fixation is often assumed to be minimal, but recent mass balance data suggest that N fixation contributes nearly 25% of crop N requirements. In natural ecosystems, N fixation inputs are highly variable and uncertain, exceeding those of legumes in some systems, and barely detectable in others. Critical knowledge gaps and priorities for research include better estimates of rhizosphere and endosphere fixation rates, defining the role of diazotrophic community composition in influencing fixation rates, and elucidating the relationship between N-fixing microbes and plants.