Seminar

Abstract

Reactive nitrogen oxides (NOy = NO + NO2 + HONO) decrease air quality and indirectly impact radiative forcing, yet the factors responsible for their emission from non-point sources (i.e., soils) remain poorly understood. Our team investigated the factors that control the production of NOy in forest soils using genomics techniques, process-based assays and inhibitor experiments, and used these data to identify putative hotspots for gas emissions across forests of the Eastern and Midwestern USA. I will discuss our findings of how nitrogen oxide soil emissions are mediated by microbial community structure (e.g., ammonium oxidizer abundances), soil characteristics and nitrogen transformation rates. We found that while nitrification rates are controlled primarily by ammonia-oxidizing archaea (AOA), production of NOy is mediated in large part by ammonia-oxidizing bacteria (AOB). Variation in nitrification rates and nitrogen oxide emissions tracked variation in root-associated fungi, with stands dominated by arbuscular mycorrhizal (AM) trees having greater N transformation rates and NOy fluxes than stands dominated by ectomycorrhizal (ECM) trees. Given mapped distributions of AM and ECM trees from 78,000 forest inventory plots, we estimate that forests of the upper Midwestern USA and Mississippi River Valley may be considered hotspots for NOy emission, which generally coincides with areas receiving the highest N deposition. Together, our results greatly improve our understanding of nitrogen oxide fluxes from forests, which should lead to improved predictions about the atmospheric consequences of tree species shifts owing to land management and climate change.

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Jonathan Raff is a professor at Indiana University with joint appointments in Chemistry and the School of Public and Environmental Affairs. His research focuses on atmospheric nitrogen chemistry, and its links to biological processes and surface chemistry. His recent recognitions include an NSF CAREER award and a DOE Early Career Research Program award.