Edited by Sarah E. Hobbie, University of Minnesota, St. Paul, MN, and approved December 14, 2018 (received for review August 24, 2018)
Reactive nitrogen oxides (NOy) can negatively impact air quality, visibility, and human health. Biogenic sources of NOy are poorly understood despite their growing importance in future scenarios of decreasing anthropogenic emissions. Using soil from two forest stands that differ in plant and soil characteristics, we show that ammonia-oxidizing bacteria and to a lesser extent, heterotrophic microbes are the primary producers of NOy. Moreover, we show that ammonia-oxidizing archaea contribute little to NOy yet play a central role in determining nitrogen-cycle rates in forests. Given known relationships between tree species and soil characteristics, we conclude that NOy emissions are spatially variable, occurring in hotspots throughout the eastern United States.
Reactive nitrogen oxides (NOy; NOy = NO + NO2 + HONO) decrease air quality and impact radiative forcing, yet the factors responsible for their emission from nonpoint sources (i.e., soils) remain poorly understood. We investigated the factors that control the production of aerobic NOy in forest soils using molecular techniques, process-based assays, and inhibitor experiments. We subsequently used these data to identify hotspots for gas emissions across forests of the eastern United States. Here, we show that nitrogen oxide soil emissions are mediated by microbial community structure (e.g., ammonium oxidizer abundances), soil chemical characteristics (pH and C:N), and nitrogen (N) transformation rates (net nitrification). We find that, while nitrification rates are controlled primarily by chemoautotrophic ammonia-oxidizing archaea (AOA), the production of NOy is mediated in large part by chemoautotrophic ammonia-oxidizing bacteria (AOB). Variation in nitrification rates and nitrogen oxide emissions tracked variation in forest communities, as stands dominated by arbuscular mycorrhizal (AM) trees had 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 broadleaf forests of the Midwest and the eastern United States as well as the Mississippi River corridor may be considered hotspots of biogenic NOy emissions. Together, our results greatly improve our understanding of NOy fluxes from forests, which should lead to improved predictions about the atmospheric consequences of tree species shifts owing to land management and climate change.
Author contributions: R.M.M., R.P.P., Z.C.P., J.R.W., D.B.R., and J.D.R. designed research; R.M.M., Z.C.P., and R.B.A. performed research; I.J., S.F., and S.E.P. contributed new reagents/analytic tools; R.M.M., Z.C.P., R.B.A., and J.D.R. analyzed data; and R.M.M., R.P.P., Z.C.P., R.B.A., and J.D.R. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1814632116/-/DCSupplemental.
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