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 NOyin 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 NOyis 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 NOyfluxes 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 NOyemissions. Together, our results greatly improve our understanding of NOyfluxes from forests, which should lead to improved predictions about the atmospheric consequences of tree species shifts owing to land management and climate change.

Pyrogenic carbon (PyC) constitutes an important pool of soil organic matter (SOM), particularly for its reactivity and because of its assumed long residence times in soil. In the past, research on the dynamics of PyC in the soil system has focused on quantifying stock and mean residence time (MRT) of PyC in soil, as well as determining both PyC stabilization mechanisms and loss pathways. Much of this research has focused on decomposition as the most important loss pathway for PyC from soil. However, the low density of PyC and its high concentration on the soil surface after fire indicates that a significant proportion of PyC formed or deposited on the soil surface is likely laterally transported away from the site of production by wind and water erosion. Here, we present a synthesis of available data and literature to compare the magnitude of the water-driven erosional PyC flux with other important loss pathways, including leaching and decomposition, of PyC from soil. Furthermore, we use a simple first-order kinetic model of soil PyC dynamics to assess the effect of erosion and deposition on residence time of PyC in eroding landscapes. Current reports of PyC MRT range from 250 to 660 years. Using a specific example-based model system, we find that ignoring the role of erosion may lead to the under-or over-estimation of PyC MRT on the centennial time scale. Furthermore, we find that, depending on the specific landform positions, timescales considered, and initial concentrations of PyC in soil, ignoring the role of erosion in distributing PyC across a landscape can lead to discrepancies in PyC concentrations on the order of several 100 g PyC m −2 . Erosion is an important PyC flux that can act as a significant control on the stock and residence time of PyC in the soil system.

Pyrogenic carbon (PyC) is an incomplete combustion by-product with longer soil residence times compared with nonpyrogenic components of the soil carbon (C) pool and can be preferentially eroded in fire-affected landscapes. To investigate geomorphic and fire-related controls on PyC erosion, sediment fences were established in three combinations of slope (high 13.9-37.3%; moderate 0-6.7%) and burn severity (high; moderate) plots within the perimeter of the Rim Fire in 2013, Yosemite National Park, California, USA. After each major precipitation event following the fire, we determined transport rates of total sediment, fine and coarse sediment fractions, and C and nitrogen (N). We measured stable isotope (δ 13 C and δ 15 N) compositions and 13 C-nuclear magnetic resonance spectra of soils and eroded sediments. The highest total and fine (<2 mm) sediment transport in high severity burned areas correlated with initial discharge peaks from an adjacent stream, while moderate burn severity sites had considerably more of the >2 mm fraction transported than high burn severity sites. The δ 13 C and δ 15 N values and 13 C-nuclear magnetic resonance analyses indicated that sediment eroded from moderate severity burn areas included fresh organic matter that was not as significantly affected by the fire, whereas sediments from high severity burn areas were preferentially enriched in PyC. Our results indicate that along a single hillslope after the Rim Fire, burn severity acted as a primary control on PyC transport postfire, with slope angle likely playing a secondary role. The preferential erosion of PyC has major implications for the long-term persistence of PyC within the soil system.

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