Microbial Soil Emissions of Nitric Oxide and Their Impact on Atmospheric Oxidative Capacity in a Mixed Deciduous Forest Clearing

Nitric oxide and nitrogen dioxide (NO_x = NO + NO₂) control the oxidative capacity of the troposphere by promoting the formation of ozone (O₃) and HO_x (OH + HO₂). As levels of anthropogenic NO_x emission decrease due to effective pollution control measures, the relative importance of NO­_x from microbial soil emissions to background NO­_x levels increases. Unfortunately, emissions inventories from natural sources of NO_x from non-agricultural plots in the North America are limited, leaving a gap in understanding the precise role these areas play on atmospheric chemistry.

In this study, we present results of a field campaign held during the summer of 2017 with the objective of understanding the relative importance of microbial soil emissions on local HO_x levels. We used in situ dynamic flux chambers to measure NO_x fluxes at three different experimental plots with varying levels of understory vegetation in a clearing at the edge of a mixed deciduous forest. Measurements of NO_x fluxes and trace gas concentrations from this site were used in a chemical box model. Model results and microbial diversity analysis provide insights on: 1) The impact of surface vegetation on soil emissions of NO; 2) the effect of soil emissions on the local oxidative capacity through the production of HO_x and 3) how dramatic changes in surface vegetation, such as those associated with wildfires or deforestation, could have a significant impact on radical cycling of the local environment.