Understanding soil NO fluxes and their impact on the forest atmosphere

An, Hyunjin and Ashworth, Kirsti and Wild, Oliver and Sayer, Emma (2024) Understanding soil NO fluxes and their impact on the forest atmosphere. PhD thesis, Lancaster University.

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Abstract

Nitrogen oxides (NOx = NO + NO2) are important atmospheric pollutants and major precursors of ozone (O3) and secondary aerosol. To address air pollution issues in industrial and urban cities, anthropogenic emissions have been the main focus of attention to improve air quality. NOx emissions from natural sources such as the soil are less intense and are frequently neglected. However, NOx is highly reactive and even small emissions to the atmosphere can make a substantial impact in locations where anthropogenic emissions are low, such as in remote or forested regions. In this work, soil NOx fluxes have been measured in the field in a number of different environments, and in lab experiments with collected soil samples, and the impacts of these fluxes have been investigated using model simulations. In-situ measurements in two forests with different vegetation and soil characteristics enabled investigation of the driving factors that govern the magnitude of soil NOx fluxes and their diel and periodic cycles. Field observations in a Eucalyptus forest and suburban greenspace found that soils are an ongoing and non-negligible emission source, and that the diel cycles are influenced by meteorology. Surprisingly, negative soil NOx fluxes (reflecting net NOx uptake) were found in nitrogen limited forest soils. To account for the effects of future climate changes, soils were collected under conditions of elevated atmospheric carbon dioxide (CO2) and temperature treatments were applied. The measurements from the field campaigns and lab experiments demonstrated that climate and pedoclimate parameters affect the magnitude of soil NOx fluxes, and that atmospheric CO2 levels alter the optimal soil NOx flux response to temperature. The soil NO fluxes measured in these studies were then applied in a canopy exchange model to investigate their impact on the forest atmosphere. Soil NO fluxes influenced O3 concentrations, but also affected peroxy radical concentrations and isoprene oxidation. The model results showed that impacts were bigger under heatwave and prolonged drought conditions. Furthermore, the structure of the forest canopy affects the vertical profile of O3 and its relationship with soil NO fluxes. As Earth’s climate changes, and as anthropogenic sources reduce under the effect of emission controls, the role of soil NOx fluxes will become increasingly important. The findings in this work motivate further interest in the role of soil fluxes on the atmosphere, and in the wider importance of biosphere and atmosphere interactions.