Controls over nitric oxide and ammonia emissions from Mojave Desert soils

Abstract

Emissions of reactive N compounds produced during terrestrial N cycling can be an important N loss pathway from ecosystems. Most measurements of this process focus on NO and N₂O efflux; however, in alkaline soils such as those in the Mojave Desert, NH₃ production can be an important component of N gas loss. We investigated patterns of NO and NH₃ emissions in the Mojave Desert and identified seasonal changes in temperature, precipitation and spatial heterogeneity in soil nutrients as primary controllers of soil efflux. Across all seasons, NH₃ dominated reactive N gas emissions with fluxes ranging from 0.9 to 10 ng N m⁻² s⁻¹ as compared to NO fluxes of 0.08–1.9 ng N m⁻² s⁻¹. Fluxes were higher in April and July than in October; however, a fall precipitation event yielded large increases in both NO and NH₃ efflux. To explore the mechanisms driving field observations, we combined NO and NH₃ soil flux measurements with laboratory manipulations of temperature, water and nutrient conditions. These experiments showed a large transient NH₃ pulse (~70–100 ng N m⁻² s⁻¹) following water addition, presumably driven by an increase in soil NH₄ ⁺ concentrations. This was followed by an increase in NO production, with maximum NO flux rates of 34 ng N m⁻² s⁻¹. Our study suggests that immediately following water addition NH₃ volatilization proceeds at high rates due to the absence of microbial competition for NH₄ ⁺; during this period N gas loss is insensitive to changes in temperature and soil nutrients. Subsequently, NO emission increases and rates of both NO and NH₃ emission are sensitive to temperature and nutrient constraints on microbial activity. Addition of labile C reduces gaseous N losses, presumably by increasing microbial immobilization, whereas addition of NO₃ ⁻ stimulates NO and NH₃ efflux.