Effects of soil water content, temperature and experimental nitrogen deposition on nitric oxide (NO) efflux from semi-arid shrubland soil

Abstract:

Southern Californian urban shrublands are exposed to large amounts of anthropogenic nitrogen (N) deposition, which is altering the carbon (C) and N cycling. It is possible that N inputs are exceeding biotic demand and increasing N losses from leaching and gaseous efflux. Nitrogen input increases microbial processes such as nitrification, which can increase soil nitric oxide (NO) efflux; however, experimentation is needed to understand the environmental controls on NO flux. Thus, a fully factorial laboratory incubation experiment was executed to see how NO efflux of chaparral soil varied as function of N addition, soil temperature, and moisture conditions. We hypothesized that the NO efflux would i) increase as a function of N addition, ii) present an optimum response as a function of soil moisture, and iii) increase as an exponential function of temperature. NO efflux increased with N addition, supporting the hypothesis that an increase in N availability will increase NO efflux. NO efflux also increased as a function of soil temperature and was well-described using the Arrhenius model, and the NO efflux followed an optimum response as a function of soil moisture. In general, the greatest NO efflux was observed from soil exposed to 5 gN/m², 25% water-filled pore space (WFPS), and a temperature of 34.4°C. There was high sensitivity to temperature changes, especially in soil moisture < 25% WFPS. These conditions are comparable to the normal environmental conditions in chaparral soil (low soil moisture and high temperature), suggesting that chaparral soils may have optimal environmental conditions for nitrification and are important sources of atmospheric NO. We applied our experimental findings to a modified version of the Dual Arrhenius and Michaelis–Menten (DAMM) model (Davidson et al. 2012; Global Change Biology 18, 371–384) and estimated an annual NO efflux of 1.1-2.5 kgN ha^-1 y^-1 depending on N deposition exposure, which is comparable to what is reported for other chaparral shrublands of southern California.