Fertilized soils have large potential for production of soil nitrogen oxide (NOx=NO+NO2), however these emissions are difficult to predict in high-temperature environments. Understanding these emissions may improve air quality modelling as NOx contributes to formation of tropospheric ozone (O3), a powerful air pollutant. Here we identify the environmental and management factors that regulate soil NOx emissions in a high-temperature agricultural region of California. We also investigate whether soil NOx emissions are capable of influencing regional air quality. We report some of the highest soil NOx emissions ever observed. Emissions vary nonlinearly with fertilization, temperature and soil moisture. We find that a regional air chemistry model often underestimates soil NOx emissions and NOx at the surface and in the troposphere. Adjusting the model to match NOx observations leads to elevated tropospheric O3. Our results suggest management can greatly reduce soil NOx emissions, thereby improving air quality.

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Drying and rewetting of soils triggers a cascade of physical, chemical, and biological processes; understanding these responses to varying moisture levels becomes increasingly important in the context of changing precipitation patterns. When soils dry and water content decreases, diffusion is limited and substrates can accumulate. Upon rewetting, these substrates are mobilized and can energize hot moments of intense biogeochemical cycling, leading to pulses of trace gas emissions. Until recently, it was difficult to follow the rewetting dynamics of nutrient cycling in the field without physically disturbing the soil. Here

Event-driven and diel dynamics of soil respiration (R s ) strongly influence terrestrial carbon (C) emissions and are difficult to predict. Wetting events may cause a large pulse or strong inhibition of R s . Complex diel dynamics include hysteresis in the relationship between R s and soil temperature. The mechanistic basis for these dynamics is not well understood, resulting in large discrepancies between predicted and observed R s . We present a unifying approach for interpreting these phenomena in a hot arid agricultural environment. We performed a whole ecosystem wetting experiment with continuous measurement of R s to study pulse responses to wetting in a heterotrophic system. We also investigated R s during cultivation of Sorghum bicolor to evaluate the role of photosynthetic C in the regulation of diel variation in R s . Finally, we adapted a R s model with sensitivity to soil O 2 and water content by incorporating two soil C pools differing in lability. We observed a large wetting-induced pulse of R s from the fallow field and were able to accurately simulate the pulse via release of labile soil C. During the exponential phase of plant growth, R s was inhibited in response to wetting, which was accurately simulated through depletion of soil O 2 . Without plants, hysteresis was not observed; however, with growing plants, an increasingly significant counterclockwise hysteresis developed. Hysteresis was simulated via a dynamic photosynthetic C pool and was not likely controlled by physical processes. These results help characterize the complex regulation of R s and improve understanding of these phenomena under warmer and more variable conditions.

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