UC Riverside

Soil nitric oxide (NO) emissions are variable in both space and time, and are important pathways for N loss in seasonally dry ecosystems that undergo abrupt transitions from dry-to-wet soil conditions. We measured soil NO emissions from a chaparral catchment to characterize seasonal variability of, and triggers for enhanced NO losses. Pulses in NO emissions were observed in the summer and autumn when dry soils (soil water content (θ) < 6%) were wetted naturally and artificially (range: 97–513 ng NO–N m−2 s−1). The rapidity and magnitude of these pulses suggest that abiotic processes may influence NO emissions. Outside of the observed pulses, NO emissions were highest during the dry season (θ < 6%; dry season mean = 3.4 ng NO–N m−2 s−1) and lowest during the winter wet season (θ > 20%; wet season mean = 0.14 ng NO–N m−2 s−1). These observed seasonal patterns contrast with previous DAYCENT simulations of NO emissions in our catchment, which predicted higher NO emissions during the wet season. Our field observations are consistent with sustained rates of nitrification, reduced plant N uptake, and high soil gas diffusivity observed during the dry season in arid environments.