Recent attention to nitrogen (N) cycling in arid and semi-arid ecosystems has revealed nitrification and denitrification can increase rather quickly following infrequent precipitation events and can result in large fluxes of N trace gases in areas of high N deposition. In the southwestern United States, widespread conversion from shrublands to exotic grasslands has potentially increased the quality and quantity of litter entering the soil following senescence and may increase rates of nitrification, denitrification, and gaseous emissions. However, changes in timing of senescence from perennial shrubs to annual grasses may alter the temporal release of pollutant nitrogen oxides (together known as “NOx”) and greenhouse gas nitrous oxide (N2O). We therefore ask the question: How do N trace gas emissions respond to seasonal variation in precipitation as semi-arid shrublands convert to exotic grasslands? To test this question, we conducted a factorial field study at Motte Rimrock Reserve (Perris, CA) in which we manipulated litter quantity and quality in coastal sage scrub- and grass-dominated plots. We performed experimental water additions to plots during the rainy and dry seasons in Summer 2016- Winter 2017 and measured NOx and N2O fluxes from plots at various time intervals over a 24-hour period of drydown.
NOx and N2O emissions peaked at different times post-wetting; N2O peaked within an hour of experimental wetting and fell to pre-wetting levels within 24 hours, while NOx peaked approximately 6 hours after wetting and remained elevated after 24 hours (in other unpublished studies, we found that NOx remained elevated in this system even after 48 hours). Emissions were higher following experimental wetting in the summer than in the winter for both NOx and N2O; this could be because N that was stored during a dry period was then processed quickly and released, or it could be that nitrification and denitrication are less efficient during infrequent wet-up/dry-down cycles. Future work will more fully quantify nitrification and denitrification to resolve this question. Further, N2O responded more strongly to litter manipulations than did NOx. Regardless of dominant live vegetation in the plot, N2O emissions were highest in plots with added grass litter compared to those of added shrub litter and no added litter. We conclude that the combination of seasonal rains and ecosystem type conversion alter ecosystem feedbacks to climate, with infrequent summer rains driving larger immediate pulses of N trace gases and conversion to grasslands driving more pronounced emissions of N2O.