Wetting-induced pulses produced unexpectedly high emissions of N2O and NOx in a desert ecosystem

Abstract:

Approximately one third of Earth’s land surface is subjected to arid conditions, and aridland soils have the potential for significant feedbacks to global climate change drivers, such as anthropogenic nitrogen deposition. This study examined wetting-induced pulses of N₂O and NO_x along a nitrogen deposition gradient in the Colorado Desert of southern California. Measurements were made before and following water (to simulate a 2 cm rain event) and nitrogen plus water additions (30 kg NH₄NO₃ ha^-1) at 15 minutes, 12 hours and 24 hours post-wetting. We found nitrogenous fluxes that were substantially higher than expected. N₂O fluxes, in particular were remarkably high reaching up to 200 ng N₂O-N m^-2 s^-1, similar to agriculture levels and in the range of peat bog emissions. There was a clear transition between N₂O emissions, which peaked 15 minutes after wetting, and NO_x emissions, which peaked at the 12 hour timepoint. NO_x emissions were also considerable, reaching as high as 350 ng NO_x-N m^-2 s^-1. Both N₂O and NO_x fluxes responded strongly to water additions, demonstrating a clear wetting-induced pulse response. While N₂O was not affected by nitrogen additions, NO_x fluxes demonstrated a significant increase with nitrogen plus water over water alone (p=0.016). These results suggest that gaseous nitrogen export, particularly N₂O emissions, is a greater form of nitrogen loss in arid systems than is currently assumed. This potential for high nitrogen emissions and the capacity for anthropogenic nitrogen deposition to increase these emissions present serious implications for local air quality and significant soil feedbacks to climate change.