Contrasting Controls Over Soil N2O and NO Emissions Across an Atmospheric N Deposition Gradient

Despite exhibiting dry conditions that should limit microbial activity, drylands can produce among the largest nitrous oxide (N2O) and nitric oxide (NO) emissions when dry soils are wetted. These pulsed N emissions contribute to the greenhouse effect and are important pathways of ecosystem N loss. Atmospheric N deposition is a major source of N in drylands that are downwind of urban areas and may stimulate microbial processes responsible for pulsed N emissions. However, microbial N transformations are also controlled by soil physiochemical propertiessuch as pHthat can co-vary with N deposition. Thus, we ask: how do controls on dryland N2O and NO emissions differ across a N deposition gradient? To answer this question, we measured in-situ N2O and NO emissions after wetting soils with 15N-labeled ammonium or nitrate solutions in four Southern Californian drylands that experience between 2 and 20 kg N ha-1 yr-1 of atmospheric N deposition. We also incubated soils in the lab to infer N2O production pathways using site preference of 15N in the N2O molecule. Cumulative N2O emissions were lowest in high N deposition sites (F3,12 = 3.1, p = 0.066), which were also more acidic. N2O emissions peaked within one hour of wetting (up to 2070 ng N-N2O m-2 s-1); only the 15N-nitrate tracer was detected in N2O. N2O site preference averaged 49.7 during the peak N2O pulse, consistent with values reported for fungal denitrification. In contrast to N2O, NO emissions plateaued within 5 hours of wetting (up to 508 ng N-N2O m-2 s-1), were produced from both ammonium and nitrate, and were higher in low N deposition sites (F3,12 = 2.9, p = 0.088). Taken together, these results suggest N deposition stimulates multiple biological NO production pathways, but other soil properties limit the capacity for N2O production with elevated N deposition.