Coupling field experiments with a model to quantify nitrogen trace gas emissions from desert soils

Abstract:

Emissions of nitric oxide (NO) and nitrous oxide (N₂O) from desert soils are increasingly described as pulses, similar to Birch effect pulses of CO₂. Although the occurrence of NO and N₂O pulses is primarily limited by soil moisture, we hypothesized that the magnitude and duration of pulses would additionally be mediated by soil temperature and substrate availability, where the strongest pulse responses to wetting would occur under high-temperature, high-substrate conditions. To test this hypothesis, we conducted five field-based experiments at a desert site in Southern California in which we tracked NO and N₂O pulse dynamics across varying combinations of biological cover, nutrient additions, seasonal climate regimes, soil temperature, and frequency of soil rewetting.

As expected, NO and N₂O produced pulses in response to experimental wetting in all field campaigns, indicating that soil moisture is the primary limiting factor to NO and N₂O emission from desert soils. For both gas species, soils under biological cover produced stronger pulses compared to soils in interspaces; pulses were stronger in the dry season compared to the wet season; and wetting with additions of both carbon and nitrogen produced stronger pulses compared to wetting alone. Although N₂O pulses showed seasonal divergence in magnitude, they did not respond strongly to intra-seasonal manipulations in rewetting frequency. Conversely, NO pulses increased as length between intra-seasonal rewetting events increased, suggesting NO pulses are more responsive to subtle changes in rewetting frequency and, presumably, to accumulation of N reserves in dry soils. N₂O and NO pulses were also positively correlated to soil temperature at multiple temporal scales: the trajectory of each pulse was related to temperature regimes at seasonal and diurnal timescales as well as temperature conditions at time of wetting. We used these case studies to build and test a modified Dual-Arrhenius-Michaelis-Menten (DAMM) model to characterize NO and N₂O emissions from desert soils, and we attempt to scale up our field measurements toward a generalized model of annual flux of these trace gases from drylands globally.