Linking NO and N2O emission pulses with the mobilization of mineral and organic N upon rewetting dry soils

- Our study revealed rapid soil N dynamics following a rewetting pulse.
- Immediately after rewetting, diffusive N flux was dominated by NO3−.
- NH4+ dominated total N diffusion 24 h after rewetting.
- Rewetting triggered rapid NO and N2O emission pulses.
- Shifts in diffusing N-compounds corresponded with shifts in NO and N2O emissions.

Drying and rewetting of soils triggers a cascade of physical, chemical, and biological processes; understanding these responses to varying moisture levels becomes increasingly important in the context of changing precipitation patterns. When soils dry and water content decreases, diffusion is limited and substrates can accumulate. Upon rewetting, these substrates are mobilized and can energize hot moments of intense biogeochemical cycling, leading to pulses of trace gas emissions. Until recently, it was difficult to follow the rewetting dynamics of nutrient cycling in the field without physically disturbing the soil. Here we present a study that combines real-time trace gas measurements with high-resolution measurements of diffusive nutrient fluxes in intact soils. Our goal was to distinguish the contribution of different inorganic and organic nitrogen (N) forms to the rewetting substrate flush and the production of nitric oxide (NO) and nitrous oxide (N2O). Diffusive flux of N-bearing substrates (NO2−, NO3−, NH4+ and amino acids) was determined in situ in hourly resolution using a microdialysis approach. We conducted an irrigation experiment in a semi-arid California grassland at the end of the dry season, and followed soil N flux and N trace gas emissions over the course of 30 h post-wetting. Upon rewetting, both inorganic and organic N diffused through the soil, with inorganic N contributing most to the rewetting N flush. Emissions of NO and N2O rapidly increased and remained elevated for the duration of our measurements, whereas diffusive soil N flux was characterized by large temporal variation. Immediately after rewetting, NO3− contributed 80% to the total diffusive N flux but was consumed rapidly, possibly due to fast microbial uptake or denitrification. Ammonium flux contributed only ∼10% to the initial diffusive N flux, but it dominated total N diffusion 27 h post-wetting, coinciding with peak N-gas emissions. This suggests nitrification may control most of the N trace gases produced during the late stages of a rewetting pulse. Nitrite contributed only 1% to total N diffusion and did not show a clear temporal pattern. Amino acids contributed roughly as much as NH4+ to the initial diffusive N flux, but the organic N pulse was short-lived, indicating that organic N did not contribute substantially to N-gas formation shortly after rewetting at our study site. Our results support the hypothesis that in semi-arid environments N-bearing substrates concentrate during dry periods and, upon rewetting, can lead to pulses of NO and N2O when they react chemically or are transformed by microorganisms.