Spatiotemporal Heterogeneity in Trace Gas Nitrogen Pulses from Desert Soils

Carbon (C) and nitrogen (N) cycling in drylands is often described using a pulse dynamics framework: labile C and N accumulate in "reserves" during dry periods and are quickly mobilized following rain, generating pulses of intense biogeochemical cycling that can transfer gases from soils to the atmosphere. Frequency and magnitude of wetting events are known to drive the size and duration of gas emission pulses; however, we suspect that pulse dynamics are also spatially heterogeneous. In deserts, the configuration of vegetation likely drives the spatial distribution of C and N reserves, and soils located under desert shrub canopies may be primed for stronger pulse responses to rain than soils in shrub interspaces. To explore spatiotemporal heterogeneity in trace gas C and N pulses from desert soils, we deployed in Boyd Deep Canyon (Palm Desert, CA) a custom array of high-resolution trace gas analyzers (CO₂, N₂O, and NO_x (NO+NO₂) connected to automated gas collection chambers which collect measurements in real time and in situ. We measured gas pulse responses to three wetting treatments added under shrub canopies and in shrub interspaces: adding water to dry soils (wetting), re-wetting 48 h after the initial wetting (re-wet), and adding water+N to dry soils (wet+N).

Wetting dry soils produced large CO₂ emission pulses, although added N did not produce additional increases in emissions. In contrast, N₂O pulses were much larger in both magnitude and duration under the wet+N treatment compared to wetting alone. CO₂ pulses from re-wetted soils were generally smaller than when first wetted, while for N₂O the pulses were similar regardless of number of wettings. Across initial wetting treatments, peak CO₂ fluxes were higher from canopy soils than those from interspaces; conversely, peak N₂O fluxes were stronger from interspaces, particularly after adding water+N. However, upon rewetting, canopy and interspace soils diverged in their pulse responses. Re-wetted canopy soils produced stronger CO₂ and weaker N₂O pulses compared to initial wetting; in contrast, re-wetted interspace soils produced weaker CO₂ and stronger N₂O pulses. While wetting seems to be the initial driver of pulse dynamics, spatial differences in nutrient reserves emerge upon subsequent re-wetting of desert soils.