N2O Production and Consumption in Semi-Arid Soils

We investigated how summer monsoon precipitation affects surface and subsurface N₂O production and consumption in semi-arid rangeland soils, southern Arizona, USA. Surface fluxes and profiles up to 2 m deep were sampled for N₂O concentration, δ ¹⁵N and δ ¹⁸O. Soil samples for incubations and C and N stable isotope composition were taken at the surface through 50 cm depth. Pre-monsoon soils were dry with small N₂O fluxes (1.6±0.6 μ g/m²/hr). Initial precipitation events wetted the surface 10 cm and triggered much larger N₂O fluxes (11.7±6.7 μ g/m²/hr). δ ¹⁵N values of N₂O surface fluxes averaged -9.7±1.9‰ after precipitation and increased as the soil surface dried (-6.5±2.1‰ ). Soil profile δ ¹⁵N values after rainfall ranged from -3.8 to -1.5‰ , with one outlier at 8.6‰ . δ ¹⁸O values of N₂O fluxes following rainfall were highly variable (29±32‰ ), but were more consistent (43.4±0.1‰ ) when the soil surface dried. Soil profile δ ¹⁸O values following rainfall averaged 44.1±1.0‰ , with one outlier 55.2‰ . Analysis of N₂O trapped from aerobic laboratory incubations of surface soil had δ ¹⁵N (-10.8‰ ) and δ ¹⁸O (22.0‰ ) values indicative of nitrification processes. Assuming that incubation isotope values represent production, we calculated that 40% of N₂O was consumed before leaving the soil. Our results suggest that after precipitation, surface produced N₂O preserved isotopic signal of production but as the surface dried, flux and soil profile N and O isotope values reflected considerable N₂O consumption. Isotopic variability of fluxes following rainfall indicates that several distinct surface production mechanisms.