The influence of dry-season processes on soil NO emissions: A δ15N-NO and δ18O-NO isotopic approach
Nitric oxide (NO) is an important regulator of atmospheric photochemistry and can also indirectly influence climate change. In soils, NO emissions are primarily controlled by nitrification and denitrification, but abiotic chemical reactions can also influence the timing and magnitude of NO fluxes. What controls whether chemistry or biology dominates NO production in soils? As soils dry, resources become concentrated (enhancing chemical reactions) but disconnected which may favor microbial reaction once soils are rewet. To understand how dry-season processes influence abiotic and biotic NO production, we studied grassland, chaparral, mixed conifer, and alpine meadow ecosystems under a Mediterranean climate in California. In our approach, we used a dual isotopic technique (δ15N-NO and δ18O-NO) to understand sources and processes producing NO, and measured inorganic N pools and microbial biomass C and N during seasonal transitions.
The δ18O-NO signature immediately after wetting was enriched compared to measurements made after 24 h, suggesting NO2- and NO3- were produced during the dry season and that these pools controlled the production of NO, though NO2- was more important than NO3-. In contrast, NH4+ concentrated over the dry season contributed little to NO emissions following wet-up. Over time, however, as hydrologically connected soils favored nitrification, NH4+ oxidation dominated NO emissions. Seasonal patterns in NO production and soil NH4+, NO2-, and NO3- pools across these Mediterranean-type ecosystems are consistent with the results from our dual isotope approach. Dry-season processes strongly influence NO emissions by: 1) concentrating substrates that contribute to rapid chemical reactions and by 2) supplying resources to nitrifiers once substrates can diffuse in hydrologically connected soils.