The Paradox of High Subsurface [N2O] and Low Surface Flux of N2O to the Atmosphere

Abstract:

Soils are the main contributor of N₂O emissions to the atmosphere, as documented by many measurements of N₂O efflux at the soil surface. Different soil layers can act as sources or sinks for N₂O, depending on N₂O consumption and production by microorganisms, diffusion through the soil profile, and dissolution in soil water. Our research sought to identify and quantify sources or sinks of N₂O in surface and deeper soils to improve our ability to predict fluxes of N₂O from soils to the atmosphere.

We monitored the vertical profiles of soil N₂O concentrations (0-160 cm depending on thickness) along two contrasting hillslopes (swale versus planar) and three landscape positions (ridgetop, midslope, and valley floor) in the Susquehanna Shale Hills Critical Zone Observatory (CZO) in central Pennsylvania. All landscape and hillslope positions had higher subsurface N₂O concentrations (~800-3,000 ppb_v) than soil surface concentrations (~320 ppb_v), which were close to background atmospheric levels. Low soil-atmosphere fluxes measured in surface chambers suggested that much of the N₂O produced at depth never reached the soil surface. Denitrification bioassays conducted on soil cores co-located with gas sampling positions revealed low potential rates of N₂O consumption (<0.02 g m^-2 yr^-1), while dissolved N₂O concentrations measured in the nearby stream were below ambient levels (<300 ppb_v). These results suggest that the flux of N₂O at the soil-atmosphere interface is low because diffusion limits transport from deeper soil layers. Models that link dynamic soil diffusivity with accumulations and depletions of N₂O in the subsurface could improve our ability to predict surface-atmosphere N₂O efflux.