A Deeper Understanding: Using the Depth Distribution of N2O in Soil to Improve Predictions of Soil-Atmosphere Emissions.

Atmospheric N₂O has been increasing at an accelerating rate since ~1750, largely due to fertilizer use and fuel combustion, leading to stratospheric ozone depletion and climate change. Soils are the main contributor of N₂O emissions to the atmosphere, as documented by many measurements of N₂O efflux at the soil surface. Yet these surface measurements are only telling part of the story; measurements of subsurface N₂O dynamics are lacking. Thus, our research linked measurements of surface N₂O efflux with N₂O concentrations deeper in 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 hillslope) and three landscape positions (ridgetop, midslope, and valley floor) in the Susquehanna Shale Hills Critical Zone Observatory (CZO) in central Pennsylvania. All three landscape positions of the swale hillslope at Shale Hills had higher subsurface N₂O concentrations than the corresponding landscape positions of the planar hillslope. The highest overall N₂O concentrations were in the mid-slope swale position (>3,000 ppb_v), with the lowest concentrations in the mid-slope planar position (~800 ppb_v). Though N₂O concentrations at depth were highly variable, concentrations near the soil surface for all landscape and hillslope positions remained close to background atmospheric levels (~320 ppb_v). These results suggest that pockets of high N₂O in subsoils are not diffusing to the soil surface, thus limiting soil-atmosphere N₂O efflux. 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.