Coupled soil oxygen and greenhouse gas dynamics under variable hydrology

Abstract

Hot spots and hot moments of greenhouse gas (GHG) fluxes can contribute significantly to overall GHG budgets. Hot spots and hot moments occur when dynamic soil hydrology triggers important shifts in soil biogeochemical and physical processes that control GHG emissions. Soil oxygen (O2), a direct control on biogenic GHG production (i.e., nitrous oxide-N2O, carbon dioxide-CO2 and methane-CH4), may serve as both an important proxy for determining sudden shifts in subsurface biogenic GHG production, as well as the physical transport of soil GHG to the atmosphere. Recent technological advancements offer opportunities to link in-situ, near-continuous measurements of soil O2 concentration to soil biogeochemical processes and soil gas transport. Using high frequency data, this study asked: Do soil O2 dynamics following short-term (<8 days) soil saturation correspond to changes in soil GHG concentrations and GHG surface fluxes? We addressed this question in a restored riparian wetland in Ohio, USA. Changes in subsurface (10 and 20 cm) CO2 and N2O concentrations were inversely related to short-term (<48 h) changes in soil O2 concentrations. Subsurface CH4 concentrations, however, did not change in response to soil O2 dynamics. Changing subsurface GHG concentrations did not necessarily translate into altered surface (soil to atmosphere) GHG fluxes; soil O2 dynamics at 10 cm did not correspond with changes in surface N2O and CH4 fluxes. However, changes in soil O2 concentration at 10 cm had a significant positive linear relationship with change in surface CO2 flux. Our study suggests that monitoring near-continuous soil O2 concentration under dynamic soil hydrology may lead to greater understanding of hot spots and hot moments of GHG emissions. This understanding is increasingly important for estimating the contribution of soil processes to atmospheric GHG concentrations.