Greenhouse gas production, diffusion and consumption in a soil profile under maize and wheat production

Agricultural soil emissions are a balance between sinks and sources of greenhouse gases (GHGs). The fluxes of GHGs from soils are complex and spatially and temporally heterogenous. While the soil surface is the exchange site with the atmosphere and is commonly where GHG fluxes are measured, it is important to consider processes occurring throughout the soil profile. To reduce emissions and improve agricultural sustainability we need to better understand the drivers and dynamics (production, consumption, diffusion) of these gases within the soil profile. Due to the heterogeneous nature of GHG processes at small to large scales, it is important to test how these processes differ with depth in different systems. In this study, we measured in situ CO2, N2O and CH4 concentration gradients as a function of soil depth over subsequent maize and wheat growing seasons with active gas samplers inserted into an arable field at 10, 20, 30 and 50 cm depths. We found N2O and CH4 concentrations increased with depth, but only CO2 concentrations differed with depth between growing seasons due likely to differences in soil diffusivity driven by soil conditions. Using the concentration gradient method (GM), the CO2 fluxes at each depth and their contribution to the surface flux were calculated and validated against a chamber method (CM) measured surface flux. We found the GM estimated surface CO2 flux was only 6 % different in the wheat, but 28 % lower than the surface measured flux in the maize growing season, due to drought conditions reducing the accuracy of the GM. Finally, we measured fluxes of CO2, N2O and CH4 in ambient and highly concentrated headspaces in laboratory mesocosms over a 72 h incubation period. We provide evidence of depth dependent CH4 oxidation and N2O consumption and possibly CO2 fixation. In conclusion, our study provides valuable information on the applicability of the GM and further evidence of the GHG production, consumption and diffusion mechanisms that occur deeper in the soil in a temperate arable context.