Modeling of dual-permeability gas and solute reactive transport in macroporous agricultural soils with a focus on GHG cycling and emissions

Agricultural soil is the domain where biogeochemical C-N turnover is affected by atmospheric and hydrologic processes, playing a vital role in anthropogenic greenhouse gas (GHG) emissions and thus affecting global climate change. GHG cycling and emissions are further complicated by the characteristics of soil structure, in particular macropores (i.e., preferential flow pathway). In this study, the processes of non-equilibrium gas diffusion and gas exchange were implemented into an existing dual-permeability flow and reactive solute transport model. The model was then used in a sensitivity analysis to evaluate the suitability of the modeling approach to account for development of anaerobic hotspots, with an emphasis on assessing N2O production and emissions. The sensitivity analysis combined with characteristic time scale analysis suggests that the development of anaerobic hotspots is controlled by a combination of both physical and geochemical parameters. Time scale analysis of O2 supply and consumption indicates that the occurrence of anaerobic hotspots is determined by the balance between characteristic times of O2 diffusion through the soil profile or exchange between macropores and the soil matrix, and the characteristic time of O2 consumption. Motivated by these findings, the present model was applied to simulate GHG cycling at a field site in Ontario, Canada, featuring macroporous agricultural soil. The model generally reproduced observed spatiotemporal variations in pore gas O2 and GHG concentrations and fluxes. Simulation results suggest that at this site, gas exchange processes between regions of preferential flow and the soil matrix play an important role in controlling the spatial variation of O2 in the soil. The simulations illustrate how the release of GHGs from the soil matrix into the macropores can lead to GHG emissions to the atmosphere. The modeling approach presented here, including dual domain flow and solute transport, as well as dual domain gas transport, shows promise for future studies with the aim to develop a more complete understanding of how soil structure affects complex GHG cycling in agricultural soils.