Assessing GHG cycling in agricultural and riparian soils using a uniform reactive transport modeling approach

Abstract

Agricultural drainage ditches are necessary for regulating moisture contents in fields for crop production, and moreover, they can provide and regulate important ecosystem services. Drainage ditches and their associated riparian zones can be significant sources of enhanced N2O, CH4 and CO2 emissions, since they receive nutrient laden runoff and drainage from adjacent fields, while providing moisture conditions suitable for the production of greenhouse gases (GHGs). In this study, a uniform reactive transport model with up-scaled rate parameters was used to assess the dynamics of GHG cycling and biogeochemical processes in a crop field and riparian soils adjacent to a drainage ditch (i.e., located on the shoulder and midslope of the ditch) in eastern Ontario, Canada. Simulations adequately reproduced observed spatial variations in pore gas GHG concentrations in the cropped field and the monitoring locations near the drainage ditch. Spatial variations can be attributed to differences in environmental factors (i.e., soil moisture content and temperature), soil nutrient supply (i.e., NH4+, NO3–), and organic C availability. Simulation results also showed that at this site, GHG emissions did not vary significantly between monitoring locations, suggesting that the ditch slopes are not significant GHG emission hotspots. Both major rainfall events and warmer summer temperatures led to the development of hot moments for CO2 and N2O emissions and corresponding soil gas concentrations. Although the uniform model was successful in reproducing long-term trends in observed data, the model was not able to fully capture in-season temporal variability of fluxes and concentrations related to acute precipitation events (i.e., hot moments). Nevertheless, the modeling approach presented shows promise for future studies comparing the effect of management styles for agricultural soils and drainage ditch zones on GHG emissions and uptake.