Modeling soil moisture and oxygen effects on soil biogeochemical cycles including dissimilatory nitrate reduction to ammonium (DNRA)

The emission of greenhouse gasses (GHG) from soils is controlled by biogeochemical reactions and the physical constraints on gas diffusion to the soil surface. Here we present and discuss a mathematical model that couples oxygen and soil water dynamics to biochemical reactions and gas transport to explore the major drivers of trace gas emission at daily time scale in unsaturated soils. The model accounts for trace gas emissions (CO2, and N2O from nitrification and denitrification), as well as for the competition for nitrate by denitrification and dissimilatory reduction of nitrate to ammonium (DNRA). Our results indicate that explicit modeling of oxygen dynamics is important when re-aeration is limited, such as under wet conditions, in particular for fine-textured soils. The balance of labile substrate, oxygen, and water availabilities explain the observed peaks in GHG emissions at moisture values around the soil field capacity. The timing of these peaks during a dry-down is delayed in fine-textured soils, due to the slower drying and limited gas exchange rates. In addition, N2O emissions may be limited by DNRA at high soil moisture.