Reactive Transport Modeling of Nitrogen Fate in a Structured Clay Loam Subject to Liquid Swine Manure Application: Implications of Tile Drainage Management.

Geochemical based reactive transport simulations were performed using MIN3P to predict the fate and transport of nitrogen species derived from land application of liquid swine manure (LSM). The results provide insight into the influence controlled tile drainage might have on nitrification. The 2D numerical model domain was 0.075 m wide and 0.8 m high and consisted of a 0.2 m top soil layer underlain by a 0.1 m plowpan, and a 0.5 m bottom soil layer. The macroporous nature of the soil was represented by a discrete, 0.005 m wide vertically continuous zone of high permeability. The simulation extended for an 864 hour period following LSM application under a transient scenario driven by hourly precipitation data. Nitrogen (as NH₄⁺) was introduced to the simulation through an initial pulse of water designed to reflect the broadcast application of LSM. Initially, the NH₄⁺ concentrations in tile drainage were highly dependent on precipitation, with a concentration peak observed in response to the first post-application rainfall. By 60 hours post application, the concentration of NH₄⁺ had decreased substantially in drainage, which was likely due to NH₄⁺ oxidation resulting from oxygen diffusion to shallow groundwater and due to the cation exchange. Because tile drainage management can influence DO levels in shallow groundwater, it is hypothesized that tile drainage management could actively influence nitrification.