Impacts of Subsurface Tile Drainage on AgeConcentration Dynamics of Inorganic Nitrogen in Soil

We explore the impacts of tile drains in agricultural fields on the coupled age and concentration dynamics of nitrate, immobile ammonium, mobile ammonia and ammonium, and nonreactive tracers such as chloride. We implement two mobile interacting pore domains to capture matrix and preferential flow paths in a coupled ecohydrology and biogeochemistry model, Dhara. We apply this model to an agricultural farm that utilizes a corn-soybean rotation in the Midwestern United States located in the Intensively Managed Landscapes Critical Zone Observatory. In general, we observe both low concentration and age of nitrate in the areas that are classified as topographic depressions even with the presence of tile drains. Also, an increase in the age of mobile ammonia/ammonium is observed after installing tile drains. This is in contrast to the cases for nitrate, immobile ammonium, and nonreactive tracer. These results arise because the depletion of mobile ammonia/ammonium due to tile drainage causes a high mobility flux from immobile ammonium to mobile ammonia/ammonium, which also carries a considerable amount of relatively old age of nitrogen from immobile ammonium to mobile ammonia/ammonium. These results illustrate how storm event scale dynamics impact spatial heterogeneity and temporal variability of the efflux, which helps in disentangling the complexity of nitrogen dynamics in the soil. This understanding can contribute to precision agriculture for nitrogen applications to reduce environmental impacts. Plain Language Summary Age-concentration dynamics for inorganic nitrogen in the soil characterizes for how long and how much nitrogen is present in the soil and how these vary in space and time. This paper shows how the presence of subsurface tile drains, a dominant hydrologic control in the midwestern United States and many other parts of the world, structures this variability. It also argues that using traditional tracer methods, such as chloride and bromide, does not capture the longer residence of nitrogen in the soil due to its complex reactive nature in the soil. By understanding what controls these variabilities, we can develop new methods for reducing the application of fertilizers.