HYDROLOGIC DRIVERS OF NITROGEN CYCLING: IMPLICATIONS FOR AN EARTH SYSTEM MODEL AND AGRICULTURAL MANAGEMENT PRACTICES

Abstract

The research in this dissertation is broadly concerned with the measurement, modeling, and management of nitrogen transport and transformation in agricultural watersheds. Our objective in the first study was to quantify nitrogen losses under different cover crop treatments to provide insights into the environmental benefits of cover crops over their lifetime. We measured nitrous oxide emissions, nitrate leaching, and plant nitrogen demand in the early spring and continued nitrous oxide measurements after tillage. We found the environmental benefit of cover crops depends on the type and period of analysis. Legumes increased nitrous oxide emissions to the atmosphere post-tillage over the non-legumes. Neither legume nor non-legume, nor a mixture of the two, was effective at reducing nitrous oxide during the growth phase relative to fallow conditions. In the second study, we examined the scale-dependency of several nitrogen cycle processes in an earth system model with previously identified scale issues in its hydrologic cycle and recognized difficulty in simulating gaseous and hydrologic-driven nitrogen losses. We hypothesized that observed discrepancies in these soil nitrogen processes across grid-scales are explained by issues in hydrologic scale-dependence. The recommended spin-up procedure generated different soil hydraulic properties for a model resolution, resulting in different drainage and runoff rates. We confirmed that the choice in grid scale affected model estimates of soil moisture, nitrification, nitrate leaching, and to a lesser degree, denitrification. In the final study, we present an exploration of flow variability and denitrifying bioreactor performance that aim to improve management of nitrogen in tile drainage water. Using synthetic data modeled after observed flow distributions in an existing bioreactor in central New York, we found that a record with a few high flow events generated greater nitrate export than a record with frequent low flow events. The finding supports the general design practice of building the reactor with appropriate size and flow control to maintain a low flow rate.