Integrative Biology

Isotopic Investigation of the Sources and Cycling of Nitrate in a Tile-Drained Corn-Soybean Rotation System

Nitrogen (N) loss from tile-drained corn–soybean systems threatens water quality and long-term soil sustainability, but the processes controlling N retention, transformation, and loss are difficult to resolve with conventional measurements alone. This dissertation combines natural-abundance nitrate N and O isotopes measured across soil, water, and crop N pools with field observations, mass-balance analysis, and the process-based agroecosystem model RZWQM2 to constrain N sources, internal cycling, and fate in a tile-drained agroecosystem. Results show that tile-drain nitrate derives from recent fertilizer and legacy N from past fertilization and recycled soybean residues, revealing a persistent legacy effect across the rotation. Isotope mass balance helped quantify difficult-to-measure internal N fluxes, showing that denitrification can be as important as nitrate leaching and that neutral soil N balance may require high soybean biological N fixation. Isotope-informed calibration of RZWQM2 also reduced uncertainty in internal N pathways and improved fertilizer-management guidance.