Quantification of Field Scale Denitrification by Stable Isotope Analysis of NO3- and H2O from Tile Drain Runoff

Nitrogen within intensively managed landscapes (IMLs) is lost through multiple pathways. Among these, denitrification is one of the most researched. Despite this attention, current techniques to quantify denitrification produce highly variable results that are attributed to the inability to account for high spatial and temporal variability of denitrification rates. Here we present an isotopic Rayleigh distillation model to quantify field-scale denitrification. Nitrate in tile drain runoff from a Midwestern corn field in Monticello, Illinois, was collected using a high sampling frequency (15 min) and δ15N and δ18O were determined using IRMS (Isotope Ratio Mass Spectrometry). We observed that during an individual hydrological event and over the course of the growing season significant changes in δ 15N (4‰) and δ 18O (4‰) values occurred and this shift was attributed to denitrification. By applying the Rayleigh model we calculated that a total of 3.3% of added nitrogen to the field site was lost through denitrification between spring and fall. This technique is less labor intensive than other methods and temporal resolution can be increased to capture changes in denitrification within individual hydrological events. Dual isotopes of NO3-, anion (NO3-, SO42-, Cl-) and cation (Na+, K+, Ca2+, Mg2+) concentrations were measured in addition to stable isotopes (δ18O and δ2H) of H2O. These measurements provided insight into how the hydrology of a tile-drained IML and how matrix and preferential flow pathways effect the rate of denitrification.