Within-Field Drainage Flow and Nitrogen Loading Variability Following Tile Drainage Installation.

Artificial agricultural drainage is required to sustain high productivity cropping systems across much of the U.S. Midwest. However, nitrate losses through subsurface drainage are a significant concern because of their direct impact on water quality for drinking water sources as well as negative impacts at the Mississippi River Basin scale. Although the spatial dynamics of within-field drainage flow and crop growth are known to be variable, few studies have evaluated key nitrogen (N) cycling processes at the field scale to assess potential relationships between drainage flow, N loading, and in-season crop N deficiencies following tile drainage installation. A new replicated drainage plot research site was established in central Illinois in fall 2016 to evaluate the combined effects of recommended water quality-improvement practices on tile drainage nutrient losses, N use efficiencies, and corn yield. Sixteen individually drained plots (0.8 ha) containing three tile laterals at 18 m spacing were hydrologically isolated using border tiles on a Virden silty clay loam (fine, smectitic, mesic Vertic Argiaquolls; drainage design coefficient: 9.5 mm/d). This study aimed to evaluate whether the within-field variability of corn growth and NDVI during vegetative growth was related to drainage flow, nitrate loads, or soil N₂O emissions among the sixteen plots during the 2017 growing season. Crop management was uniform across the field. Drainage samples were collected from each plot weekly and analyzed for nitrate, while flow was continuously monitored. Soil N₂O fluxes were measured by the closed-static chamber method from sidedress N application to harvest. UAV imagery was obtained at V6 and R1-R2 to evaluate spatial differences in corn N deficiencies estimated using NDVI. Results are expected to advance our understanding of the spatial variability of N cycling processes under uniform crop management, which could help inform the development of within-field management strategies to reduce N losses.