Edge-of-Field Nitrogen and Phosphorus Export in Tile-Drained Fields Managed As Corn for Silage.

Agricultural subsurface tile drainage is an important practice for many farms with poorly drained soils and can substantially increase crop yields. While tile drains can facilitate loss of nitrate-N, impacts on phosphorus (P) transport are less clear. We initiated a USDA-NRCS funded edge-of-field (EoF) water quality monitoring study in the fall of 2014 to compare nutrient losses from two adjacent tile-drained fields managed as corn for silage. The site is located in Northern NY, approximately 50 km south of Montreal, CA in the Lake Champlain Basin. Fields are hydrologically isolated using a series of berms and ditches. Runoff monitoring has occurred year-round since late October 2015. Soluble reactive P (SRP), total P (TP), ammonium-N, nitrate-N, total N (TN) and total suspended solids (TSS) export are estimated by multiplying event mean concentrations (flow proportional sampling) by runoff volume. Results show that approximately 59% of mean cumulative runoff has occurred via tile drain flow. Approximately 84 and 96% of mean TN and nitrate-N export, respectively, was from tile drain flow. Whereas tile drains were the main pathway for N export from fields, surface runoff dominated P loss accounting for approximately 94 and 92% of mean TP and SRP export, respectively. In addition, nearly 95% of TSS export occurred via surface runoff, suggesting that surface runoff and erosion has been the main P transport mechanism to date. The relatively low P loss at the site (mean TP loss = 0.61 kg ha^-1 yr^-1) could be related to low soil test P status of the fields (Morgan extractable P = 1.5 to 2.5 mg kg^-1). Results show the importance of hydrologic flow path on N and P transport, and suggest that while tile drains may increase nitrate-N transport risk, P loss was largely driven by surface runoff.