Storm nitrogen dynamics in tile-drain flow in the US Midwest

Storm losses of N via tile-drainage in the US Midwest are a major concern for water quality in the Mississippi River Basin (MRB). This study investigates the impact of precipitation characteristics on NO3 (-), NH4 (+) and DON concentrations and fluxes for spring storms in tile-drains in a Midwestern agricultural watershed. Bulk precipitation amount had little impact on solute median concentrations in tile-drains during storms, but clearly impacted Mg2+, K+ and NO3 (-) concentration patterns. For large storms (> 6 cm of bulk precipitation), large amounts of macropore flow (43-50% of total tile-drain flow) diluted Mg2+ and NO3 (-) rich groundwater as discharge peaked. This pattern was not observed for NH4 (+) and DON or for smaller tile-flow generating events (< 3 cm) during which macropore flow contributions were limited (11-17% of total tile-drain flow). Precipitation amount was positively (P < 0.01) correlated to NO3 (-) and NH4 (+) export rates, but not to DON export rates. Limited variations in antecedent water table depth in spring had little influence on N dynamics for the storms studied. Although significant differences in flow characteristics were observed between tile-drains, solute concentration dynamics and macropore flow contributions to total tile-drain flow were similar for adjacent tile-drains. Generally, NO3 (-) represented > 80% of N flux during storms, while DON and NH4 (+) represented only 2-14% and 1-7% of N flux, respectively. This study stresses the non-linear behavior of N losses to tile drains during spring storms in artificially drained landscapes of the US Midwest, at a critical time of the year for N management in the MRB.