Storm NO<sub;>3</sub><sup>-</sup> and NH<sub;>4</sub><sup>+</sup> Exports in Stream, Overland Flow, and Tile Drains of the US Midwest
Keywords:subsurface drainage, nitrogen, export rates, precipitation, runoff, overland flow
A better understanding of the dynamics of nitrogen (N) losses to streams during storms in agro-ecosystems of the US Midwest is critical to better understand how to mitigate N pollution in the Mississippi River Basin. This study investigates storm NO3- and NH4+ concentrations and fluxes in tile drains, overland flow and stream water in relation to bulk precipitation and antecedent moisture conditions. For moderate size spring storms (1.0 - 4.5 cm bulk precipitation), the occurrence of overland flow was primarily associated with high antecedent moisture conditions, but had no direct effect on stream NO3- and NH4+ concentrations. Mean storm NO3- and NH4+ concentrations in the stream and tile drains were also not significantly correlated (p > 0.05) to either bulk precipitation or antecedent moisture conditions. Nevertheless, mean stream NO3- concentrations (7.50 mg N/L) were on average 28% lower than in tile drains (10.38 mg N/L). No significant difference in NH4+ concentrations were observed between the stream (0.06 mg N/L) and tile drains (0.05 mg N/L). NO3- and NH4+ fluxes were positively correlated with bulk precipitation (p < 0.05) and high fluxes were typically associated with wet antecedent moisture conditions. Specific NO3- fluxes in tile drains (750 g N/ha/storm) were approximately 2 times larger than in the stream (398 g N/ha/storm). Such differences were not observed for NH4+ fluxes. Considering the positive correlation between storm NO3- fluxes and stream NO3- baseflow concentrations (r = 0.87, p < 0.05), it is likely that one of the most efficient strategies for reducing N losses at the watershed scale may simply lie in reducing N inputs to cropland, as opposed to trying to manage N after it is applied to fields.
Permanent URL: http://hdl.handle.net/2047/d20002606