Climate Effects on Nitrogen Losses from a Tile Drained Watershed in Southern Minnesota.

Agricultural drainage in the Upper Midwestern United States has been linked to excess nitrogen in rivers and subsequently to hypoxic conditions in the Gulf of Mexico. This study evaluated the effects of a changing climate on N losses from the Le Sueur River watershed in Southern Minnesota. The calculations for N losses are based on the premise that baseflow in rivers is a reflection of tile flow in the landscape. We tested this hypothesis using seven year average baseflow from the Le Sueur River against the corresponding average tile flow data of Randall and Vetsch (2005) for Waseca, Minnesota. Average baseflow over seven growing season equaled 127±49 mm as compared to measured tile flow of 150±85 mm from corn and 135±89 mm from soybean plots. The measured tile flow corresponded to the fall applied fertilizer treatment in Randall and Vetsch (2005) study. Since growing season length varies from year to year, we used the relationship between growing season baseflow vs. annual baseflow for seven years to estimate growing season baseflow from 1950 to 2009. The predicted seasonal baseflow was then multiplied with 0.125 kg-N/ha/mm of baseflow (Randall and Vetsch, 2005) to estimate annual N losses from the corn-soybean rotation in the Le Sueur River watershed. Predicted N losses for 1976-2009 equaled 13.2 kg-N/ha as compared to 7.6 kg-N/ha for 1950-1975. This is equivalent to a 74% increase in N losses relative to the 1950-1975 period. Plots of annual N losses vs. precipitation for 1950-1975 and 1976-2009 were not statistically different (p=0.085). A plot of five year moving averages of N losses vs. precipitation showed that the relationship between N losses and precipitation is about the same for two periods and the higher N losses in recent years are due to higher precipitation levels.