THE RELATIONSHIP OF NITRATE-NITROGEN CONCENTRATIONS IN TROUT STREAMS TO ROW CROP LAND USE IN KARSTLAND WATERSHEDS OF SOUTHEAST MINNESOTA

The relationship between row crop land use and nitrate-nitrogen concentration in baseflow was evaluated for 96 trout stream watersheds in the karstlands of southeast Minnesota. The watersheds range in size from 9 to 830 square kilometers; the percentage of land in row crop (planted to corn or soybean in 2009) varied from 1.4 to 70.9%. Mean baseflow nitrate-nitrogen concentrations ranged from 0.21 to 17.4 mg/l. Preliminary results indicate that nitrate-nitrogen concentrations are directly related to the percentage of row crop in the watershed (r-squared = 0.69). A linear regression showed a slope of 0.17, suggesting that the average baseflow nitrate-nitrogen concentration in the trout stream watersheds of southeast Minnesota can be approximated by multiplying a watershed’s row crop percentage by 0.17. The baseflow nitrate-nitrogen concentration in the trout streams exceeds the 10 mg/l standard when about 60% of the watershed is in row crops. This suggests that nitrogen applied to the land surface over a span of decades impacts the condition of the underlying aquifer; this condition is measured in private and municipal wells, at points of spring discharge, and in the baseflow of trout streams.