FROM FIELD TO STREAM: EXAMINATION OF NITROGEN TRANSPORT AND THE PERFORMANCE OF BMPS IN GLACIAL TILL AND KARST LANDSCAPES OF SOUTHEAST MINNESOTA

The Root River Field to Stream Partnership is a multi-organizational effort to evaluate agricultural practices and water quality. This long-term study is taking place in small watersheds that represent Glacial Till, Karst and Bluffland Karst landscapes in southeast Minnesota. Results over the past ten years have provided important insights about field to stream nitrogen transport.

Nitrogen loss from edge-of-field row crop sites nested within each study area were examined. Results showed that on average 10 lb ac^-1 yr^-1 of total nitrogen (TN) was lost in surface runoff of which 70% was sediment bound. On a corn-soybean field located in the Till landscape with subsurface drainage tile, both surface and subsurface TN loss was measured. On average 44 lb ac^-1 yr^-1 of TN was lost; 15% was contained in surface runoff and 85% in subsurface drainage. Nitrate-nitrogen (nitrate) made up 98% of the subsurface TN with April through June accounting for 55% of the annual subsurface loss. On this same field, the performance of new nitrogen rate and timing practices are now being evaluated. Preliminary results indicate that split applying nitrogen can result in lower nitrogen inputs, improve profitability by $17 ac^-1 and reduce subsurface TN loss by 20%. A recently installed edge-of-field prairie strip will further reduce sediment and nutrients in surface runoff. On a different field in the Karst landscape, soil water nitrate concentrations were measured from lysimeters in the vadose zone. When the corn-soybean field transitioned to alfalfa, nitrate levels were reduced 78%.

In-stream flow weighted mean nitrate concentrations were significantly different (p<0.05) between each small watershed and averaged 12.5 mg L^-1, 9.1 mg L^-1 and 3.4 mg L^-1 respectively in the Till, Karst and Bluffland Karst landscapes. These differences can be explained by: 1) differences in the fraction of the landscape in row crops, 2) differences in the porosity and permeability of the sub-surface sediments and bedrock, and 3) differences in groundwater flow paths, residence time/lag time, and mixing.

Measuring field to stream nutrient transport has helped gauge water quality expectations, inform watershed protection strategies and develop trust and adoption of additional conservation practices by farmers in the study watersheds.