Phosphorus and Nitrogen Losses in Runoff from Fields with and without Tile Drainage

Leanna Thalmann, University of Vermont


Nutrient losses in surface and subsurface drainage from crop fields have important water quality implications. The deterioration of water quality in segments of Lake Champlain has led to efforts to reduce phosphorus (P) and nitrogen (N) export from agricultural fields. This thesis presents data from two years of edge-of-field monitoring in two adjacent corn (Zea mays L.) silage fields in Keeseville, New York. One field has only surface drainage improvements with monitoring equipment, and the other has both surface and subsurface drainage modifications and monitoring equipment. The study took place from October 2018 to September 2020 and quantified flow and concentrations of total phosphorus (TP), dissolved reactive phosphorus (DRP), total nitrogen (TN), nitrate (NO3--N), ammonium (NH4+-N), and total suspended solids (TSS), in a Tile Drained Field (TD) and an Undrained Field (UD). For the events fully monitored, the non-growing season (NGS; October 15-April 15) dominated runoff with 96% (326.7 mm) and 90% (283.7 mm) of the total runoff occurring for TD and UD, respectively. The NGS also exported the greatest amount of nutrients; TD discharged more runoff and N, while UD dominated in P export. The TD discharged 170.6 mm/year of runoff and exported 11.5 kg TN/ha/yr and 0.135 kg TP/ha/yr. The Tile Drained Surface contributed 48% (82.6 mm/yr) of the total runoff and 13% (3.0 kg/ha/yr) of the TN export and 73% (0.099 kg/ha/yr) of the TP export. Comparatively, UD discharged 158.4 mm/year of runoff and 44% less TN export (6.5 kg TN/ha/yr). The UD field exported 0.144 kg TP/ha/yr, which was 6.8% more TP export than TD. The presence of tile drainage reduced the total surface runoff volume from TDS, which reduced nutrient loading from Tile Drained Surface. Five large runoff events all occurred during the NGS, and contributed the majority of nutrient export and discharge. Overall, the Tile Drained Surface and the Undrained Surface had higher concentrations and loadings of P, while the Tile Drained Tile had higher concentrations and loadings of N. The NGS was a critical period for discharge contributed the greatest amount of runoff and nutrient export. Management practices should be put in place to reduce these losses during the late fall, winter and early spring. Optimizing the timing and application rates of N and P should be implemented to reduce the risk of nutrient export.