Soil water and solute movement in response to temperature gradients can redistribute soluble nutrients in the soil profile. In soils overlying shallow aquifers, nitrate redistribution during winter months may be important to the prevention of groundwater contamination. The Big Sioux Aquifer in eastern South Dakota has high nitrate concentrations and in some areas is not suitable for drinking water. The objective of this study was to determine the effects of vegetation (residue and species) on soil nitrate redistribution during winter months. The field study was conducted near Aurora, SD on an Estelline soil overlying the Big Sioux Aquifer. N15 enriched potassium nitrate was used as a tracer to determine nitrate movement. The tracer was placed at a depth of 120 cm in the soil profile in late October. The following April, soils were sampled to a depth of 120 cm and divided into 7.5 cm increments to be analyzed for 15N. Vegetative treatments included residues from switchgrass, corn, alfalfa, and non-nodulating alfalfa. The migration of nitrate-N in the soil was related to frost depth. The 15N atom abundance in the frozen zone was 0.371% compared to 0.491%, 0.851%, 0.953%, and 1.007% at depths of 75, 90, 105, and 120 cm, respectively. In the switchgrass treatment, large quantities of residue trapped snow and frost development lagged behind other species. Average snow depth in the switchgrass was 50 cm compared to 10 cm in the other crops. During maximum frost penetration, frost depth was 50 cm for switchgrass compared to 60 cm for the alfalfas and 70 cm for the corn. In the switchgrass treatment, N15 movement was limited to the deepest increment.

Diane H. Rickerl
NPB 247B, Box 2140C
South Dakota State University
Brookings, South Dakota  57007

Phone:  605-688-5541
Fax:    605-688-4452