The Effects of Winter Cover Cropping on Nutrient Leaching Through Repacked Soil Columns

Abstract. Winter cover cropping has been identified as a conservation tool with the potential to decrease nutrient transport from agricultural fields. This study utilized laboratory-scale soil columns and the one-dimensional water and solute transport model HYDRUS-1D to quantify and understand the potential decrease in nitrogen and phosphorus loss through tile drainage due to cover crops, as well as compare the amount of nitrogen and phosphorus available in the soil at cash crop planting, with and without a cover crop. A 50/50 mixture of oats (Avena sativa) and hairy vetch (Vicia villosa) was grown in Flanagan silt loam soil columns August – January in Urbana, IL. In August, 200 kg ha^-1 NO₃-N and 50 kg ha^-1 PO₄-P were applied to a growing cover crop. A 65% increase in cover crop growth was shown to significantly decrease nitrate leaching within 95% confidence limits. A 9325 kg ha^-1 stand of oats/hairy vetch cover crop had 12.7 times less nitrate leaching (total mass) over the growing period than bare soil. Both drainage volumes and nitrate concentrations were reduced as a result of cover cropping. However, average nitrate concentrations from both bare and cover cropped soil columns were above 10 mg L^-1, which is the acceptable limit for drinking water. This experiment simulates a “worst-case scenario” for nitrate leaching by combining fall fertilizer application with soil directly above a tile, but a “best-case scenario” with a profile initially free of macropores. Orthophosphate leaching was 2 – 3 orders of magnitude less than nitrate leaching, but also decreased with increasing cover crop growth. The HYDRUS-1D model was successfully calibrated and validated for water flow from cover cropped and bare soil columns. However, the model under-predicted the effects of cover cropping on water and nitrate leaching.