Evaluating Nitrate Loss From Manure-Amended Sandy Soils in Wisconsin.

A two-year study was conducted in the central sands of Wisconsin to determine the impact of dairy manure application on groundwater quality. Two sites with no manure history were selected. A split plot design with four replications was used where manure treatment was the main plot (separated solid manure, separated liquid, separated liquid plus a nitrification inhibitor, and two controls with no manure) and fertilizer N application was the subplot. Each manure treatment plot was split into six subplots with three receiving a single sidedress fertilizer application of 0, 56, or 112 kg N ha^-1 while the others had two sidedress applications totaling 112 kg N ha^-1 with varying percentage of N being applied in each split.  Subplots in one of the control treatments received 0 to 280 kg N ha^-1 in increments of 56 kg N ha^-1 while subplots in the second control plot received 224 kg N ha^-1 in differing percentages between two sidedressings.

In 2012, following manure application and planting, suction cup lysimeters were installed five feet below the soil surface to collect water leaching below the root zone in the 0 and 112 kg N ha^-1 subplots of all treatments as well as the 224 kg N ha^-1 subplot of the no manure treatment. Water samples were collected weekly from installation until two weeks after harvest and were analyzed for nitrate concentration using a single reagent with vanadium chloride as the reductant.

Nitrate concentrations ranged from below the detection limit of 0.02 mg L^-1 to above 300 mg L^-1.  As expected, fields amended with higher rates of nitrogen fertilizer generally had greater nitrate concentrations. In 2012, less nitrate leached from plots amended with separated solid manure compared to separated liquid manure.  Nitrate leaching from plots where a nitrification inhibitor was used varied by location. 2013 results are expected to show similar patterns.