Animal agriculture often has been associated with water contamination. In particular, intensively managed pastures have been implicated in ground water contamination by nitrate, especially in humid regions, on thin or sandy soils, and where N additions through fertilization or symbiotic fixation are high. We found little increase in ground water nitrate under pastures on central Wisconsin farms where little N was applied, and hypothesized that denitrification may be contributing to nitrate removal. We evaluated ground water chemistry and gas composition using pumping induced ebullition under one pasture and one conventionally cropped field. Responses to dairy cow urine and dung were monitored under controlled environment conditions with intact cores from the pasture site. The solute and dissolved gas composition of ground water under the pasture indicated that conditions were highly variable, but generally favorable for denitrification, and that a large portion of leached nitrate was converted to dinitrogen gas. Similar patterns were not evident at the corn site, due to insufficient dissolved organic carbon substrate. Nitrate leaching was lower under pasture than corn, as evidenced by lower total nitrate concentrations (excess dinitrogen gas plus nitrate) in ground water under pasture. In the intact core experiment, nitrous oxide emissions were equal to about 3% of total nitrate accumulation by the end of the month-long incubation, with losses of 0.5 mg N in the control, 1.8 mg in the dung treatment, and 5.4 mg N with urine addition. Given that excess dissolved dinitrogen gas was about equal to solution nitrate in the field, these results support the idea that denitrification actively mitigates ground water nitrate contamination under pastures managed with frequent rotational grazing, compared to conventional cropping.