Nitrous oxide emissions from cool-season pastures under managed grazing

High stocking densities on grazed pastures may promote nitrous oxide (N2O) loss from soil to the atmosphere. However, studies of N2O fluxes in cool-season pastures of North America are lacking. We performed two experiments in which measured N2O fluxes were bootstrapped with re-sampling (n = 100, with 10,000 iterations), which allowed us to generate an empirical distribution of mean fluxes to understand how pasture management strategies might affect N2O emissions. In Experiment 1, N2O fluxes were estimated in southern Wisconsin pastures under rotational grazing, continuous grazing, haymaking, and no agronomic production. Nitrous oxide fluxes were significantly positive under rotational grazing at our research farm [21.6 (se = 10.3) mu g m(-2) h(-1)], but not significantly different than zero under the other three treatments or rotationally grazed paddocks across eight working farms. In Experiment 2, we measured N2O fluxes in eastern Nebraska before, during, and after two rotational grazing events under two N-input treatments-inorganic N fertilizer and supplemented dried distillers grains-and an unfertilized control. Nitrous oxide fluxes were positive (20-100 mu g m(-2) h(-1)) in periods following rain, but otherwise not significantly different than zero. Post-grazing, N2O emissions were lower from the control than fertilized or supplemented treatments. These experiments show cool-season pastures can be a source of N2O to the atmosphere, but primarily following grazing events that coincide with significant precipitation. However, even though on-farm paddocks are in varying states of recovery from defoliation, farm scale emissions, although episodic, are likely to be positive in years with above average precipitation.