Nitrate Leaching Potential in a Semi-Arid Region Affected More By Crop Rotation Than Nitrogen Fertilizer Rate.

Nitrate leaching into groundwater is a growing concern in many agricultural systems, including dryland systems in semiarid regions of the northern Great Plains (NGP) of North America where summer fallow is common. Despite this concern, the effects of agricultural practices on nitrate leaching have been relatively unstudied. Nitrate concentrations were measured in a sandy clay loam at 30-cm increments to a depth of 180 cm after the 4^th year of a study investigating effects of cropping system and available nitrogen (N) rate (0, 0.5x, 1x, and 1.5x the recommended rate of 0.83 kg N Mg-grain^-1) on net revenue. The mean nitrate-N pool in the 90 – 120 cm depth increment was 17.6 kg ha^-1, two-fold greater (analysis of variance; P=0.08) in the fallow-winter wheat system than the pulse grain-winter wheat system when analyzed across N rate, suggesting fallow increased nitrate leaching potential. The effect of nitrogen rate was less important than cropping system in the 90-120 cm depth, with about 30% greater soil nitrate (17.9 kg N ha^-1) at the 1.5x rate (1.25 kg N Mg-grain^-1) than at the recommended rate. Cropping system did not have an effect in the two lowest depths (120-150 and 150-180 cm), likely because of insufficient deep percolation to move much nitrate that far after four years. Notably, the highest soil nitrate-N levels across all treatments were in the 150-180 cm depth increment (15-28 kg ha^-1), below the effective root zone (~100 cm) estimated from volumetric soil water contents, suggesting the long-term practice of summer fallow - cereal (at least ~30 years prior to the study) had accumulated nitrate at that depth. The pulse grain – winter wheat system’s lower nitrate leaching potential and higher net revenue across N rates suggests this system could be a win-win from both an environmental and economic perspective.