Tillage and nitrogen fertilization influence grain and soil nitrogen in an annual cropping system

Increasing the frequency of cropping in dryland systems in the northern Great Plains requires the application of N fertilizer to maintain optimum crop yields. A 12-yr annual cropping rotation [spring wheat (Triticum aestivum L.)-winter wheat-sunflower (Helianthus annuus L.)] under dryland conditions was monitored to determine the influence of tillage system [conventional till (CT), minimum till (MT), and no till (NT)] and N fertilizer rate (34, 67, and 101 kg N ha -1 ) on N removed in grain and annual changes in postharvest soil NO 3 -N. Nitrogen removal in the grain increased with increasing N rate in most years. Total grain N removal was lowest with NT at the lowest N rate and highest with NT at the highest N rate compared with CT. Total grain N removal after 12 cropping seasons was 144, 84, and 61% of the total N applied for the 34, 67, and 101 kg N ha -1 fertilizer rates, respectively. Residual soil NO 3 -N levels were not affected by N rate or tillage system in the first 3 yr, but they increased significantly following consecutive drought years. Residual NO 3 -N in the 150-cm soil profile tended to be higher with CT and MT than with NT. Soil NO 3 -N movement below the crop root zone may have occurred in 1 or 2 yr when precipitation was above average. Results indicate that NT, with annual cropping, may reduce the quantity of residual soil NO 3 -N available for leaching compared with MT and CT systems.

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