SOIL MANAGEMENT Tillage and Nitrogen Fertilization Influence Grain and Soil Nitrogen in an Annual Cropping System

ABSTRACT to optimize crop yields in a dryland annual cropping system. Long-term N fertility studies have shown thatIncreasing the frequency of cropping in dryland systems in the residual soil NO3–N levels increase when N fertilizationnorthern Great Plains requires the application of N fertilizer to main-tain optimum crop yields. A 12-yr annual cropping rotation [spring rates exceeded that needed for maximum yield (Halvor-wheat (Triticum aestivum L.)–winter wheat–sunflower (Helianthus son and Reule, 1994; Porter et al., 1996; Raun and John-annuus L.)] under dryland conditions was monitored to determine son, 1995; Westerman et al., 1994). Black et al. (1981) the influence of tillage system [conventional till (CT), minimum till reported more efficient water use with more intensive (MT), and no till (NT)] and N fertilizer rate (34, 67, and 101 kg N cropping systems and increased yields with N fertiliza-ha21) on N removed in grain and annual changes in postharvest soil tion. In addition to more efficient water use, more inten-NO3–N. Nitrogen removal in the grain increased with increasing N sive MT and NT cropping systems have the potentialrate in most years. Total grain N removal was lowest with NT at the to be more profitable and reduce soil erosion potentiallowest N rate and highest with NT at the highest N rate compared (Dhuyvetter et al., 1996; Merrill et al., 1999).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 ha21 Long-term applications of N fertilizer to dryland crop-fertilizer rates, respectively. Residual soil NO3–N levels were not ping systems can influence the level of residual soil affected by N rate or tillage system in the first 3 yr, but they increased NO3–N in the profile. Increasing levels of residual soi