Field studies were initiated in the fall of 1992, to evaluate the effects of nine alternative nitrogen (N), tillage, and crop management strategies on N loss to the shallow groundwater. The tillage and N management treatments included use of the late-spring nitrate-nitrogen (NO3-N) test and a reduced N fertilizer application rate of 100 lb/ac applied to corn grown in a corn and soybean rotation with either no-till or chisel plowing as the primary tillage practice (Treatments 1 to 4); and the use of swine manure as the N source for corn grown in rotation with no-till soybean (Treatment 5). Alternate crop management strategies included continuous corn fertilized with either swine manure or 120 lb N/ac (Treatments 6 and 7), a narrow-stripping cropping configuration that included corn, soybean, and oat followed by a N-fixing berseem clover cover crop (Treatment 8), and alfalfa (Treatment 9). Experiments were conducted on 40 one-acre plots in north central Iowa. The first two years of data clearly indicate that lower NO3-N concentrations can be obtained in the shallow groundwater by reducing N application rates to 100 lb/ac. Use of the late-spring N test and differential N fertilization rates based on that test resulted in the lowest NO3-N concentrations in subsurface drainage water under both no-till and chisel plow treatments when compared with manure application of single N application rates of 100 lb/ac. The alfalfa and narrow-strip crop plots had the lowest NO3-N concentrations (less than 10 ppm) in the subsurface drainage water at this research site. Corn following soybean on plots fertilized with swine manure had an average yield that was 1 bu/ac higher than with the LSNT, but because of the difficulty in applying the intended amount of N with manure, NO3-N concentrations in the tile drainage water were much higher. When averaged for the two years, no-till yield was slightly lower, but for both tillage practices, use of the LSNT resulted in equal or better yields than either preplant fertilization or application of swine manure.
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