DEPLETED N-15 CARRYOVER, LEACHING AND UPTAKE FOR 3 YEARS OF IRRIGATEDCORN

Deep percolation of nitrate can contribute to the deterioration of gro undwater resources. Leaching of nitrate is a complex process affected by fertilizer and irrigation practices, efficiency of N use by the cro p, and how the soil's water holding capacity and water transmission pr operties are affected by soil texture. Depleted ((NH4)-N-15)(2)SO4 fer tilizer at N rates of 0, 125, 250 and 375 kg ha(-1) was applied annual ly for 3 years to continuous corn grown within three different water r egimes. This time period and the labeled N permitted an evaluation of N use efficiency by the crop and NO3 leaching and carryover on a Weld silty clay loam, a fine-textured soil, typical of the ''hardland'' soi ls of the semi-arid Great Plains. Three water regimes, W-1, (similar t o 1.5 ET), W-3 (similar to ET) and W-3 (similar to 0.8 ET), were used. Beneath each plot within each water regime, Duke-Haise vacuum trough extractors were installed under undisturbed soil profiles at 1.22-m de pth to measure weekly percolate and the NO3 concentration in the perco late. The corn was harvested in the fall in the dent stage to measure the total above-ground biomass N uptake. Soil profiles (1.8 m) were sa mpled annually in the fall after crop harvest to determine NO3-N in th e soil or carryover. Great variability was encountered in measuring th e amount of extractor water and its NO3 content under each water regim e, which made estimates of NO3 leaching losses unreliable. Also, the v ariability demonstrates formidable problems in quantifying percolation losses with vacuum trough extractors under undisturbed fine-textured soil profiles. With the highest N rate of 376 kg ha(-1) yr(-1) and wit hin the water regime W-1, where leaching was expected to be greatest, only 1% of the cumulative labeled N applied was found in extractor wat ers and most movement of the labeled N into extractors occurred the th ird year. The 125-kg-ha(-1)-yr(-1) fertilizer N rate significantly inc reased the crop yield over the unfertilized plots without increasing r esidual NO3-N accumulation; whereas fertilizer N rates of > 125 kg ha( -1) yr(-1) did not appreciably increase plant yields over the 125-kg-h a(-1)-N rate, but did appreciably increase residual NO3.