Depleted 15N carryover, leaching and uptake for three years of irrigated corn

Deep percolation of nitrate can contribute to the deterioration of groundwater resources. Leaching of nitrate is a complex process affected by fertilizer and irrigation practices, efficiency of N use by the crop, and how the soil's water holding capacity and water transmission properties are affected by soil texture. Depleted (15NH4)2SO4 fertilizer at N rates of 0, 125, 250 and 375 kg ha-1 was applied annually for 3 years to continuous corn grown within three different water regimes. This time period and the labeled N permitted an evaluation of N use efficiency by the crop and NO 3 leaching and carryover on a Weld silty clay loam, a fine-textured soil, typical of the "hardland" soils of the semi-arid Great Plains. Three water regimes, W 1 (approximately 1.5 ET), W 2 (approximately ET) and W3 (approximately 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 depth to measure weekly percolate and the NO3 concentration in the percolate. 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 sampled annually in the fall after crop harvest to determine NO3-N in the soil or carryover. Great variability was encountered in measuring the amount of extractor water and its NO3 content under each water regime, which made estimates of NO3 leaching losses unreliable. Also, the variability 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 within the water regime W1, where leaching was expected to be greatest, only 1% of the cumulative labeled N applied was found in extractor waters and most movement of the labeled N into extractors occurred the third year. The 125-kg-ha-1 yr -1 fertilizer N rate significantly increased the crop yield over the unfertilized plots without increasing residual NO3-N accumulation; whereas fertilizer N rates of > 125 kg ha-1 yr-1 did not appreciably increase plant yields over the 125-kg-ha 1-N rate, but did appreciably increase residual NO3.