Depleted N 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 NO₃ 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, W₁ ( 1.5 ET), W₂ ( ET) and W₃ ( 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 N03 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.