N-Source and Irrigation Effects on Nitrate Leaching

Efficiency of N fertilizer sources and N leaching below the root zone of irrigated crops are concerns of agronomists and environmentalists alike. Practical data comparing the efficacy and leaching susceptibility of various N sources are necessary for resource management and as input to current nutrient transport models. This study was conducted to evaluate the effects of N fertilizer source and irrigation management on distribution of N and on leaching of NO₃-N within the root zone of a small-grain crop. Undisturbed columns of Maddock fine sandy loam, a well-drained Udorthentic Haploboroll, were ciopped to wheat (Triticum aestivum L. cv. 'Era') in a greenhouse study. Three N source, Ca(N0₃)₂, (NH₄)₂SO₄, and CO(NH₃)₂, and a check treatment were compared in factorial combination with two irrigation levels. Nitrate and NH₄+-N concentrations in leachate were measured throughout the study. Total-N and NO₃--N in the soil columns were measured at harvest. Percolation from the columns was the same for all treatments. Essentially, none of the applied N leached below the rooting depth as either NH₄+-N or N0₃--N. Maximum depth of NO₂-- N leaching, which was approximately 60 cm, occurred with Ca(N0₃)₂ and high volume-low frequency irrigations. No N0₃--N accumulated below 20 cm when (NH₄)₂SO₄, and CO(NH₂)₂ were applied, followed by low volume-high frequency irrigations. Total amount and depth of N0₃-₃-N leached were dependent on irrigation method and N source. High volume-low frequency irrigations leached solutes to a greater depth than did low volume-high frequency irrigations. Contribution of N source to N0₃--N leaching was generally CO(NH₂)₂≤ check<(NH₄)₂S0₄<Ca(N0₃)₂, although N0₂--N leaching from urea treatments may have been offset by ammonia volatilization following treatment applications. Nitrate leaching from N fertilizers under irrigation may be minimized by timely applications of ammonia based fertilizers and low volume irrigations to minimize leaching below the crop root zone.