Effects of Irrigation Water Quality and Nitrogen Rate on the Recovery of 15N Fertilizer by Sorghum in Field Study

The effects of N-15-labeled ammonium nitrate on yield, uptake of nitrogen (N) by sorghum (Sorghum sudanense, Piper), and on N remaining in the soil were studied in a field experiment with different N rates (0, 50, and 100 kg N ha(-1)) and with two irrigation water qualities, well water (WW) and treated wastewater (TWW). Treated wastewater irrigation increased dry matter and N yield compared to WW. At equal N rates, recovery of N-15-labeled fertilizer by plants increased with TWW irrigation compared to WW (36% versus 23%). Neither fertilizer rate nor water quality had an effect on the N-15-labeled fertilizer remaining in the 0- to 60-cm layer of soil. On average 41% in the TWW treatment (49-33%) and 38% in WW treatment was mostly present in the surface 20-cm layer. Losses of N-15-labeled fertilizer were unaffected by irrigation water quality (35%) and increased with N application rate in TWW (4% versus 31%).