Flood irrigation of wheat on a transitional red-brown earth. II. Effect of duration of ponding on availability of soil and fertilizer nitrogen

Abstract

Grain yield of wheat growing on a transitional red-brown earth was reduced by long periods of ponding during irrigation. To determine whether fertilizer N loss was a major cause of this yield decline, 15N-labelled urea was applied to microplots at the same time and rate as the crop was topdressed with urea (end of tillerfng, 100 kg N ha-1). In addition, 15N-labelled nitrate was used to assess denitrification potential during and after each irrigation. Prior to the first irrigation (19days after urea application), 76% of the urea N was immobilized in the plants (33%) and soil (43%), 15% was present as soil mineral N, and 9% was not accounted for. The majority (85%) of the urea-derived mineral N was present as ammonium in the 0-0.1 m soil layer. After the first irrigation, amounts of mineral N in the soil remained very low at 3-6 kg N ha-l in the 0-0.2 m layer, with only 5-15% of this in the nitrate form. There was no significant effect of duration of ponding on plant or soii recovery of urea N after any irrigation. Mean plant recovery increased to 52%, over the first 55 days following urea application, while soil recovery declined to 22%. Beyond this stage changes in plant and soil recoveries were negligible. Plant total N content increased throughout the season due to ongoing mineralization of native soil N; however, there was negligible net mineralization of recently immobilized 15N beyond day 55. At physiological maturity, 46% of the N acquired by the plants was derived from the fertilizer. Losses of urea N increased to 25% over the first 55 days, and appeared to be due to nitrification-denitrification. Field studies with labelled nitrate indicated that denitrification potential in the soil was high throughout the experimental period. After the final two irrigations there were some significant (P < 0.05) effects of duration of ponding on N loss from urea, with lowest losses in the sprinkler, 1 h and 12 h treatments. The data indicate that, on the transitional red-brown earth, the adverse effect of long periods of ponding on wheat yield was not due to decreased availability or uptake of fertilizer or soil N in the longer term.