Fate of nitrogen-15-labelled urea applied to wheat on a waterlogged texture-contrast soil

Fertilizer N losses on waterlogged texture-contrast soils (sand over clay) are usually attributed to denitrification and leaching. In this experiment, waterlogging events were imposed on 25-cm-diameter, 75-cm-long columns of texture-contrast soil planted to wheat (Triticum aestivum L.). Treatments included 6, 12, and 18 d of aerobic conditions between fertilization using N-15-labelled urea (5.0 g m(-2)) and 3-d waterlogging events. Denitrification, measured by N-15-chamber methods, was the largest loss mechanism identified during waterlogging. Dinitrogen was the main product of denitrification. Longer aerobic periods prior to waterlogging increased denitrification losses from 3.1 to 9.4% of the urea-N added. Leaching losses of (NO3)-N-15 (3.1-5.3%) between 20 and 70 cm were less than denitrification fluxes. Total N-15 recovery in the wheat plants and soil was 87.9% before waterlogging and decreased to 72.3% after waterlogging. The balance of added fertilizer N was reasonably well reconstructed if it is assumed that NH3 volatilization accounted for the early loss of 12% of the urea-N, and that in addition to the measured surface fluxes of N-2 + N2O, some of these gases remained entrapped in the soil. This study confirms that texture-contrast soils cropped to wheat have a high potential for N losses through denitrification and leaching during waterlogging events.