Transformation and recovery of urea applied to a grass pasture in south-eastern Queensland

Abstract

The fate of urea-N during the 14 d after four seasonal applications to a Setaria sphacelata cv. Nandi pasture in south-eastern Queensland was traced. The aim was to explain the variations in the responses by grass, which have been observed after applications of urea. The total losses of urea-15N from the soil-plant system and the fluxes of NH, into the atmosphere were respectively 29 and 12% of the urea-N applied in summer, 45 and 42% in autumn, 23 and 13% in winter, and 20 and 9% in spring. Losses of nitrogen by leaching were probably negligible as mineral-N derived from the fertilizer remained mostly in the 0-2 cm layer of soil. Hydrolysis of urea to NH4+-N was complete after 1 d in summer, 3 d in spring, 6 d in autumn and 7 d in winter. Losses of urea-N were influenced by the water content of the surface soil (0-0.5 cm) at the time of the application and by the subsequent pattern of rainfall. The loss was large when urea was applied to wet soil and rainfall during the next 7 d did not exceed 1 mm, as in autumn. Losses were much smaller when rain (5 mm or more) fell soon after the applications, as in spring and summer. In winter urea was applied to dry soil and fluxes of NH, continued at a moderate rate until rain fell on the sixth day. Losses of 15N were generally about double the losses of NH,. Work on this discrepancy is needed to fully understand the fate of urea-N broadcast onto pastures. Losses of NH, from urea broadcast onto pastures could be minimized in practice by making applications to dry soil just before rain is expected or before irrigation.