Fate of urea nitrogen applied in solution in furrows to sunflowers growing on a red-brown earth: transformations, losses and plant uptake

Abstract

Nitrogen transformations and transfers were investigated after applying a urea solution to a crop of sunflowers by ponding in furrows. The fertilizer was applied 24 days after sowing. Distribution of the applied nitrogen (N) in the soil, N uptake by plants, and losses by ammonia (NH3) volatilization and denitrification were measured. This method of fertilizer application resulted in concentration of the applied N in the furrow and shoulder zone of the soil beds; little of the applied N moved across to the centre of the beds. Inorganic N was not leached into the clay B horizon, but was retained by the surface 0-200 mm layer. A 15N mass balance showed that 28 kg of the 80 kg N ha-1 applied (35%) was lost during the experiment. Less than 5 kg N ha-1 (6% of the applied N) was lost as NH3 and the remainder (22 kg N ha-1 or 29% of the applied N) appeared to be lost by denitrification. The 15N balance data suggest that c. 4.6 kg ha-1 fertilizer N were lost after each irrigation up to 44 days after fertilizer application. The study indicates that N loss could be markedly reduced by delaying the bulk of the application to coincide with the period of rapid uptake.