Transformations and transfers of nitrogen after irrigating a cracking clay soil with a urea solution

Abstract

The effectiveness of applying urea fertilizer (120 kg N ha-1) in irrigation water was investigated in a commercial sunflower crop on a calcareous, cracking clay soil. This method of fertilizer application resulted in fairly even distribution of applied nitrogen (N) within the soil profile down to the bottom of the plough layer and along the field in the direction of flow. At physiological maturity (76 days after fertilization) the data suggest that 57 kg of the applied N per hectare was recovered in the plant tops and the bulk of this (42 kg) was in the grain. Less than 2 kg N was lost as ammonia (NH,) and c. 2 kg N was emitted as nitrous oxide (N2O) during the first 23 days after fertilizer application, when most of the N not recovered by the sunflowers was lost. Slightly more than half of the N2O emitted was lost during the 11 days after fertilization, when NH+4 was being nitrified; the remainder was lost after a second irrigation by which time most of the urea had been converted to nitrate. A partial N balance suggests that within 20 h of the second irrigation a substantial amount of N (between 22 and 30 kg ha-1) was lost as a result of denitrification. Thus, although the method of application of fertilizer N is simple and cost effective, produces an even distribution of applied N across the field, and reduces NH3 loss, it still appears to be subject to substantial losses of N, presumably by denitrification, during subsequent irrigations.