Fate of urea nitrogen applied at planting to grain sorghum grown under sprinkler and furrow irrigation on a cracking clay soil

Abstract

Uptake of nitrogen (N) fertilizer and yields of grain by sorghum on Cununurra clay in north-western Australia have been found previously to be higher under sprinkler than under furrow irrigation. The cause of this was investigated using a 15N balance technique on field microplots. Additional data on sequential changes in the distribution of mineral-N in the soil, and on volatilization of ammonia, were collected from outside the 15N microplots. The improved effectiveness of N fertilizer under sprinkler irrigation was associated with less upward movement of 15N into the top of the ridge and thus out of the rooting zone of the crop. Furrow irrigation caused 22% of the 15N applied to move into the top of the ridge, sprinkler irrigation caused only 2% of the 15N to do this. The urea appeared to move upwards during the first irrigation, and this N became unavailable to the sorghum because of soil dryness, high temperatures, and osmotic effects in the surface soil. Loss of 15N from the soil-plant system was approximately 27% under both systems of irrigation. This loss was attributed to denitrification, because there was no evidence for large losses of N by leaching or volatization of ammonia.