Effect of timing of simulated rainfall on ammonia volatilization from urea, applied to soil of varyingmoisture content

SUMMARY Ammonia volatilization from granular urea applied at 10gNm −2 to pasture was investigated using an enclosure method. Misting 0, 4 or 16 mm of water on to the soil at field capacity within 3 h of urea application resulted in total NH 3 losses of 2.81, 0.92 and 0.18 g N m −2 respectively. Further delaying the watering reduced this effect until at 48 h, volatilization was lowered from 3.33 to only 3.09gNm −2 with 16mm of water. Hydrolysis and NH 3 loss were rapid. Similar trends occurred at a lower initial soil moisture content. On air‐dry soil (0.06 g H 2 O/g soil), hydrolysis was slow (73 ± 14% of the urea remained after 30 days) and volatilization, while gradual, accounted for 33% of applied urea‐N after 30 days. Addition of 16 mm of water 48 and 96 h after urea application was followed by a period of rapid hydrolysis and volatilization, resulting in a total loss of 2.59 and 2.40gNm −2 respectively. Repeated addition of 2mm of water produced bursts of hydrolysis and NH 3 loss until completion of hydrolysis when additional water had no effect. A total loss after 30 days of 3.94 g N m −2 occurred in this 2 mm treatment.