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



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 NH3 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 NH3 loss were rapid. Similar trends occurred at a lower initial soil moisture content.

On air-dry soil (0.06 g H2O/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 NH3 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.