AMMONIA VOLATILIZATION FROM SOILS AMENDED WITH SURFACE-APPLIED UREA

Urea is the most commonly used solid form of nitrogen fertilizer and is unique because it is hydrolyzed to ammonia and carbon dioxide by the enzyme urease. The resulting high ammoniacal-nitrogen content and pH may promote ammonia volatilization. The purpose of this research was to investigate the soil properties and environmental conditions that influence urea hyrolysis and subsequent ammonia loss.; The effects of air drying and sampling depth on urea hydrolysis rates from soils in pasture and cultivation were determined. For cultivated soils, urea hydrolysis rates were highly correlated with total nitrogen and organic carbon and were not affected by air-drying or sampling depth within the Ap horizon. For field-moist soils in pasture, urea hydrolysis rates were moderately correlated with total nitrogen and organic carbon. Drying generally decreased hydrolysis rates and reduced correlations.; Soil properties related to urea hydrolysis rates also affect ammonium retention, but their relative importance on ammonia loss has not been compared. The capacity of a soil to retain ammonium had a greater effect than the rate of urea hydrolysis in controlling ammonia loss from urea.; Ammonia loss from urea is influenced by water, but the effects of soil moisture, evaporation, and drying have not been separated. A system was designed to allow evaporation with or without soil drying. On a Captina sil (Typic Fragiudult), immediate drying inhibited urea hydrolysis and ammonia loss. Drying after hydrolysis was complete resulted in losses equivalent to those measured during evaporation without drying. Drying at a critical time during hydrolysis increased the hydrolysis rate and ammonia loss. Air at 85% relative humidity was sufficient to cause ammonia loss from an initially dry soil.