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INFLUENCE OF UREASE ACTIVITY AND SOIL PROPERTIES ON AMMONIA VOLATILIZATION FROM UREA

When urea is hydrolyzed to ammonia and carbon dioxide by the enzyme urease, nitrogen can be lost via ammonia volatilization. The purpose of this study was to evaluate the influence of soil physical and chemical properties and urease activity on ammonia volatilization from surface-applied urea. Seven soil samples from the 0–15-cm depth of cultivated fields were selected to provide a wide range in urease activity, percentage of clay, CEC, hydrogen ion buffering capacity, and organic carbon levels. Fertilizer-grade urea was applied to these samples at a rate equivalent to 100 kg N ha−1 in laboratory chambers at 28 ± 2°C. Air flow of two chamber volume exchanges per minute at a relative humidity of 30% and a soil moisture potential of −0.034 MPa were maintained. Volatilized ammonia was trapped in acid and determined by steam distillation. Ammonia volatilization from the soils was characterized by a period of nearly linear ammonia less before volatilization was negligible. The ammonia volatilization rates ranged from 0.101 to 0.416% h−1 and were negatively correlated with percentage of clay, total nitrogen, CEC, organic carbon, hydrogen ion buffering capacity, and urease activity. Results from this study suggested that a low hydrogen ion buffering capacity was a better indicator of a high potential for ammonia volatilization from surface-applied urea than soil pH or urease activity in the seven soils studied.