NH3 volatilization, soil NH4+concentration and soil pH following subsurface banding of urea at increasing rates

Abstract

Subsurface banding of urea can result in large ammonia (NH3) emissions following a local increase in soil ammonium (NH4+) concentration and pH. We conducted a field experiment to determine how application rates of subsurface banded urea impact NH3 volatilization. Urea was banded at a 5 cm depth to a silty loam soil (pH = 5.5) at rates of 0, 6.1, 9.2, 13.3 and 15.3 g N m -1. Ammonia volatilization (wind tunnels), and soil NH4+ concentration and pH (0-10 cm) were monitored for 25 d following urea application. Volatilization losses increased exponentially with urea application rate to 11.6% of applied N for the highest urea rate, indicating that as more urea N was added to the soil a larger fraction was lost as NH3. Cumulative NH3-N emissions were closely related (R2≥ 0.85) to maximum increases in soil NH4+ concentration and pH, and their combined influence likely contributed to the nonlinearity of the volatilization response to urea application rate. However, the rapid increase in NH3 losses when soil pH rose above 7 suggests that soil pH was the main factor explaining the nonlinear response of NH3 volatilization. When compared with previous studies, our results suggest that the response of NH3 volatilization losses to urea application rate in acidic soils are controlled by similar factors whether urea is broadcasted at the soil surface or subsurface banded.