Effect of Soil Moisture on Ammonia Volatilization from Urea Applied Alfisol in the Dry Zone of Sri Lanka

Soil moisture is one of the major factors that determines volatilization of urea. In this study, we investigated the effect of soil moisture on urea volatilization from an Alfisol (Reddish Brown Earth soil). A bulk soil sample was collected from the research field of the Faculty of Agriculture, Rajarata University of Sri Lanka. A pot experiment was arranged under completely randomized design with four replicates per treatment. Soil moisture levels were maintained at 0 kPa (volumetric water content (VWC): 33%), 33 kPa (VWC: 16%), 44 kPa (VWC: 8%), and 54 kPa (VWC: 2%) as the treatments. A 50% overhead shade net was hung at 1.8 m aboveground covering the treatment pots. Urea was applied to each pot at a rate of 1,276.5 mg kg-1 and emitted NH3 was collected using the enclosure method. Soil NO3−-N, NH4+-N, pH, and EC were determined at six times at two-day intervals. The effect of soil moisture, time, and their interaction effect on NH3 volatilization was analyzed using repeated measure ANOVA with post-hoc Tukey HSD test for mean comparison. The NH3 volatilization significantly decreased with time (p < 0.05). Time and treatment interactions effect were significant for volatilized NH3 and other soil parameters measured except for soil NO3−-N (p < 0.05). A decreasing trend was observed for NH3 volatilization at 0 kPa, 33 kPa, 44 kPa & 54 kPa metric suctions as 6.3 g m-2 day-1, 4.9 g m-2 day-1, 4.7 g m-2 day-1, and 0.065 g m-2 day-1 respectively. Soil NH4+-N and EC fluctuated significantly among the treatments. Maintaining the soil moisture at 33 kPa during urea application is recommended for achieving high fertilizer use efficiency.