Poor crop establishment associated with urea-induced ammonia volatilization inhibits the optimum plant growth of dry direct-seeded rice. The objective of this study was to examine the reduction of soil ammonia volatilization by urease inhibitors when urea is applied at seeding and to identify the sensitivity of shoot and root growth of dry direct-seeded rice to ammonia volatilization. Two Petri-dish incubation experiments were conducted using three different soils with three urease inhibitors. N as urea was applied to the Petri-dish at the rate of 1mg g-1 of soil. To simulate soil water content suitable for direct-seeded rice, soil inside the Petri-dishes was kept under aerobic conditions. Ammonia volatilization due to application of urea at seeding reduced early seedling growth of dry direct-seeded rice. Urease inhibitors significantly reduced ammonia volatilization. Among the three urease inhibitors, N-(N-butyl) thiophosphoric triamide (NBPT) was the most effective one in reducing ammonia volatilization, followed by phenyl phosphorodiamidate (PPD) and hydroquinone (HQ), respectively. The findings imply that application of suitable urease inhibitors can significantly improve seedling growth of dry direct-seeded rice by reducing ammonia volatilization from added urea. This study also suggests that root growth was more sensitive to ammonia volatilization than shoots when urea was applied at seeding.
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