Relative effects of ammonia and nitrite on the germination and early growth of aerobic rice

Recent studies have documented adverse affects of urea on the establishment and growth of aerobic rice when applied at seeding. The following experiments were conducted to examine the relative importance of ammonia and nitrite (NO$ _2^- $) toxicities as mechanisms contributing to poor germination and early growth of aerobic rice. Soil was collected from an experiment in the Philippines where aerobic rice was grown continuously for 7 years. Subsamples of the soil were: (1) pretreated with sulfuric acid (0.5 M H₂SO₄ added at 75 mL kg⁻¹), (2) oven-heated at 120°C for 12 h, or (3) left untreated. In a greenhouse study N was applied to the untreated, acidified, and oven-heated soils as either urea or ammonium sulfate (0.0 or 0.3 g N kg⁻¹). Plant height, root length, total biomass, and number of seminal roots were evaluated after 10 d. Microdiffusion incubations were used to assess the effects of soil pretreatment, N source, and N rate (0, 0.5, 1.0, 1.5 g N kg⁻¹) on ammonia (NH₃) volatilization and germination. Nitrite incubations were conducted to establish a critical level for NO$ _2^- $ toxicity and measure the extractable NO$ _2^- $ and germination trends as affected by soil pretreatment, N source, and N rate. On untreated soil, urea reduced early growth and germination while ammonium sulfate caused no adverse effects. Progressively higher rates of urea increased NH₃ volatilization and inhibited germination, while oven-heating and acidification minimized the adverse effects. All treatment combinations (soil pretreatment, N source, N rate) had extractable NO$ _2^- $ levels below the critical level of 0.2 g N kg⁻¹, suggesting that ammonia and not NO$ _2^- $ toxicity was the principal cause of inhibition. Since the risk of NH₃ toxicity is highest just following urea hydrolysis, strategies to optimize the timing and placement of urea should be considered.