Ammonia Toxicity Associated With Urea Application In Dry Directed-seeded Rice And The Mitigating Strategies

The dry direct-seeded rice technology has been developed to reduce water input, save labor demand and increase water-use efficiency in china. However, poor seed germination and early seedling growth are major constraints in the adoption of dry direct-seeded rice. Urea is the most widely used N fertilizer because of its high N content and low cost. Upon application to soil, urea is rapidly hydrolyzed by urea enzymes to form ammonia, the accumulation of excess ammonia can result in toxicity to dry direct-seeded rice. But until now, the particular causes of poor seed germination and reduced early seedling growth of dry direct-seeded rice after urea-N fertilizer application are still unclear.The objectives of this study were:(1) to examine the effects of soil ammonia volatilization on seed germination and early seedling growth of dry direct-seeded rice,(2) to examine the effect of soil ammonia volatilization on growth and yield of dry direct-seeded rice,(3) to identify the mitigating strategies which may alleviate ammonia toxicity induced by urea application at sowing. In this study, field experiments were conducted at Dajin and Huaqiao village, Wuxue, Hubei Province. Pot experiments and petri-dish incubation experiments were conducted at Huazhong Agricultural University. The following results were obtained:(1) The effect of different nitrogen management practice on soil ammonia, seed germination and early seedling growth of dry direct-seeded, plant growth and grain yield of dry direct-seeded rice was different. Ammonia volatilization increased as urea rates increased. The amount of ammonia volatilized in the treatment with1.0g N pot-1as urea was2.5times higher than that in the treatment with0.5g N pot-1as urea in pot experiment. However, ammonia volatilization in the treatment without urea application at sowing was negligible. In open field experiment, N management practices, such as, reducing rate of urea applied at sowing, delaying the first urea application, or application of ammonium sulfate significantly increased grain yield, total biomass production, grain filling and1000-grain weight, compared with higher rate of urea application at sowing.(2) Application of urease inhibitors significantly reduced soil ammonia volatilization, promoted seed germination and early seedling growth of dry direct-seeded rice. In petri-dish experiment the results showed that NBPT was the most effective inhibitor, followed by PPD and HQ. Root growth of dry direct-seeded rice was more seriously inhibited by ammonia volatilization than shoot growth, suggesting that the roots of dry direct-seeded rice were more sensitive to ammonia toxicity than the shoots.(3) Results from a series of pot and field experiments showed that soil ammonia volatilization was significantly reduced when urea was replaced by ammonium sulfate at sowing. At the same N rate, seed germination percentage, early seddling growth, and grain yield of dry direct-seeded rice under the ammonium sulfate application at sowing were markedly improved, compared with under urea application. In a field experiment, grain yield with ammonium sulfate at sowing was more than1t ha-1higher than that with urea at sowing. This study showed that grain yield of above6t ha-1was achieved in dry direct-seeded rice production system.(4) Application of urease inhibitors, reducing rate of urea applied at sowing, delaying the first urea application, or application of coated urea or ammonium sulfate significantly alleviated ammonia toxicity to dry direct-seeded rice to some extent.