Use of urease, algal inhibitors, and nitrification inhibitors to reduce nitrogen loss and increase the grain yield of flooded rice (Oryza sativa L)

We studied the interacting effects on NH3 loss and grain yield of adding (1) urease inhibitors to retard the hydrolysis of urea (2) the algicide terbutryn to limit floodwater pH increases, and (3) C2H2 (provided by wax-coated calcium carbide) to prevent NH3 oxidation. The algicide treatment maintained the floodwater pH values below 8 for the first 3 days after the urea application and depressed the maximum values below 8.5 on subsequent days. As a consequence, NH3 loss was significantly (P<0.05) reduced in all treatments containing algicide. The addition of wax-coated calcium carbide effectively inhibited nitrification, as judged by the increased ammoniacal (NH3+NH4) N concentrations in the floodwater. However, these increased ammoniacal-N concentrations resulted in large losses of NH3. The results also showed that the effectiveness of a urease inhibitor cannot be judged solely from the ammoniacal-N concentrations in the floodwater of a single treatment with the inhibitor. Additional treatments with an algicide and a nitrification inhibitor are required to determine whether the low ammoniacal-N concentrations are caused by NH3 losses and nitrification. Thus N-(n-butyl)thiophosphorictriamide (NBPT) appeared to retard urea hydrolysis when judged by the low ammoniacal-N concentrations in the floodwater; however, treatments with NBPT, algicide, and C2H2 showed that the low concentrations were mainly a result of NH3 volatilization and nitrification. Even though NBPT did not completely inhibit urea hydrolysis, some treatments with this compound reduced NH3 losses and increased grain yields by up to 31%.