Inhibition of algal photosynthesis to control pH and reduce ammonia volatilization from rice floodwater

When urea or ammoniacal-N fertilizers are applied to the floodwater of a rice crop, fertilizer use efficiency is often reduced because there are substantial losses of NH₃ by volatilization. As pH rises the potential loss increases exponentially due to the increasing dominance of volatile NH₃ gas in equilibrium with NH ₄ ⁺ . We postulate that the daytime pH rise is caused mainly by photosynthesis of algae and Cyanobacteria, and that addition of a suitable photosynthetic inhibitor, concurrently with fertilizer, should suppress the pH rise, thus conserving N in the form of the non-volatile NH ₄ ⁺ . We selected terbutryne (2-(tert-butylamino)-4-(ethylamino)-6-(methylthio)-s-triazine) as the most promising inhibitor. In rice floodwater fertilized with urea the addition of terbutryne dampened the diurnal fluctuation in pH for 6 days and significantly increased the ammoniacal-N (AN) concentration measured in the floodwater. The concentration of ammonia gas in the air in equilibrium with the water, ₀, which is proportional to the gaseous flux of NH₃ at a given wind speed, was substantially reduced by terbutryne addition. Maximum values were reduced by over 50%. Terbutryne reduced the calculated cumulative NH₃ emission by 43%, relative to the fertilizer (N + P) control. Terbutryne also suppressed photosynthetic oxygen production. Therefore, it may reduce N fertilizer losses by inhibiting nitrification, an aerobic process, so retarding subsequent denitrification losses of gaseous nitrogen and nitrogen oxides.Part of the supporting documentation for Fertilization of Crops. International Patent Application PCT/AU86/00093 filed 11 April 1986.