THE ROLE OF AZOLLA IN CURBING AMMONIA VOLATILIZATION FROM FLOODED RICE SYSTEMS

By the year 2020, an additional 300 million tons of rice are needed an nually to meet the demands of a growing population. If our natural res ource base is to be preserved, intensification strategies should rely on integrated nutrient management, making full use of biological nitro gen fixation. The Azolla-Anabaena complex is amongst the most effectiv e systems of fixing nitrogen. In this paper we present evidence from g reenhouse studies on the potential of Azolla to curb the volatilizatio n of NH3 following the application of urea to a mixed Azolla-rice cult ure, providing a new incentive for developing ways of integrating Azol la in intensive rice cultivation systems. The results of a series of s hort term greenhouse experiments show that a full cover of Azolla can significantly reduce losses of applied urea-N from 45 and 50% to 20 an d 13% for the 30 and 60 kg N ha(-1) treatments, respectively. About on e-quarter of the applied N was tied up in the Azolla biomass. The appl ied N inhibited Azolla growth as well as the amount of N fixed. Inocul ation with smaller quantities of Azolla allowing for more vigorous Azo lla multiplication was equally effective in reducing NH3 volatilizatio n and doubled the amount of N-15 tied-up by Azolla. The reduction in N H3 volatilization is largely related to the depression by Azolla of th e floodwater pH, which in its absence may reach values between 9 and 1 0 as a result of algal activity. Early rice growth responded positivel y to urea as well as the large quantities of applied Azolla and increa sed the yield potential of the crop. Smaller quantities of Azolla alon e were not effective in this regard. The conservation of fertilizer N by Azolla, particularly when it fully covered the water, was reflected in a synergistic effect on rice dry matter production, amounting to 9 % at the 30 kg N rate and 16% at the 60 kg N rate. In all likelihood t his interaction is attributable to the higher efficiency of the applie d N. The benefits of Azolla in conserving basal urea-N even in small q uantities (200-500 kg fresh material ha(-1)), outweighed competition f or the applied N and may be as important as its BNF. The most promisin g integrated Azolla/rice management systems emerging from our studies should be given further attention under field conditions.