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 annually to meet the demands of a growing population. If our natural resource base is to be preserved, intensification strategies should rely on integrated nutrient management, making full use of biological nitrogen fixation. The Azolla-Anabaena complex is amongst the most effective systems of fixing nitrogen. In this paper we present evidence from greenhouse studies on the potential of Azolla to curb the volatilization of NH3 following the application of urea to a mixed Azolla-rice culture, providing a new incentive for developing ways of integrating Azolla in intensive rice cultivation systems. The results of a series of short term greenhouse experiments show that a full cover of Azolla can significantly reduce losses of applied urea-N from 45 and 50% to 20 and 13% for the 30 and 60 kg N ha(-1) treatments, respectively. About one-quarter of the applied N was tied up in the Azolla biomass. The applied N inhibited Azolla growth as well as the amount of N fixed. Inoculation with smaller quantities of Azolla allowing for more vigorous Azolla multiplication was equally effective in reducing NH3 volatilization and doubled the amount of 15N tied-up by Azolla. The reduction in NH3 volatilization is largely related to the depression by Azolla of the floodwater pH, which in its absence may reach values between 9 and 10 as a result of algal activity. Early rice growth responded positively to urea as well as the large quantities of applied Azolla and increased the yield potential of the crop. Smaller quantities of Azolla alone 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 this interaction is attributable to the higher efficiency of the applied N. The benefits of Azolla in conserving basal urea-N even in small quantities (200-500 kg fresh material ha-1), outweighed competition for the applied N and may be as important as its BNF. The most promising integrated Azolla/rice management systems emerging from our studies should be given further attention under field conditions.