The role of Azolla cover in improving the nitrogen use efficiency of lowland rice

The development of management techniques to improve the poor N use efficiency by lowland rice (Oryza sativa L.) and reduce the high N losses has been an important focus of agronomic research. The potential of an Azolla cover in combination with urea was assessed under field conditions in Laguna, Philippines. Two on-station field experiments were established in the 1998-1999 dry season and eight on-farm experiments per season were carried out in the 2000-2001 wet and dry seasons. Treatment combinations consisting of N levels applied alone or combined with Azolla were evaluated with respect to floodwater chemistry, N-15 recovery, crop growth, and grain yield. A full Azolla cover on the floodwater surface at the time of urea application prevented the rapid and large increase in floodwater pH and floodwater temperature. As a consequence, the partial pressure of ammonia (rhoNH(3)), which is an indicator of potential NH3 volatilization, was significantly depressed. N-15 recovery was higher in plots covered with Azolla where the total N-15 recovery ranged between 77 and 99%, and the aboveground (grain and straw) recovery by rice ranged between 32 and 61%. The tiller count in Azolla-covered plots was significantly increased by 50% more than the uncovered plots at all urea levels. Consequently, the grain yield was likewise improved. Grain yields from the 16 on-farm trials increased by as much as 40% at lower N rates (40 and 50 kg N ha(-1)) and by as much as 29% at higher N rates (80 and 100 kg N ha(-1)). In addition, response of rice to treatments with lower N rates with an Azolla cover was comparable to that obtained with the higher N rates without a cover. Thus, using Azolla as a surface cover in combination with urea can be an alternative management practice worth considering as a means to reduce NH3 volatilization losses and improve N use efficiency.