Effect of azolla cover on urea broadcast in flooded soils

Greenhouse and filed experiments were carried out to determine the effect of azolla cover on urea broadcast in flooded soils, and on solution's and flood water's pH, temperature and nitrogen dynamics governing volatilization losses. results from the greenhouse experiment showed that the presence of azolla cover retained greater amount of ammonium nitrogen in the flood water and reduced nitrogen losses by about 80-90% as revealed in the nitrogen balance. Full azolla cover on floodwater surface (100%-150%) upon broadcasting Urea was the most effective as physical barrier for reducing ammonia nitrogen volatilization. The higher ammonium nitrogen retained with azolla cover had caused higher pH of the culture solution as against no azolla treatment. Initially, partly covering (50%-75%) the floodwater was as ineffective as without azolla cover but with time, the presence of azolla cover regardless of density (50%-150%) proved effective. Azolla caroliniana was better than Azolla pinnata var. pinnata and Azolla pinnata var. imbicata as physical barrier. A pinnata var. pinnata and SA. pinnata var. imbricata were most efficient at 150% azolla cover but 100% azolla cover was better in A. caroliniana. On the other hand, the growth rate of Azolla was reduced as azolla cover density increased. At 50% cover, growth had almost tripled (doubling time of 2.6 days) its initial biomass within 7 days. The total nitrogen content of azolla tissues was reduced in the thicker azolla mat of 150%. Field experiments carried out fields without rice and with the presence of standing rice at maximum tillering stage confirmed the positive effect of azolla cover in holding more ammonium nitrogen concentration in the floodwater as well as the factors that affect volatilization such as pH and temperature. Floodwater pH was reduced by 0.5 - 1.4 units while the floodwater temperature was reduced by 2 degrees celcius to 5 degrees celcius. Presence of azolla cover seemed to have a more significant effect when there was no rice than with the presence of full rice canopy.