Effect of soil submergence on urea hydrolysis

Reports on the effects of soil submergence on urea hydrolysis are divergent, and hydrolysis of urea in the components (floodwater; oxidized and reduced soils) of the wetland soil system is not completely understood. In laboratory experiments, we evaluated the effect of soil submergence on urea hydrolysis with time in the floodwater, the oxidized soil, and the reduced soil. In a submerged soil system with about 1 mm of floodwater and incubated for <24 h, the depletion of O2 seemed to retard the hydrolysis. With a longer submergence time, soil Eh decreased, and soil urease activity also decreased to a nearly stabilized value. However, on reoxidation of the reduced soil under a continuous 1 cm of floodwater, the soil urease activity showed a marked increase. In general, the order of the urea hydrolysis in the main three components of the wetland soil system was: oxidized soil > reduced soil > floodwater (without algae). The presence of algae increased urease activity in the floodwater. The iron oxides (FeO-Fe2O3, Fe2O3, and αFeOOH), which may be present in the floodwater and the underlying oxidized layer, did not seem to influence urease activity in the floodwater. It is apparent from these results that urease activity will show temporal and spatial variations in the wetland rice soils. The amount of urea hydrolyzed in a given time, therefore, will be an overall consequence of (1) the differences in the urease activities in the floodwater and in the underlying oxidized-reduced soil components of the system, and (2) the relative distribution of urea in those components as influenced by method of application used. We suggest that, for a better understanding of in situ urea hydrolysis in a wetland rice field, along with the effect of soil submergence time, the effects of rice plant rhizosphere and sunlight need to be considered.