Urea fertilization and the N-cycle of rice-fields in the Camargue (S France)

The use of urea as N-fertilizer for rice production has been studied for European conditions. In field and pot experiments, urea appeared to be more efficient than ammonia or nitrate. The rice yield could be maintained at maximal local production values by broadcasting 100 kg ha(-1) of urea-N instead of using 200-200 kg ha(-1) of ammonia or nitrate nitrogen. The best results were obtained by adding the urea about 5-8 weeks after sowing. When this was done, splitting the dose did not increase the efficiency. I In laboratory experiments it was shown that urea diffuses and hydrolyses rapidly to ammonia in the anoxic layers of the flooded soils. Because this happens in the anoxic zones, the ammonia cannot be oxidised to nitrate, and is therefore less prone to denitrification, but it is adsorbed onto the soil particles. The upward diffusion of ammonia is slow, rendering it available to the rice roots for a relatively long time. The diminished use of N fertilizer is profitable for the rice farmer and helps to protect the surrounding environment.