Control of gaseous nitrogen losses from urea applied to flooded rice soils

This paper reports field experiments designed to determine whether the two main processes responsible for nitrogen (N) loss from flooded rice (ammonia volatilization and denitrification) are independent or interdependent, and glasshouse studies which investigated the effect of soil characteristics on gaseous nitrogen loss. In the first field experiment ammonia (NH3) loss from the floodwater was controlled using algicides, biocides, frequent pH adjustment, shade or cetyl alcohol, and the effect of these treatments on total N loss and denitrification was determined. Most treatments reduced NH3loss through their effects on algal growth and floodwater pH. Total gaseous N loss (54% to 35%) and NH3loss (20% to 1.2%) were affected similarly by individual treatments, indicating that the amount lost by denitrification was not substantially changed by any of the treatments. In a subsequent field experiment NH3and total N loss were again affected similarly by the treatments, but denitrification losses were very low. In control treatments with different rates of urea application, NH3and total N loss were each a constant proportion of the urea applied (NH3loss was 17% and total N loss was 24%). These results indicate that techniques which reduce NH3loss can be expected to reduce total gaseous N loss. The glasshouse experiment showed that gaseous N losses could be reduced by draining off the floodwater, and incorporating the urea into the 0–0.05 m soil layer before reflooding. Even with this method, losses varied widely (6–27%); losses were least from a cracking clay and greatest from a coarse sand which allowed the greatest mobility of the applied N. Incorporation of applied urea can therefore be expected to prevent losses more successfully from clay soils with high ammonium retention capacity.