I. R. P. Fillery

4. Reappraisal of the significance of ammonia volatilization as an N loss mechanism in flooded rice fields

The role of ammonia volatilization as a nitrogen loss mechanism in lowland rice (Oryza sativa L.) has recently been extensively reevaluated using techniques that do not disturb the field environment. This paper summarizes methodologies used in this research and discusses findings from recently conducted micrometeorological studies on ammonia volatilization. Factors affecting ammonia loss and the contribution of this process to the overall nitrogen loss from lowland rice systems are also outlined. Suggestions for future research are included.

Contribution of ammonia volatilization to total nitrogen loss after applications of urea to wetland rice fields

AbstractThe contribution of NH3 volatilization to the total N loss after the application of urea to flooded rice fields was evaluated in a series of experiments at two locations in the Philippines. Urea was applied in three ways:A.Broadcast and surface incorporated before transplanting (BI treatments), orB.Broadcast into the floodwater 14–21 days after transplanting (AT treatments), orC.Broadcast into the floodwater 5–7 days before panicle initiation (PI treatments). Total N loss was determined by using15N balance methods on microplots within fields where NH3 loss was measured concurrently by a direct, nondisturbing technique.The total15N losses in the AT studies at the completion of the NH3 loss measurements at Muñoz and Los Baños accounted for 45 and 60% of the15N applied, respectively. Ammonia volatilization accounted for all of the15N lost in the Muñoz study but only 45% of that lost at Los Baños. In comparison with the AT studies, lower N losses (18–26% of N applied) were obtained in the BI treatments. At Los Baños, NH3 loss again accounted for about half of this N loss. In the PI study at Muñoz, NH3 loss and total N loss accounted for 11 and 13% of the N applied, respectively.Thus, NH3 volatilization appeared to be the only important loss mechanism at Muñoz. In contrast, loss by an alternative mechanism, most probably involving nitrification-denitrification, was of equal importance to NH3 volatilization at Los Baños. Differences in windspeeds, temperatures, and soil properties at the two sites may account for the variation in the relative importance of the two N loss mechanisms.

Results from recent studies on nitrogen fertilizer efficiency in wetland rice

Yields of rice can be substantially increased by the use of nitrogenous fertilizers, but the return to the producer depends very much on the mode of application. This article reviews the factors affecting the uptake of nitrogen by wetland rice, in terms of grain production, and the means by which experimentally achieved results can be translated into everyday farming practice.

Effect of phenyl phosphorodiamidate on the fate of urea applied to wetland rice fields

The effect of phenyl phosphorodiamidate (PPD) on floodwater properties, N uptake,15N recovery, and grain yield of wetland rice (Oryza sativa L.) was evaluated in a series of field studies conducted at Muñoz and Los Baños, Philippines. Prilled urea and PPD-amended urea were applied to soil and incorporated immediately prior to transplanting or applied to floodwater after transplanting. Urea was also deep-placed or added in a coated form in two studies.The addition of PPD with urea retarded urea hydrolysis by 1–3 days, depending on the time and method of application. Significant reductions in the concentration of ammoniacal-N in floodwater resulted when PPD-amended urea was applied between 18 and 26 days after transplanting (DT). In contrast, PPD did not appreciably affect the concentration of ammoniacal-N in floodwater when applied with urea either immediately before or after transplanting of the seedlings.Plant N uptake and grain yield were not significantly affected by the addition of PPD with urea in three of the four experiments conducted, even though PPD substantially reduced the concentration of ammoniacal-N in the floodwater in several treatments in these studies. The15N balance studies conducted at both field locations showed PPD to increase total15N recovery by between 10% and 14% of the15N applied, 14 days after the application of urea. No further loss of15N occurred between the initial sampling (40 DT) and grain harvest at Los Baños. An increase in15N recovery occurred at grain harvest at Muñoz because15N-labeled urea was applied at 50 DT in the study. PPD increased the amount of15N in the plant and nonexchangeable soil N fraction at all harvests at Los Baños. In contrast, at Muñoz, PPD increased the quantity of15N in the KCL-extractable pool 14 days after urea was applied. Reasons for the discrepancies in results between experiments and the overall failure of PPD to increase grain yield are discussed.