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Effects of Phenylphosphorodiamidate and Dicyandiamide on Nitrogen Loss from Flooded Rice¹

ABSTRACT

The effects of a urease inhibitor, phenylphosphorodiamidate (PPD), and a nitrification inhibitor, dicyandiamide (DCD), on the losses, tranformations, and recovery of nitrogen (N) were studied when urea was applied to a flooded rice field near Griffith, New South Wales (NSW). When PPD was applied with urea to the floodwater, urea hydrolysis was not prevented but merely delayed by 4 d. Consequently, the accumulation of ammoniacal N in the floodwater was also delayed. The maximum ammoniacal N concentration was reached after 3 d in the urea control, and after 9 d in the urea + PPD treatment. The relative losses of ammonia (NH₃) from the treatments thus depended on the temperatures and pH of the floodwater and the windspeeds prevailing during the respective periods when the concentration of ammoniacal N was high. Low windspeeds at 8 to 10-d after application caused NH₃ loss on the PPD-treated plots to be less than on urea-only plots, but this was fortuitous and an opposite effect might have occurred. The ¹⁵N balance on microplots which received labelled urea showed that PPD addition increased recovery of urea N in the plant-soil system from 60 to 82%. Plant uptake of labelled N was increased by 56%. After subtracting NH₃ volatilization and any other potential losses, the resultant N balance suggested that PPD addition had allowed more urea N to be retained in the soil surface layer (0–10 mm) in such a way that nitrification-denitrification loss was substantially reduced. The nitrification inhibitor (DCD) had no significant effect on nitrate concentrations in the floodwater, NH₃ loss, or recovery of fertilizer N in plants and soil.

¹ Joint contribution from CSIRO, Division of Plant Industry, G.P.O. Box 1600, Canberra, A.C.T., 2601 Australia, CSIRO, Centre for Irrigation Research, Griffith, N.S.W., 2680 and CSIRO, Division of Environmental Mechanics, G.P.O. Box 821, Canberra, A.C.T.

² Senior Principal Research Scientists, Division of Plant Industry; Principal Research Scientist, Centre for Irrigation Research; Senior Research Scientist, Division of Environmental Mechanics (now at CSIRO, Div. of Forest Research, P.O. Box 4008, Queen Victoria Terrace, A.C.T., 2600), respectively.

Received for publication September 27, 1984. Accepted for publication June 13, 1985.