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We studied the interacting effects on NH3 loss and grain yield of adding (1) urease inhibitors to retard the hydrolysis of urea (2) the algicide terbutryn to limit floodwater pH increases, and (3) C2H2 (provided by waxcoated calcium carbide) to prevent NH3 oxidation. The algicide treatment maintained the floodwater pH values below 8 for the first 3 days after the urea application and depressed the maximum values below 8.5 on subsequent days. As a consequence, NH3 loss was significantly (P

Ammonia (NH3) volatilization is an important mechanism for nitrogen (N) loss from flooded rice fields following the application of urea into the floodwater. One method of reducing losses is to use a urease inhibitor that retards the hydrolysis of urea by soil urease and allows the urea to diffuse deeper into the soil. The two chemicals that have shown most promise in laboratory and greenhouse studies are phenylphosphorodiamidate [PPD] and N-(n-butyl)thiophosphorictriamide [NBPT], but they seldom work effectively in the field. PPD decomposes rapidly when the pH departs from neutrality, and NBPT must be converted to the oxygen analogue [N-(n-butyl)phosphorictriamide, NBPTO] for it to be effective. Our field studies in Thailand showed that NH3 loss is markedly reduced when PPD is added with the algicide terbutryn. The studies also showed that a mixture of PPD and NBPT was even more effective than either PPD or NBPT alone. It appears that initially PPD inhibited urease activity, and during this time at least part of the NBPT was converted to NBPTO; then as the activity of PPD declined, NBPTO inhibited the hydrolysis of urea. The combined urease inhibitor treatment reduced NH3 loss from 15 to 3% of the applied N, and increased grain yield from 3.6 to 4.1 t ha−1.

This paper reports a study, in a flooded rice field in Thailand, on the effects of two urease inhibitors, cyclohexylphosphorictriamide (CHPT) and N-(n-butyl)phosphorictriamide (NBPTO), the nitrification inhibitor phenylacetylene and an algicide treatment, consisting of alternate additions of copper sulfate and terbutryn at ~3 day intervals, on nitrogen (N) transformations and transfers, and grain yield. The addition of algicide reduced the growth of algae and maintained the pH of the floodwater below that of the control for 11 days. Judging from the ammoniacal N concentrations of the floodwater, phenylacetylene inhibited nitrification. The two urease inhibitors markedly reduced urea hydrolysis and CHPT was more effective than NBPTO. Addition of CHPT maintained the ammoniacal N concentration of the floodwater below 2 g m−3 for 11 days and reduced ammonia loss by ~90%. All urease inhibitor treatments in combination with algicide and / or nitrification inhibitor significantly (p < 0.05) increased the recovery of applied N by the plant. Addition of NBPTO or CHPT in combination with phenylacetylene and algicide resulted in a 2 or 3 fold increase of applied N in the grain, and significantly (p < 0.05) increased grain yield.

Poor N fertilizer use efficiency by flooded rice is caused by gaseous losses of N. Improved fertilizer management and use of nitrification inhibitors may reduce N losses. A microplot study using15N-labelled urea was conducted to investigate the effects of fertilizer application method (urea broadcast, incorporated, deep-placed) and nitrification inhibitor [encapsulated calcium carbide (ECC)] treatments on emission of N2+N20 and total loss of applied N on a grey clay near Griffith, NSW, Australia. Both incorporation and deep placement of urea decreased N2+N2O emission compared to urea broadcast into the floodwater. Addition of ECC significantly (P < 0.05) reduced emission of N2+N20 from incorporated or deep-placed urea and resulted in increased exchangeable ammonium concentrations in the soil in both treatments. Fifty percent of the applied N was lost when urea was broadcast into the floodwater. Total N loss from the applied N was significantly (P < 0.05) reduced when urea was either incorporated or deep placed. In the presence of ECC the losses were reduced further and the lowest loss (34.2% of the applied N) was noted when urea was deep-placed with ECC.

The effectiveness of wax-coated calcium carbide (as a slow-release source of acetylene) and nitrapyrin in inhibiting nitrification and emission of the greenhouse gases N2O and CH4 was evaluated in a microplot study with dry-seeded flooded rice grown on a grey clay near Griffith, NSW, Australia. The treatments consisted of factorial combinations of N levels with nitrification inhibitors (control, wax-coated calcium carbide, and nitrapyrin). The rate of nitrification was slowed considerably by the addition of wax-coated calcium carbide, but it was inhibited only slightly by the addition of nitrapyrin. As a result, the emission of N2O was markedly reduced by the application of wax-coated calcium carbide, whereas there was no significant difference in rates of N2O emission between the control and nitrapyrin treatments. Both nitrification inhibitors significantly reduced CH4 emission, but the lowest emission rates were observed in the wax-coated calcium carbide treatment. At the end of the experiment 84% of the applied N was recovered from the wax-coated calcium carbide treatment compared with ∼ 43% for the nitrapyrin and control treatments.

Seven different sample preparation methods (five ashing and fusion methods, an acid digestion method, an oxygen flask combustion method) and the AOAC extraction method (975.04) were compared with a newly developed sealed chamber digestion method for the determination of fluoride (F) in vegetation. Subterranean clover tissue was the test material. Fluoride concentration in digests was measured using a F ion‐selective electrode. The sealed chamber digestion carried out with nitric acid in Teflon chambers at 120°C for 6 hours gave the highest F concentration in the plant material and the highest recovery of added F of all methods. Tests on several types of plant material confirmed the superiority of the closed chamber digestion. The superiority of the method is attributed to the complete solubilisation of F in the sample and elimination of losses of volatile F.

Field studies were conducted over two seasons to examine the effect of Leucaena leucocephala as a green manure on the N uptake and yield of rice grown under lowland conditions. The treatments were 0, 4, 8, and 12t Leucaena ha-1 with 0,44, and 88 kg N ha-1 as urea in a factorial combination. N uptake was evaluated at maximum tillering, panicle initiation, and harvest. The incorporation of Leucaena increased N uptake throughout the vegetative period in both seasons, irrespective of the mineral-N level. At all levels of N, the grain yield increased significantly following the incorporation of Leucaena, and in both seasons the Leucaena treatment of 8 t ha-1 was almost as effective as the highest mineral-N application.