E. T. Craswell

Analytical methods in15N research

Although the stable isotope15N is an indispensable tool in research to trace the fate of fertilizer nitrogen in soil/plant systems, the analytical methods used in this research are time consuming and prone to many errors. This paper outlines the methods used in an international program of nitrogen research coordinated by the International Fertilizer Development Center (IFDC). The different steps in the digestion, distillation, and isotope ratio analysis of15 N-labeled soils, plant material, and fertilizers are described. Details on the use of a series of controls to check the precision and accuracy of the methods are also given. It is hoped that this comprehensive description of procedures will encourage the expanded and proper use of15N.

Ammonia volatilization from flooded soils

Ammonia volatilization from flooded soils has been studied for over half a century. In reviewing the literature on this subject, it becomes clear that there is no consensus on the importance given to this loss mechanism. In part, the differences of opinion can be explained by the fact that ammonia losses were studied in different environments, but to a great extent it seems due to the wide diversity of techniques used to study this loss mechanism.The many factors that influence ammonia volatilization from flooded soils are chemical, biological, and environmental in nature. These various factors are reviewed in depth and discussed with respect to their implications for measurement techniques and for soil, fertilizer, and water management.The major objective of this paper is to familiarize the reader with the most current developments in thinking about the mechanisms and extent of ammonia loss and hopefully to stimulate meaningful research on ammonia volatilization from flooded soils. Such research should be conducted in a wide range of agroclimatic conditions utilizing measurement techniques that are valid or for which the limitations are clearly understood. A better appreciation for the importance of ammonia volatilization will provide the impetus to research and development in fertilizer technology and management aimed at preventing such losses.

Fate of fertilizer nitrogen applied to wetland rice

Dramatic increases in the production and use of fertilizer N in rice-growing countries have occurred during the past decade. In developing Asia, fertilizer N production has increased from 3.6 × 106 t in 1970 to an estimated 18 × 106 t in 1982/83, of which 85% will be urea (Stangel 1979). Expanded use of N fertilizer is being combined with fertilizer-responsive rice varieties and increased areas under irrigation in an all-out effort to produce enough food for the vast Asian population. Unfortunately, N fertilizer, which is already a costly input for the rice farmer, is becoming more expensive in response to the rising cost of the oilbased feedstocks used in fertilizer production. It is therefore imperative that N fertilizer is used efficiently.

Time and mode of nitrogen fertilizer application to tropical wetland rice

In experiments with transplanted rice (Oryza sativa L.) at the International Rice Research Institute, Philippines, two methods of split application of urea and ammonium sulfate were compared with deep, point placement (10 cm) of urea supergranules and broadcast application of a slow-release fertilizer sulfur-coated urea (SCU). Comparisons were made in the wet and dry seasons and were based on rice yield and N uptake. Urea- and ammonium-N concentrations and pH of the floodwater were measured to aid interpretation of the results.Split applications of urea were generally less efficient than ammonium sulfate. The split in which the initial fertilizer dose was broadcast and incorporated into the soil before transplanting was more effective than the split in which the fertilizer was broadcast directly into the floodwater 21 days after transplanting. Both split applications were inferior to the urea supergranules and SCU, in terms of both yield and N uptake efficiency; average apparent N recoveries ranged from 30% for the delayed split urea to 80% for the urea supergranule.Broadcast applications of urea and ammonium sulfate produced high floodwater concentrations of urea- and ammonium-N, which fell to zero within 4–5 days. Floodwater pH was as high as 9.3 and fluctuated diurnally due to heavy algal growth. Ammonia volatilization and algal immobilization of N in the floodwater were probably responsible for the poor efficiency of the split applications; the supergranules and SCU on the other hand produced low floodwater N concentrations and were efficiently used by the rice crop.

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.

Fate and efficiency of nitrogen fertilizers applied to wetland rice. I. The Philippines

Urea is the main form of fertilizer nitrogen applied to wetland rice. As part of an effort to evaluate the efficiency of nitrogen fertilizers, conventional urea and modified urea products such as sulfur-coated urea (SCU), urea supergranules (USG), and sulfur-coated urea supergranules (SCUSG) were compared with ammonium sulfate on an Aquic Tropudalf at the experimental farm of the International Rice Research Institute (IRRI) in the Philippines. The sulfur-coated materials were prepared in the laboratory and were not completely representative of commercial SCU. Two experiments were conducted in the wet season (1978, 1979) and one in the dry season (1979). All fertilizers were labeled with 5% or 10% excess15N so that the fertilizer-N balance at two or three sampling times during the growing season could be constructed and the magnitude of N loss assessed. The SCU, USG, and SCUSG were applied at transplanting, and the whole dose of nitrogen was15N-labeled. The urea and ammonium sulfate applications were split: two-thirds was broadcast and incorporated at transplanting, and one-third was broadcast at panicle initiation; only the initial dose was15N-labeled.Deep-point placement (10 cm) of urea supergranules (USG) between the rice hills consistently provided the highest plant recovery of15N in all experiments and at all harvest times; recoveries ranged from 48% to 75% with an average of approximately 58% at maturity. Among the fertilizers broadcast and incorporated before transplanting, average plant recoveries of15N were only approximately 34% and 26% from urea and ammonium sulfate, respectively. Plant recovery of15N from the broadcast and incorporated SCU (37%) was far inferior to that from USG. Sulfur coating of supergranules did not improve plant recovery over USG alone although sulfur coating delayed the plant uptake of15N from the USG.The15N not accounted for in the plant and soil was presumed lost. Loss of N from urea and ammonium sulfate was high (63%) in the dry season. Coating with sulfur gave a slight improvement, and deep placement of USG and SCUSG greatly reduced the losses. Losses of N were substantially lower in the wet season than in the dry season for broadcast and incorporated urea, SCU, and ammonium sulfate (9%–30%), whereas losses from deep-placed urea remained more or less the same as in the dry season. Net immobilization of15N from the broadcast fertilizers in the wet season ranged from 49% to 53% in the first experiment and from 16% to 32% in the second experiment, presumably because of aquatic weeds and green algae; immobilization was proportionally less at higher rates of fertilizer application. Deep placement reduced the extent of15N immobilization in the soil plus roots to less than 21% in all experiments.

8. New developments in nitrogen fertilizers for rice

The efficiency of nitrogen (N) fertilizer products and practices currently used on rice is low, and improving this efficiency would be very beneficial to rice-growing countries. The development of new N fertilizers is best achieved by following a logical sequence of testing and evaluation procedures in a variety of settings from the laboratory to the farmers field. Novel N fertilizers currently at various stages of testing include urea supergranules for deep placement, urea coated with various materials to control the N release rate, mixtures of a urease inhibitor with urea to reduce losses, and organic N sources other than urea.

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.