Ammonia volatilization losses from fertilizers applied to acid soil in the field

Losses of ammonia by volatilization from ammonium sulphate and urea applied to soil were studied in field conditions.Losses from ammonium sulphate generally were not large; ammonia volatilization is thus unlikely to be an important pathway of nitrogen loss from cropped soils, and does not explain the low responses to nitrogen fertilizer of wheat grown in the higher rainfall cropping areas of South-Eastern Australia.Losses of nitrogen from ammonium sulphate were not greatly affected by meteorological variables, rate of application, water applicaton or incorporation into soil.The above variables all affected losses of nitrogen from urea, by influencing the rates of solution and hydrolysis of urea, and volatilization of ammonia. Losses ranged from 4 to 50% of the applied urea-nitrogen. Losses of urea-nitrogen were large when evaporation rates were high, and large variations occurred in the rates at which urea could be hydrolyzed.Extrapolation of the results to grazing conditions suggests that ammonia volatilization may result in large losses of nitrogen from short pastures in dry conditions.     

Nitrogen losses and fertilizer N use efficiency in irrigated porous soils

Porous soils are characterized by high infiltration, low moisture retention and poor fertility due to limitation of organic matter and nitrogen (N). However, wherever irrigated and properly managed, these are among the most productive soils in the world. For sustained productivity and prevention of N related pollution problems, fertilizer N management in porous soils needs to be improved by reducing losses of N via different mechanisms. Losses of N through ammonia volatilization are not favoured in porous soils provided fertilizer N is applied before an irrigation or rainfall event. Ammonium N transported to depth along with percolating water cannot move back to soil surface where it is prone to be lost as NH₃. Under upland conditions nitrification proceeds rapidly in porous soils. Due to high water percolation rates in porous soils, continuous flooding for rice production usually cannot be maintained and alternate flood and drained conditions are created. Nitrification proceeds rapidly during drained conditions and nitrates thus produced are subsequently reduced to N₂ and N₂O through denitrification upon reflooding. Indirect N-budget estimates show that up to 50% of the applied N may be lost via nitrification-denitrification in irrigated porous soils under wetland rice.High soil nitrate N levels and sufficient downward movement of rain water to move nitrate N below the rooting depth are often encountered in soils of humid and subhumid zones, to a lesser extent in soils of semiarid zone and quite infrequently, if at all in arid zone soils. The few investigations carried out with irrigated porous soils do not show substantial leaching losses of N beyond potential rooting zone even under wetland rice. However, inefficient management of irrigation water and fertilizer N particularly with shallow rooted crops may lead to pollution of groundwater due to nitrate leaching. At a number of locations, groundwater beneath irrigated porous soils is showing increased nitrate N concentrations. Efficient management of N for any cropping system in irrigated porous soils can be achieved by plugging losses of N via different mechanisms leading to both high crop production and minimal pollution of the environment.     

NPK-fertilizer efficiency — a situation analysis for the tropics

This paper examines the efficiency of applied N, P, and K fertilizers under tropical conditions. To meet their food demands, tropical countries are importing large quantities of fertilizers at an enormous cost. There is a need for improving crop yields at a reduced cost and a better understanding of the factors that contribute to the overall efficiency of applied fertilizers. It is estimated that under tropical condition, the efficiency of applied N is less than 50%, less than 10% for P and for K it is somewhere around 40%. Losses of N are mainly due to leaching, runoff and volatile losses of ammonia. Under flooding and in alternate wetting and drying conditions of rice lands and low lands, dentrification and volatile ammonia losses are considerable. The N losses from these soil could be minimized by proper management such as rate, methods and time of application. The coating of urea with S has shown some improvement in increasing efficiency. Nitrification and urea hydrolysis inhibitors can improve fertilizer efficiency in certain situations provided they are properly used. The efficiencies of these inhibitors depend on the nature of the chemical compounds, soil properties, and method of application. Low efficiency of applied P fertilizer is mainly due to retention of P by soil clay fractions and iron and aluminum hydroxides. Even though retained P is not available to the first crop, it is made available to a certain extent to the succeeding crops. The rate and methods of P applications and forms of P determine the efficiency of applied P fertilizers. The use of native rock phosphate along with P fertilizers on acid soils appears to be an attractive alternative in reducing the fertilizer cost. The loss of K in tropical soils is largely attributed to leaching and runoff. To reduce K loss by leaching, it is more advisible to apply K in split doses than a single dose. Liming has a beneficial effect in retention of K and reducing P fixation in acid soils.Senior author formerly was a Research Advisor to EMBRAPA/IICA/World Bank program at National Corn and Sorghum Research Center, Sete Lagos, MG, Brazil.     

Leaching of nitrate from cropped rainfed terraces in the mid-hills of Nepal

Intensification of crop production in the mid-hills of Nepal has led to concerns that nitrogen loss by leaching may increase. This study estimated the amount of N leached during two years from rainfed terraces (bari-land) at three locations in Nepal. Maize or upland rice grown in the monsoon season was given either no nutrient inputs or inputs via either nitrogen fertilizer or farmyard manure. Nitrate concentration in soil solution was measured regularly with porous ceramic cup samplers and drainage estimated from a simple soil water balance. Estimated losses of nitrogen by leaching ranged from 0 to 63.5 kg N ha^–1 depending on location and the form of nitrogen applied. Losses from plots receiving no nutrient inputs were generally small (range: 0–35 kg N ha^–1) and losses from plots where nitrogen was applied as manure (range: 2–41 kg N ha^–1) were typically half those from plots with nitrogen applied as fertilizer. Losses during the post-monsoon crops of finger millet were small (typically <5% of total loss) although losses from the one site with blackgram were larger (about 13%). The highest concentrations of nitrate in solution were measured early in the season as the monsoon rains began and immediately following fertilizer applications. Leaching losses are likely to be minimised if manure is applied as a basal nutrient dressing followed by fertilizer nitrogen later in the season.     

Ammonia volatilization from point-placed urea in upland,sandy soils

The upland fertilization practice in Africa of placing N fertilizer below the soil surface near the plant might be facilitated through use of urea supergranules (USG). Since little is known about N losses from point-placed urea on light-textured African soils, laboratory studies were conducted in a forced-draft system to determine (a) the influence of soil properties on ammonia loss from USG and (b) to compare N loss from USG with that from broadcast N sources. Ammonia loss from 1.1 g USG placed at a 4-cm soil depth ranged from 2.9 to 62% of the added N on six light-textured soils. Ammonia loss was correlated with soil clay content (r = –0.93^**) but not with pH. A more detailed study on a soil from Niger revealed significantly less ammonia loss from either surfaced applied urea (18%) or surface-applied calcium ammonium nitrate (7%) than from USG placed at a 4-cm depth (67%). Amendment of surface-applied urea with 1.7% phenyl phosphorodiamidate (PPD), a urease inhibitor, essentially eliminated ammonia loss (1.9%). An¹⁵N balance confirmed that ammonia volatilization was the major loss mechanism for all N sources. The results suggest that point-placed urea may be prone to ammonia volatilization loss on light-textured African soils moistened by frequent light rainfall. In such cases, broadcast application of urea, CAN, or urea amended with PPD may be less prone to N loss.     

Recent developments on the use of urease inhibitors in the tropics

Urea has become the most widely used form of N fertilizer in the world, particularly in the tropics. Its efficiency, however, is decreased by losses of N through ammonia volatilization when the urea is not incorporated into the soil. High temperatures and high biological activity at the soil surface promote rapid hydrolysis of urea to ammonia and carbonate species by the soil enzyme urease, leading to large ammonia losses. These conditions have generated interest in materials that can inhibit the urease enzyme, slowing urea hydrolysis and allowing the urea to move away from the soil surface to where it is not as susceptible to ammonia loss. The phosphoryl di- and triamides, which are structural analogs of urea, meet the requirements for effective soil urease inhibition to varying degrees depending on the conditions of their use. Until the discovery of these compounds, there was little hope that urease inhibition could be achieved either economically or in an environmentally acceptable way. Included in this group is N-(n-butyl) thiophosphoric triamide (NBTPT), which is that most widely tested proinhibitor or precursor of the actual inhibitor N-(n-butyl) phosphoric triamide. Recent research in tropical rice systems indicates that urease inhibitors such as N-(n-butyl) phosphoric triamide and cyclohexylphosphoric triamide can play an important role in increasing urea efficiency. In some experiments where urease inhibition was only partially successful, better results were obtained when the phosphoroamides were used in conjunction with an algicide, to restrict ammonia loss, and nitrification inhibitors, to reduce loss of N by denitrification. Further research on tropical soils in different environments is required to determine the most suitable combination of inhibitors to reduce N loss and increase the efficiency of fertilizer N use.     

Strategies to reduce gaseous emissions of nitrogen from irrigated agriculture

Fertilizer nitrogen (N) is not used efficiently in irrigation agriculture because much of the N applied is lost from the plant-soil system by emission of gaseous compounds to the atmosphere. Nitrogen may be emitted by ammonia volatilization, and as nitrous oxide, nitric oxide and dinitrogen during nitrification, biological denitrification and chemodenitrification. Nitrogen emitted to the atmosphere as ammonia may be returned to the biosphere and recycled thus adding to the nitrous oxide and nitric oxide burden in the atmosphere. Thus ammonia volatilization needs to be controlled as well as nitrification-denitrification to limit emission of nitrogen oxides. Many approaches have been suggested for controlling losses of fertilizer N including optimal use of fertilizer form, rate and method of application, matching N supply with demand, supplying fertilizer in the irrigation water, applying fertilizer to the plant rather than the soil, and use of slow-release fertilizers. While these techniques have the potential to increase the effectiveness of the applied N none of them have a large impact on gaseous loss of N. However, the results of recent experiments in tropical and temperate regions with flooded rice, and irrigated cotton, wheat and maize show that use of newly developed urease and nitrification inhibitors has the capacity to prevent loss of N and increase the yield of crops.     

Evaluation of current fertilizer practice and soil fertility in vegetable production in the Beijing region

A survey on current fertilizer practices and their effects on soil fertility and soil salinity was conducted from 1996 to 2000 in Beijing Province, a major vegetable production area in the North China Plain. Inputs of the major nutrients (NPK) and fertilizer application methods and sources for different vegetable species and field conditions were evaluated. Excessive N and P fertilizer application, often up to about 5 times the crop requirement in the case of N, was very common, especially for high-value crops. Potassium supply may have been inadequate for some crops such as leafy vegetables. Urea, diammonium orthophosphate ((NH₄)₂HPO₄) and chicken manure were the major nutrient sources for vegetable production in the region. Over 50% of N, 60% of P and nearly 90% of K applied originated from organic manure. Total N application rate for open-field Chinese cabbage from organic manure and inorganic fertilizers ranged from 300 to 900 kg N ha^–1 on 78% of the farms surveyed. More than 35% of the surveyed greenhouse-grown tomato crops received > 1000 kg N ha^–1 from organic and inorganic sources. A negative K balance (applied K minus K removed by the crop) was found in two-thirds of the surveyed fields of open-field Chinese cabbage and half of the surveyed fields of greenhouse-grown tomato. Plant-available N, P and K increased with increasing length of the period the greenhouse soils had been used for vegetable production. Similarly, soil salinity increased more in greenhouse soils than in open-field soils. The results indicate that balanced NPK fertilizer use and maintenance of soil quality are important for the development of sustainable vegetable production systems in this region.     

Effect of fertilizer placement on nitrogen loss from sugarcane in tropical Queensland

This paper reports on the fate of nitrogen (N) in a first ratoon sugarcane (Saccharum officinarum L.) crop in the wet tropics of Queensland when urea was either surface applied or drilled into the soil 3–4 days after harvesting the plant cane. Ammonia volatilization was measured with a micrometeorological method, and fertilizer N recovery in plants and soil, to a depth of 140 cm, was determined by mass balance in macroplots with ¹⁵N labelled urea 166 and 334 days after fertilizer application. The bulk of the fertilizer and soil N uptake by the sugarcane occurred between fertilizing and the first sampling on day 166. Nitrogen use efficiency measured as the recovery of labelled N in the plant was very low. At the time of the final sampling (day 334), the efficiencies for the surface and subsurface treatments were 18.9% and 28.8%, respectively. The tops, leaves, stalks and roots in the subsurface treatment contained significantly more fertilizer N than the corresponding parts in the surface treatment. The total recoveries of fertilizer N for the plant-trash-soil system on day 334 indicate significant losses of N in both treatments (59.1% and 45.6% of the applied N in the surface and subsurface treatments, respectively). Drilling the urea into the soil instead of applying it to the trash surface reduced ammonia loss from 37.3% to 5.5% of the applied N. Subtracting the data for ammonia loss from total loss suggests that losses by leaching and denitrification combined increased from 21.8% and 40.1% of the applied N as a result of the change in method of application. While the treatment resulted in increased denitrification and/or leaching loss, total N loss was reduced from 59.1% to 45.6%, (a saving of 13.5% of the applied N), which resulted in an extra 9.9%of the applied N being assimilated by the crop. This revised version was published online in November 2006 with corrections to the Cover Date.     

Ammonia volatilization from urea nitricphosphate and urea applied to the soil surface

Urea nitricphosphate (UNP) is an N-P fertilizer prepared by solubilizing phosphate ore with nitric acid and conditioning the product with urea. The product is acidic, and its nutrient analysis is 23-12-0. Urea makes up 74% of the N component of this material and the remainder comes from the nitrate added as nitric acid. In volatilization trials, UNP lost significantly less N than did urea in a noncalcareous soil (13 and 31% respectively). In calcareous soils the urea-N component of UNP exhibited loss patterns similar to those of urea. Soil pH remained stable at the center of the granule placement site during UNP hydrolysis, thereby reducing NH₃ loss, whereas the pH of the same soil treated with urea rose almost 1.9 units. The urea component of UNP appeared to diffuse from the center of the acidic microsite allowing hydrolysis to take place and permitting limited NH₃ volatilization to occur. UNP appears to be an attractive NP fertilizer in terms of nutrient analysis and resistance of the N component to volatile N losses as NH₃.     

The effect of the urease inhibitor N-(n-butyl) thiophosphoric triamide on the efficiency of urea application in a flooded rice field trial in Thailand

The compound N-(n-butyl) thiophosphoric triamide (nBTPT) was tested for its ability to reduce the rate of urea hydrolysis in applications of urea at 10 d after transplanting flooded rice. The rates of urea hydrolysis were relatively slow, and nBTPT caused a 1-d reduction in the rate of disappearance of urea from the floodwater. Despite this, the vapor pressures of ammonia in the floodwater were significantly lower in the plots with nBTPT than without for the first 5 d following the N application. The vapor pressures of ammonia measured in the afternoons indicate that ammonia volatilization losses were considerable from the treatments without nBTPT and low from the treatments with nBTPT. There was no nitrogen response in this wet-season crop, apparently because of the high availability of N in the soil. N conserved from ammonia volatilization losses by use of the inhibitor was apparently susceptible to denitrification loss, and 50% of the fertilizer was lost in the 37 d following the application of¹⁵N-labeled urea both with and without the inhibitor.     

The availability of nitrogen from sugarcane trash on contrasting soils in the wet tropics of North Queensland

Sugarcane crop residues (‘trash’) have the potential to supply nitrogen (N) to crops when they are retained on the soil surface after harvest. Farmers should account for the contribution of this N to crop requirements in order to avoid over-fertilisation. In very wet tropical locations, the climate may increase the rate of trash decomposition as well as the amount of N lost from the soil–plant system due to leaching or denitrification. A field experiment was conducted on Hydrosol and Ferrosol soils in the wet tropics of northern Australia using ¹⁵N-labelled trash either applied to the soil surface or incorporated. Labelled urea fertiliser was also applied with unlabelled surface trash. The objective of the experiment was to investigate the contribution of trash to crop N nutrition in wet tropical climates, the timing of N mineralisation from trash, and the retention of trash N in contrasting soils. Less than 6% of the N in trash was recovered in the first crop and the recovery was not affected by trash incorporation. Around 6% of the N in fertiliser was also recovered in the first crop, which was less than previously measured in temperate areas (20–40%). Leaf samples taken at the end of the second crop contined 2–3% of N from trash and fertilizer applied at the beginning of the experiment. Although most N was recovered in the 0–1.5 m soil layer there was some evidence of movement of N below this depth. The results showed that trash supplies N slowly and in small amounts to the succeeding crop in wet tropics sugarcane growing areas regardless of trash placement (on the soil surface or incorporated) or soil type, and so N mineralisation from a single trash blanket is not important for sugarcane production in the wet tropics.     

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.     

Ammonia losses from ammonium-containing and -forming fertilizers after surface application at different rates on alkaline soils

The objective of this investigation was to compare the susceptibility of different ammonium containing and forming fertilizers to NH₃ losses and to determine the effect of application rates on NH₃ volatilization. Losses of NH₃ from five fertilizers, namely (NH₄)₂SO₄, CAN, urea, MAP and DAP were determined. The fertilizers were surface-applied to a sandy clay loam Arniston soil and a clayey Gelykvlakte soil of which the pH values were respectively 9.0 and 8.9. The application levels were equivalent to 0, 15, 30, 60, 120 and 240 kg N ha^–1. After a contact period of 3 days NH₃ losses were determined. Ammonia was lost from both soils under all treatments. More NH₃ was lost from the clayey Gelykvlakte soil compared with the sandy clay loam Arniston soil. Loss of NH₃ from the various fertilizers was ranked as follows: Urea > DAP > (NH₄)₂SO₄ > MAP > CAN. Ammonia losses increased with increasing application rates, but the proportion of N lost, decreased. Losses of NH₃ may be reduced by selective choice of fertilizer type and application rate.     

Impact on ammonia volatilization losses of mixing KCl of high pH with urea

Ammonia volatilization associated with urea hydrolysis has been shown to be primarily associated with the pH of the soil solution and its buffering ability in the immediate zone of the fertilizer granule. Numerous studies have also shown that these losses can be reduced significantly by the addition of large amounts of KCl with the urea. Because the pH of commercial sources of potash ranges from 6.5 to 9.5, investigations were conducted to determine if the high pH of these K sources had an effect on the ammonia lost from three contrasting soils. Despite large ammonia losses (approximately 50% of N applied) and a significant reduction in loss due to the use of KCl (30%-50% reduction), the experiments showed no effect of potash pH on ammonia loss. It may be concluded that no risk of enhanced ammonia loss can be associated with the use of high-pH potash sources.     

The effect of fertilizer placement on nitrogen uptake and yield of wheat and maize in Chinese loess soils

Field trials were carried out to study the fate of¹⁵N-labelled urea applied to summer maize and winter wheat in loess soils in Shaanxi Province, north-west China. In the maize experiment, nitrogen was applied at rates of 0 or 210 kg N ha^–1, either as a surface application, mixed uniformly with the top 0.15 m of soil, or placed in holes 0.1 m deep adjacent to each plant and then covered with soil. In the wheat experiment, nitrogen was applied at rates of 0, 75 or 150 kg N ha^–1, either to the surface, or incorporated by mixing with the top 0.15 m, or placed in a band at 0.15 m depth. Measurements were made of crop N uptake, residual fertilizer N and soil mineral N. The total above-ground dry matter yield of maize varied between 7.6 and 11.9 t ha^–1. The crop recovery of fertilizer N following point placement was 25% of that applied, which was higher than that from the surface application (18%) or incorporation by mixing (18%). The total grain yield of wheat varied between 4.3 and 4.7 t ha^–1. In the surface applications, the recovery of fertilizer-derived nitrogen (25%) was considerably lower than that from the mixing treatments and banded placements (33 and 36%). The fertilizer N application rate had a significant effect on grain and total dry matter yield, as well as on total N uptake and grain N contents. The main mechanism for loss of N appeared to be by ammonia volatilization, rather than leaching. High mineral N concentrations remained in the soil at harvest, following both crops, demonstrating a potential for significant reductions in N application rates without associated loss in yield.     

Increasing nutrient use efficiency in rice with changing needs

The use of N fertilizer in Asia has increased from 24 to 39% of the world's total consumption between 1973 and 1987/88. Approximately 60% of the N fertilizer consumed in Asia is used on rice (Oryza sativa L.). However, the N applied to rice, primarily as urea, is not effectively utilized by the crop. Ammonia volatilization is recognized as a major mechanism of N loss, causing ineffective N utilization. Basal incorporation of urea without standing water; deep placement of urea; and modification of urea with algicides, urea inhibitors or coatings are strategies to reduce ammonia loss. Loss of N by nitrification-denitrification may be a serious problem particularly when soil is dried between rice crops, then flooded for the subsequent rice crop. The use of organic N sources, such as green manure and organic manures, as partial substitutes of inorganic N fertilizer is receiving renewed research interest.The use of P fertilizers for rice is most necessary on Oxisols and Ultisols with high P-fixing capacity. Phosphate rock and partially acidulated phosphate rock are alternatives to soluble P sources used on these soils. Response to K is normally highest on light-textured soils. The limited available information suggests that in lowland rice-upland crop rotations, K fertilizers should be applied to the non-rice crop. Zinc deficiency can be overcome through (a) use of varieties more tolerant to zinc deficiency, (b) application of zinc sulfate, and (c) dipping seedling roots in a zinc oxide suspension.Increasing use of S-free fertilizers, intensive cropping, and use of high yielding rice varieties have led to S deficiency in many rice growing countries. Sulfur deficiency can be corrected by applying S-containing materials even with elemental S. Residual effects have also been reported even at a low rate of 20 kg S/ha. Thus, S does not need to be applied every season.To address the unresolved integrated nutrient management issues, both strategic and applied research are required on interacting soil-plant-water-nutrient-climate processes. Long-term sustainability is one of the parameters that must be considered in evaluating the desirability of alternative rice technologies.Paper presented at the Fertilizer Asia Conference and Exhibition, 15–18 October 1989, Manila.     

Effect of wheat straw application on ammonia volatilization from urea applied to a paddy field

A proper amount of nitrogen (N) fertilizer is critical for the ideal production in the wheat-rice rotation in the Yangtse Delta region of China and straw retention is important for sustaining soil quality and productivity. However, the effects of straw retention on paddy field ammonia volatilization from applied urea are unclear. The objectives of this study were to explore the effect of wheat straw retuned with urea and to evaluate how floodwater ammonium concentration and pH, soil Eh influence on flooded rice field ammonia volatilization. The study was conducted for 2?years using a lysimeter experiment included 5 treatments, urea applied at rates of 0, 180, 240?kg?N?ha^?1 with no retained straw, and at rates of 180 and 240?kg?N?ha^?1 with 6.5?t?ha^?1 of retained wheat straw. Urea was split into three applications: incorporated at transplanting, tillering, and topdressing at panicle emergence. Rice was flooded to a depth of 5?cm and grown in rotation with irrigated wheat as a source of straw. Averaged over the two levels of applied N, straw incorporation increased the floodwater ammonium concentration by 11.5?C22.5?%, pH by 0.13?C0.70 units but reduced topsoil Eh by 1.0?C47?mv. Ammonia volatilization increased with the increasing amounts of urea applied and with straw incorporated. With no retained straw, the average ammonia volatilization from the fertilized treatments was 40.4?kg?N?ha^?1, accounting for 15.8?% of the fertilizer-N. With retained wheat straw, the average ammonia volatilization from the fertilized treatments was 51.9?kg?N?ha^?1, accounting for 21.3?% of the fertilizer-N. The increase in ammonia volatilization caused by straw incorporation may be partly attributed to the presence of urease in the straw and to the increased pH in the floodwater. It is unclear whether the reduced redox potential also contributed to ammonia volatilization.     

Effect of green manure on the yield,N uptake and floodwater properties of a flooded rice,wheat rotation receiving15N urea on a highly permeable soil

Field studies were conducted for two years on a rapidly percolating loamy sand (Typic Ustochrept) to evaluate the effect of green manure (GM) on the yield,¹⁵N recovery from urea applied to flooded rice, the potential for ammonia loss and uptake of residual fertilizer N by succeeding crops. The GM crop ofSesbania aculeata was grownin situ and incorporated one day before transplanting rice. Urea was broadcast in 0.05 m deep floodwater, and incorporated with a harrow. Green manure significantly increased the yield and N uptake by rice and substituted for a minimum of 60 kg fertilizer N ha^–1. The recovery of fertilizer N as indicated by¹⁵N recovery was higher in the GM + urea treatments. The grain yield and N uptake by succeeding wheat in the rotation was slightly higher with GM. The recovery of residual fertilizer N as indicated by the¹⁵N recovery in the second, third and fourth crops of wheat, rice and wheat was only 3, 1 and 1 per cent of the urea fertilizer applied to the preceding rice crop. Floodwater chemistry parameters showed that the combined use of the GM and 40 kg N ha^–1 as urea applied at transplanting resulted in a comparatively higher potential for NH₃ loss immediately after fertilizer application. The actual ammonia loss as suggested by the¹⁵N recoveries in the rice crop, however, did not appear to be appreciably larger in the GM treatment. It appeared the ammonia loss was restricted by low ammoniacal-N concentration maintained in the floodwater after 2 to 3 days of fertilizer application.     

Ammonia volatilization from nitrogen fertilizers applied to cereals in two cropping areas of southern Australia

As farmers in southern Australia typically apply nitrogen (N) to cereal crops by top-dressing with ammonia (NH₃) based fertilizer in late winter or early spring there is the potential for large losses of NH₃. This paper describes the results of micrometeorological measurements to determine NH₃ loss and emission factors following applications of urea, urea ammonium nitrate (UAN), and ammonium sulfate (AS) at different rates to cereal crops at two locations in southern Australia. The amounts of NH₃ lost are required for farm economics and management, whilst emission factors are needed for inventory purposes. Ammonia loss varied with fertilizer type (urea?>?UAN?>?AS) and location, and ranged from 1.8 to 23?% of N applied. This compares with the emission factor of 10?% of applied N advocated by IPCC ( 2007). The variation with location seemed to be due to a combination of factors including soil texture, soil moisture content when fertilizer was applied and rainfall after fertilizer application. Two experiments at one location, 1?week apart, demonstrated how small, temporal differences in weather conditions and initial soil water content affected the magnitude of NH₃ loss. The results of these experiments underline the difficulties farmers face in timing fertilization as the potential for loss, depending on rainfall, can be large.