Ammonia volatilization from a flooded rice field fertilized with amended urea materials

The extent of ammonia volatilization from prilled urea, urea supergranule and urea amended with neem seed cake, shell-lac and dicyandiamide was studied in a field experiment on flooded rice. The ammonia loss was measured by the closed acid trap method. The collected ammonia was highest from unamended prilled urea, accounting for 19 to 20 per cent of the applied N in 1983 and 20 to 24 per cent of the applied N in 1984. Coating of urea prills was either coaltar soaked neem seed cake or shell-lac was more effective than coating with dicyandiamide in reducing ammonia loss. Deep placement of urea as a supergranule was the most effective method of reducing ammonia volatilization. A diurnal variation in the pH and temperature of floodwater was observed. The quantity of ammonia collected in the acid trap was closely related to ammoniacal-N concentration and pH of the floodwater.     

Incorporation of urea in puddled rice soils as affected by tillage implements

Thorough incorporation of urea into the soil is known to reduce the high N losses associated with the traditional practice of broadcasting urea onto puddled lowland rice fields. Few studies have, however, examined the effectiveness of farm-level implements for incorporating broadcast urea during final land preparation in small rice fields. A field experiment was conducted at two sites in the Philippines to compare the effectiveness of several commonly available and experimental tillage implements for basal incorporation of urea. The relative effectiveness of N incorporation and relative susceptibility of N to ammonia volatilization losses were assessed from floodwater (urea + ammoniacal)-N and partial pressure of ammonia (pNH₃) following urea application to puddled, unplanted soil.Conventional water buffalo- and single axle tractor-drawn comb harrows were equally, but only partially, effective in reducing floodwater (urea + ammoniacal)-N andpNH₃ by 42 to 56% of the values for broadcast prilled urea (PU) without incorporation. Removal of the comb harrow from the single axle tractor did not reduce the effectiveness of PU incorporation, indicating that the cagewheel rather than the comb harrow was largely responsible for fertilizer incorporation. An experimental conical puddler was slightly more effective than the conventional comb harrow. The movement of transplanters through the field did not effectively incorporate PU. A power weeder, frequently used by researchers to incorporate fertilizer in small experimental plots, was less effective than traditional comb harrows, reducing floodwater (urea + ammoniacal)-N by only 35%. No tillage implement for incorporating urea reducedpNH₃ as effectively as did the liquid urea band injector.Results suggest that sizeable losses of fertilizer N still occur following incorporation of PU with tillage implements commonly used by small rice farmers. Considerable scope remains for the improvement of fertilizer incorporation in puddled rice soils.     

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.     

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.     

Lowering ammonia volatilization losses from urea application by activation of nitrification process

The aim of this work was to lower ammonia volatilization losses by increasing the rate of nitrification. This was achieved by eliminating the gap in timing between urea hydrolysis and ammonium nitrification. Soils were pretreated with a small amount of ammonium salt which led to the activation of the nitrification process. When nitrification passed its lag period, urea was applied to the soils. Ammonium produced by urea hydrolysis was quickly oxidized into nitrate and did not accumulate in the soil. This resulted in decreased ammonium concentrations in soil, and consequently, in decreased ammonia volatilization losses.This report is part of a doctoral thesis by the first author.     

Estimating ammonia volatilization from unsaturated urea fertilized and urine affected soils by an indirect method

An indirect method for evaluating the emission of ammonia from urea-fertilized upland crops or urine-affected pastures, without affecting the plant's environment, was evaluated at Yanco, N.S.W., Australia and near Lincoln University, New Zealand. The parameters measured were the ammoniacal nitrogen concentration, pH and temperature in the aqueous phase at the soil's surface, and windspeed at a reference height. The combined effect of these influences on volatilization rate were incorporated into a linear relationship of the form F =k u _z 0, where F is the vertical flux of ammonia, determined by a micrometeorological method,u _z is the wind speed at some reference height above the soil,0 is the ammonia concentration in equilibrium with the liquid phase (calculated from the ammoniacal nitrogen concentration, pH and temperature) andk is a proportionality constant. Strong linear relationships of this kind were found for the data sets from both experiments. The respective correlation coefficients for the relationships at the two sites were 0.870 and 0.879, and the respective k values were 6.3 × 10^–5 and 7.5 × 10^–5. The field measurements require little in the way of specialized equipment (e.g. flat - surface pH electrode, ammonia electrode, anemometer) and are comparatively easy to carry out. The results suggest that with some further refinement, this technique could achieve application in the calculation of ammonia losses from small plots in the field.     

Recovery of nitrogen by spring barley from ammonium nitrate,urea and sulphur-coated urea as affected by time and method of application

The objective of this study was to increase the efficiency of fall-applied N either by placement in bands or by using a slow-release fertilizer. Four field experiments were conducted in north-central Alberta to determine the influence of N source, time of application and method of placement on the recovery of fall-applied N as soil mineral N in May, and on yield and recovery of N in grain of spring-sown barley. The recovery in soil of mineral N by May from the fall-applied fertilizers varied among treatments. More specifically, the recovery was lowest with topdressed application, highest with banding, and tended to be less with incorporation application as compared to banding. Recovery of mineral N was least for sulphur-coated urea (SCU) compared with A.N. and urea, regardless of method of application. The loss of fall-applied N was substantial, but leaching did not go beyond 60 cm deep.Yield and recovery of N in barley grain were much greater with spring application than with fall application at the 4 sites for ammonium nitrate (A.N.) and at 3 sites for urea. The SCU treatments were inferior. The A.N. and urea had greatest yield and N recovery with banding, followed by incorporation and then with topdressing for both fall- and spring-applied N. Method of application had little effect on yield and N uptake with SCU. In all, the greatest yield or crop N uptake was obtained with spring banding of A.N. or urea, while SCU did not function well as a fall- or spring-applied N fertilizer.(Contribution No. 680)     

Effect of urea granule size on ammonia volatilization from surface-applied urea

Urea powder and granules of varying size (1 to 8 mm diameter) were surface applied to a ryegrass/white clover pasture. Evolution of NH₃ was measured using a continuous air flow enclosure method. At 30 kg N ha^–1, the percentage of urea-N lost as NH₃ from powder or granules of 1–2, 3–4, 5.6 and 8 mm diameter was 18, 17, 20, 22 and 32 respectively. As the particle size increased, the rate of urea hydrolysis decreased and delayed the time at which the maximum rate of volatilization occurred. Mineral-N and soil surface pH measurements confirmed that during the period of volatilization, urea moved less than 30 mm from the application point.For the powder and 3–4 mm granule treatments, when the application rate was increased from 30 to 300 kg N ha^–1, the percentage of urea-N volatilized increased, but at any particular rate there was no significant difference in percentage loss between the powder and 3–4 mm granules.     

Dynamics of ammonia volatilization from simulated urine patches and aqueous urea applied to pasture. II. Theoretical derivation of a simplified model

Theoretical considerations for the development of a simplified model for predicting volatilization losses of ammonia gas (NH_3(g)) from the urine patches of grazing herbivores in a pasture ecosystem are presented. The volatilization of NH_3(g) is treated as a physico-chemical phenomenon based on the soil solution chemistry of urine patches to develop a general equation to describe the rate of volatilization from a pasture surface. A semi-empirical approach was then used in which published data define typical limits for the parameters appearing in the volatilization equation. This led to the simplification of the general volatilization equation into a more useable and more readily verifiable form.The dominant factor in determining the rate of volatilization of NH_3(g) was shown to be the soil surface pH. To better understand the dynamics of pH changes within urine patches, the more extensive literature dealing with volatilization losses from flooded soils was reviewed. From the apparent similarities between the two systems a procedure was described by which a careful monitoring of soil surface pH as a function of time could be used to solve the simplified equation.To calculate NH_3(g) fluxes this model requires the following as input data: a knowledge of the disposition of the applied-N within the soil profile; the rate of urea hydrolysis in the topsoil; and soil surface pH and temperature measurements throughout the duration of a volatilization event.     

Effect of magnesium sulphate addition to urea on nitrogen loss due to ammonia volatilization

Fertilizer was applied as urea alone or as a mixture of urea and magnesium sulphate (MgSO₄·1H₂O) to study the effect on ammonia volatilization under laboratory conditions in relation to soil texture, N:Mg ratio, air flow rate, fertilizer form (solid or liquid) and organic material. When the mixture of urea and magnesium sulphate (UMM) was applied at a ratio of 1:0.21, significantly lower NH₃-N losses than from urea were found in 2 of 6 soils, and 4 soils showed a similar tendency. Increasing the N:Mg ratio to 1:0.5 resulted in significantly lower NH₃-N loss. Lower air flow rates reduced ammonia loss from UMM more than from urea alone. The effectiveness of UMM over urea was not improved in the liquid form. Increase of organic material had no influence on NH₃-N loss from urea alone or UMM.     

Incorporation of urea for transplanted rice as affected by floodwater and growing season

Incorporation of urea into puddled rice soils is known to reduce ammoniacal-N buildup in floodwater and the subsequent loss of N as ammonia. Little is known, however, about seasonal and temperature effects on the effectiveness of basal urea incorporation in puddled soils. A field experiment was conducted in northern Vietnam on an Aquic Ustifluvent in the spring season (February to June) and summer season (July to November) to determine the effect of the presence of floodwater and method of fertilizer incorporation on floodwater ammoniacal-N, floodwater urea-N, andpNH₃ following urea application. During the 4 d following basal urea application, floodwater temperature at 1400 h was 7 to 15°C higher in summer (July) than that in spring (February), and floodwater pH at 1400 h was 0.5 to 1.0 higher in summer than that in spring. ThepNH₃ was much higher in summer than that in spring, suggesting a high potential for ammonia volatilization in summer. The movement of transplanters through the field did not reducepNH₃, irrespective of floodwater depth (0 or 5 cm) and season. Harrowing and subsequent transplanter movement partially reducedpNH₃ in the summer;pNH₃ reduction, however, was greater when floodwater depth was 0 rather than 5 cm during harrowing and transplanting. This partial reduction ofpNH₃ in summer did not result in a corresponding increase in rice yield, presumably because N losses were only slightly reduced and because yield was constrained by additional factors, such as the adverse climate. In spring, the removal of floodwater before urea application and incorporation increased grain yield by 0.2 Mg ha^–1, even thoughpNH₃ was consistently low and was not reduced by urea incorporation. This result suggests that water management and tillage during basal urea application may influence rice growth and yield in ways other than reduced N loss.     

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.     

Ammonia losses from surface-applied urea as related to urea application rates,plant residue and calcium chloride addition

Volatile losses of NH₃ from surface-applied urea are known to decrease in the presence of soluble Ca-salts or with a decrease in easily decomposable organic matter content (EDOM), both of which influence urease activity. How these factors interact to affect NH₃ losses is not fully understood. Studies were conducted to determine the effect CaCl₂ in sand with varying rates of EDOM on NH₃ losses from surface applied urea. The same effects were examined on agricultural soils containing partially decomposed native organic matter (NOM). Determinations were made in the laboratory on field soils, sand free of organic matter and sand with known amounts of grass clippings (GC, EDOM). Low levels of GC in sand with low amounts of added urea resulted in little NH₃ loss. Ammonia loss increased as more N was applied at the low levels of GC. The loss was independent of urea application rates at high levels of GC. Ammonia losses were reduced more effectively at low EDOM and NOM in the presence of Ca. Incubation of sand with GC at low rates prior to urea addition increased NH₃ losses relative to high levels of non-incubated GC. For the above situation incubation for as high as 24 days resulted in equivalent NH₃ losses. The amount and state of decomposition of existing organic matter affected the degree of NH₃ loss from surface placed urea and its control by added Ca-salts. Microbial decomposition of EDOM, such as might occur in the spring prior to urea addition, led to greater NH₃ losses. Greater loss of NH₃ from urea might be an indication of a larger ureolytic microbial population leading to increased urease production.     

Ammonia volatilization from grassland receiving nitrogen fertilizer and rotationally grazed by dairy cattle

The micrometeorological mass balance method was used to measure ammonia (NH₃) volatilization from rotationally grazed swards throughout the 1987 and 1988 growing seasons. In both years the swards were dressed with calcium ammonium nitrate (CAN) split over 7 dressings. In 1987 the sward received a total of 550 kg N ha^–1, in 1988 a total of 550 or 250 kg N ha^–1. For the 550 kg N ha^–1 treatments there were 8 and 9 grazing cycles, respectively, in 1987 and 1988 and 7 for the 250 kg N ha^–1 treatment. Losses from the 550 N sward were 42.2 and 39.2 kg N ha^–1 in 1987 and 1988, respectively; this was equivalent to 8.5 and 7.7% of the N returned to the sward in the excreta of the grazing cattle. The NH₃ loss from the 250N sward was 8.1 kg N ha^–1 in 1988, which was equivalent to 3.1% of the N returned to the sward in excreta during the growing season. There was a wide variation in NH₃ volatilization between the individual grazing periods. This indicates the necessity of continued measurements throughout the growing season to obtain reliable data on NH₃ volatilization. Soil humidity is suggested to be a key factor, because emissions were high from wet soil, and low from drier soil. Results of a Monte Carlo simulation study showed that the measured NH₃ loss from the 250 and 550 N swards had a standard deviation of 13 and 5% of the mean, respectively.     

Ammonia volatilization from urea and ammoniacal fertilizers surface applied to winter wheat and grassland

Ammonia volatilization from urea, diammonium phosphate, ammonium sulphate and calcium ammonium nitrate surface applied to winter wheat and grassland was determined with windtunnels. The fertilizers were applied at a rate of 8–12 g N m^–2 to plots on a non-calcareous sandy loam. Five experiments were carried out during March to June 1992, each experiment including 2 to 4 treatments with two or three replications. The daily ammonia loss rate was measured during 15 to 20 days. Cumulated daily loss of ammonia from urea followed a sigmoidal expression, while the cumulated ammonia loss from diammonium phosphate showed a logarithmic relationship with time from application. For ammonium sulphate and calcium ammonium nitrate no significant loss could be determined, because daily loss of ammonia were at the detection limit of the wind tunnels. Mean cumulated ammonia loss from plots receiving urea, diammonium phosphate, ammonium sulphate and calcium ammonium nitrate were 25%, 14%, <5% and <2%, respectively, during a 15–20 day measuring period.     

A comparative study of urea hydrolysis rate and ammonia volatilization from urea and urea calcium nitrate

A comparing of urea hydrolysis and NH₃ volatilization from urea supergranules and urea calcium nitrate (UCN, a new fertilizer produced by Norsk Hydro A/S, Norway) was made on two different flooded soil types, a high-CEC clay loam (Ås) and an intermediate-CEC clay loam (Kinn).Nitrogen loss by ammonia volatilization was reduced from 17% by surface application of urea supergranules (USG) on flooded Ås soil to 3% and 6% by UCN briquettes at either the same urea or nitrogen concentration as USG. A significant reduction was even found with the surface application of prilled UCN, 12% and 18% N-loss for prilled UCN and urea, respectively. The floodwater pH and NH ₄ ⁺ content was lower with UCN than urea, which reduced the potential for ammonia volatilization.NH₃-loss (5%) was significantly less when USG was surface applied on Kinn soil, while NH₃-loss from UCN briquettes was independent of soil type. The reduction in NH₃-loss from USG on Kinn soil was due to a decrease in the pH and NH ₄ ⁺ content of the floodwater caused by a reduced rate of urea hydrolysis.The rate of urea hydrolysis was lower with UCN than USG in both soils, but the difference between UCN and USG was greater in the Ås soil than in the Kinn soil. Three days after deep placement (10 cm), 18% of UCN urea and 52% of USG urea were hydrolyzed in Ås soil, while only 12% UCN and 17% USG were hydrolyzed in the Kinn soil.The surface application of USG on flooded soil reduced the rate of urea hydrolysis as compared to deep placement. 30% and 17% of USG urea was hydrolyzed after four days on Ås and Kinn soil, respectively. During the first few days the rate of hydrolysis of UCN was more affected by the soil type than the application method. Four days after surface application 32% and 13% UCN urea was hydrolyzed on Ås and Kinn soil, respectively. The rate of urea hydrolysis exhibited a zero-order reaction when USG and UCN-briquettes were point placed in flooded soils.     

Growth and yield of rice as affected by spacing,time and depth of placement of urea briquettes

A field study was conducted on a sandy clay soil (Entisol) in India to examine urea briquettes (UB) for lowland rice during 1986 and 1987. Grain yield was significantly greater for UB than a split application of prilled urea. At equal rates of N application, a spacing of 30 cm between two UBs was significantly better than 60 to 90 cm spacing. Two applications (10 DAT and at panicle initiation) of UB was no better than a single application (10 DAT). Placement of UB at 3–4 cm depth was significantly better than its surface application or placement at 0–1 cm depth.     

Predicting the effect of soil-water-air dynamics on ammonia volatilization from applied urea with a mechanistic model

To assess the influence of varying soil water and soil air contents on ammonia volatilization from surface applied urea, a mechanistic model is used to simulate the system. The results are discussed in terms of the effects of soil-water-air dynamics on the movement of urea, ammoniacal-nitrogen and soil base, and on the rate of urea hydrolysis, and their influence on ammonia volatilization. Changing the soil moisture between 90% and 125% of field capacity did not have a marked influence on ammonia volatilization. The predicted losses were at their minimum with a moisture content slightly above field capacity, and increased sharply as the soil moisture fell below 90% of the field capacity. Ammonia volatilization losses measured by experiment at differentf values agreed very well with those predicted by the model. The relative contribution of the liquid pathway over the gaseous pathway of movement of NH₃ through soil increased with increase inf, and, at a givenf, decreased with increase in the pH.     

Urea management for lowland transplanted rice as affected by application of farmyard manure

Farmyard manure (FYM) applied to rice-growing soils can substitute for industrial fertilizers, but little is known about the influence of FYM on the effectiveness and optimal management for industrial N fertilizers. A field experiment was conducted in northern Vietnam on a degraded soil in the spring season (February to June) and summer season (July to November) to determine the effect of FYM on optimal timing for the first application of urea. The experimental design was a randomized complete block with two rates of basal incorporated FYM (0 or 6 Mg ha^–1) in factorial combination with two timings of the first application of 30 kg urea-N ha^–1 (basal incorporated before transplanting or delayed until 14 to 16 d after transplanting). The FYM was formed by composting pig manure with rice straw for 3 months. Basal incorporation of FYM, containing 23 kg N ha^–1, increased rice grain yield in both seasons. The yield increase cannot be attributed to reduced ammonia loss of applied urea-N, because FYM did not reduce partial pressure of ammonia (pNH₃) following urea application in either season. Basal and delayed applications of urea were equally effective in the absence of FYM, but when FYM was applied rice yields in both seasons were higher for delayed (mean = 3.2 Mg ha^–1) than basal (mean = 2.9 Mg ha^–1) application of urea. Results suggest that recommendations for urea timing in irrigated lowland rice should consider whether farmers apply FYM.     

The influence of sward mass,defoliation and watering on ammonia volatilization losses from an Italian ryegrass sward topdressed with urea

Using intact pasture sods with 0.15m² surface area and sealed in volatilization chambers, the influence of the mass of herbage on the ammonia volatilization losses following a surface application of urea was determined. Ammonia volatilization loss increased as sward mass decreased. This effect was still evident in poorly established pastures.Under drying conditions the timing of defoliation and water applications relative to the application of urea was also shown to influence ammonia volatilization patterns and magnitude of loss. Delaying defoliation promotes a reduction in ammonia volatilization losses while delayed watering resulted in increased losses.