Fate of urea nitrogen applied to a banana crop in the wet tropics of Queensland

This paper reports a study in the wet tropics of Queensland on the fate of urea applied to a dry or wet soil surface under banana plants. The transformations of urea were followed in cylindrical microplots (10.3 cm diameter × 23 cm long), a nitrogen (N) balance was conducted in macroplots (3.85 m × 2.0 m) with ¹⁵N labelled urea, and ammonia volatilization was determined with a mass balance micrometeorological method. Most of the urea was hydrolysed within 4 days irrespective of whether the urea was applied onto dry or wet soil. The nitrification rate was slow at the beginning when the soil was dry, but increased greatly after small amounts of rain; in the 9 days after rain 20% of the N applied was converted to nitrate. In the 40 days between urea application and harvesting, the macroplots the banana plants absorbed only 15% of the applied N; at harvest the largest amounts were found in the leaves (3.4%), pseudostem (3.3%) and fruit (2.8%). Only 1% of the applied N was present in the roots. Sixty percent of the applied N was recovered in the soil and 25% was lost from the plant-soil system by either ammonia volatilization, leaching or denitrification. Direct measurements of ammonia volatilization showed that when urea was applied to dry soil, and only small amounts of rain were received, little ammonia was lost (3.2% of applied N). In contrast, when urea was applied onto wet soil, urea hydrolysis occurred immediately, ammonia was volatilized on day zero, and 17.2% of the applied N was lost by the ninth day after that application. In the latter study, although rain fell every day, the extensive canopy of banana plants reduced the rainfall reaching the fertilized area under the bananas to less than half. Thus even though 90 mm of rain fell during the volatilization study, the fertilized area did not receive sufficient water to wash the urea into the soil and prevent ammonia loss. Losses by leaching and denitrification combined amounted to 5% of the applied N.     

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 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.     

A comparison of the sulfur and oxygen analogs of phosphoric triamide urease inhibitors in reducing urea hydrolysis and ammonia volatilization

A variety of compounds have been tested as urease inhibitors with the goal of providing a means of reducing ammonia volatilization losses from urea fertilizers when they are applied to the soil surface. Four phosphoric triamide compounds were studied in laboratory experiments to assess their effect on urea hydrolysis, soil ammonium levels, and ammonia volatilization. The compounds N(n-butyl) thiophosphoric triamide (nBTPT), cyclohexyl thiophosphoric triamide (CHTPT), and their oxygen analogs [N-(n-butyl) phosphoric triamide (nBPT) and cyclohexyl phosphoric triamide (CHPT), respectively] were mixed with urea at 0.1% and 0.01% w/w ratios, and the products were applied to the soil surface. A forced-draft apparatus was used to measure ammonia loss. The urea treatment lost 47% of applied N as ammonia in 14 d. The inhibitors applied at 0.1% w/w showed losses of 7%–10% in 14 d; at 0.01%, losses ranged from 13%–30% in the same period. At the 0.1% level, no significant difference was found among the inhibitors in terms of ammonia loss or urea hydrolysis trends. At the 0.01% concentrations, the oxygen analogs showed better urea urease inhibition than did the thio compounds, and their ammonia losses were half those of their sulfur analogs.     

Nitrogen use and losses in agriculture in subtropical Australia

This review examines the use of nitrogen (N) fertilizer on sugar-cane, summer and winter grain crops, cotton, tropical fruit crops and pastoral areas in the four subtropical zones in eastern Australia. The pathways for N loss from the various crops grown in these zones are also examined and estimates of N loss given.Sugar-cane is the most important crop grown in the subtropical humid northern and southern zones, using 77% of all N fertilizer applied in 1988–89. Urea is the most widely used form of N fertilizer with about 50% of the applied N often lost via ammonia volatilization, denitrification and leaching. Losses of N via ammonia volatilization can be reduced by either irrigating after application, applying urea in subsurface bands or delaying application until after canopy development. Denitrification losses of 20% of applied N have been measured on clay soils in sugar- cane areas while leaching losses may occur by movement of solutes down preferential pathways (e.g. soil fauna, root channels and structural weaknesses in the soil profile). Tropical fruit crops also make a significant contribution to the economy of the humid northern and southern zones. The livestock industry is well established in the subtropical northern zones, with beef and dairy production relying on leguminous as well as N fertilized pastures. Urea is again the most widely used form of N and is susceptible to large losses via ammonia volatilization. Over a 12 month period, losses of between 9% and 42% of the N applied were recorded from a subtropical pasture.Wheat is the major winter crop of the sub-humid northern and southern zones with grain sorghum the main summer crop. Urea is the principal form of N fertilizer applied to both crops and is essential for increasing or maintaining economic yields from both regions. This decrease in soil fertility in grain producing areas is due mainly to a decrease in the amount of soil organic matter available for mineralization. Cotton is another major crop of both areas and relies heavily on N fertilizer application. Nitrogen fertilizer losses have been recorded from all cropping areas, although nitrification inhibitors such as wax coated calcium carbide and 2-ethynylpyridine have reduced denitrification losses from soils growing wheat and cotton respectively.Subtropical agriculture relies heavily on N fertilizer, principally urea, to maintain and increase crop yields. Losses of N from soils sown to crops and from native and sown pasture occur although management practices are being developed to help minimize this loss.     

Ammonia volatilization losses from different fertilizers and effect of several urease inhibitors,CaCl2 and phosphogypsum on losses from urea

The extent of ammonia volatilization losses from urea, ammonium sulphate (AS), and diammonium phosphate (DAP) were determined in soil incubation studies. The effects of some urease inhibitors (thiourea, hyroquinone, 2–4 dinitro phenol and boric acid) and CaCl₂ and phosphogypsum additions on ammonia loss from urea were also studied. Total ammonia volatilization losses were 32.6%, 3.1% and 2.3% of the N applied to the soil as urea, AS and DAP, respectively. Among the chemicals examined in the study, 500 mg H₃BO₃ in 1 kg of the soil decreased the ammonia loss from urea by 21% in comparison with the control. When 50 mg/kg soil of thiourea, 2–4 dinitro phenol or hydroquinone were applied, ammonia volatilization losses were found to be 10%, 3% and 0% less than urea applied alone, respectively. When 2500 mg CaCl₂ was applied to 1 kg of soil with urea, ammonia loss was decreased by 5%. The lowest hydrolysis rate (65%) occurred with the boric acid treatment. The differences between the hydrolysis rates of the other treatments were not statistically significant. Phosphogypsum was found the most effective agent in reducing ammonia losses from urea. When phosphogypsum was mixed at 2.3 times as much as the urea, ammonia loss was about 85% less than that of urea applied alone. Obviously, further work is needed to find out the potential of both boric acid and phosphogypsum as reducing agents of ammonia losses from urea.     

Ammonia volatilization from fertilizers applied to irrigated wheat soils

A series of experiments using flow chambers was undertaken in the field to investigate the effects of stubble and fertilizer management, soil moisture and precipitation on ammonia volatilization following nitrogen application on chromic luvisols. In the first factorial experiment, urea at 100 kg N ha^–1 was applied to the soil surface one, three and six days following irrigation; there were four rice stubble management systems comprising stubble burnt, stubble burnt then rotary hoed, stubble rotary hoed into the soil and stubble retained on the surface. Cultivation almost halved ammonia loss. The higher loss from uncultivated plots was ascribed to an alkaline ash bed on burnt plots, and to higher soil moisture and some retention of urea prills in the crop residue above the soil surface of the stubble retention plots. Average volatilization over a 12 day period following urea application from plots fertilizer one, three or six days after irrigation was 16, 15 and 4 kg N ha^–1, respectively. Daily application of up to 1.7 mm of water did not reduce volatilization and 35 kg N ha^–1 was lost within five days of fertilization. Daily precipitation of 6.8 mm reduced loss to 14 kg N ha^–1. This quantity of rain is uncommon in the region and it was concluded that showery conditions are unlikely to reduce volatilization. The third experiment demonstrated that the quantity of stubble on the soil surface had no effect on volatilization, and all plots lost 25% of applied nitrogen. In the fourth experiment, 100 kg N ha^–1 as urea or ammonium nitrate was either broadcast onto the surface or stubble retention plots, or placed, and partly covered to simulate topdressing with a disc implement. Partial burial of urea reduced ammonia volatilization from 36 to 7 kg N ha^–1, while partial burial of ammonium nitrate reduced loss from 4 to 0 kg N ha^–1.     

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.     

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.     

Loss of ammonia after application of urea at different times to dry-seeded,irrigated rice

Ammonia loss from urea applied to dry-seeded rice, determined using a micrometeorological technique, varied considerably depending on the time of application. Ammonia volatilization was negligible, before and after flooding, when urea was applied to the dry soil surface two days before permanent flood. Before flooding, the urea prills remained undissolved and urea hydrolysis could not proceed. Thus there was no source of fertilizerderived ammonia for volatilization to occur. Upon flooding, the urea prills were washed into cracks in the soil which subsequently closed. Therefore the movement of soluble nitrogen into the floodwater was prevented, and again there was no ammonia source for the volatilization process.When urea was broadcast into the floodwater a few days after permanent flood, ammonia losses were high and varied from 11–21% of the nitrogen applied. These losses were associated with high floodwater pHs and high wind speeds near the water surface.However, when urea was applied into the floodwater at panicle initiation, ammonia losses were low (3–8% of the applied nitrogen). At this stage of growth the plant canopy shaded the floodwater, inhibiting algal photosynthesis and consequent pH elevation, thus resulting in low ammonia gas concentrations at the floodwater surface. In addition, the plant canopy restricted air movement at the water surface, thereby reducing ammonia transport away from the air-water interface.These findings provide basic information required for improving current fertilizer management practices.     

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.     

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.     

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.     

Fate of fertilizer nitrogen applied to wheat under simulated mediterranean environmental conditions

Triticum aestivumThe fate of fertilizer nitrogen applied to dryland wheat was studied in the greenhouse under simulated Mediterranian-type climatic conditions. Wheat, L., was grown in 76-cm-deep pots, each containing 50–70 kg of soil, and subjected to different watering regimes. Two calcareous clay soils were used in the experiments, Uvalde clay (Aridic Calciustoll) and Vernon clay (Typic Ustochrept). Fertilizer nitrogen balance studies were conducted using various¹⁵N-labeled nitrogen sources, including ammonium nitrate, urea, and urea amended with urea phosphate, phenyl phosphorodiamidate (a urease inhibitor), and dicyandiamide (a nitrification inhibitor). Wheat yields were most significantly affected by available water. With additional water during the growing period, the recovery of fertilizer nitrogen by wheat increased and the fraction of fertilizer nitrogen remaining in the soil decreased. In the driest regimes, from 40 to 65% of the fertilizer nitrogen remained in the soils. In most experiments the gaseous loss of fertilizer nitrogen, as estimated from unaccounted for¹⁵N, was not significantly affected by water regime. The¹⁵N not accounted for in the plant and the soil at harvest ranged from 12 to 25% for ammonium nitrate and from 12 to 38% for regular urea. Direct measurement of labeled ammonia loss from soil indicated that ammonia volatilization probably was the main N loss mechanism. Low unaccounted-for¹⁵N from nitrate-labeled ammonium nitrate, 4 to 10%, indicated that N losses due to denitrification, gaseous loss from plants, or shedding of anthers and pollen were small or negligible. Amendment of urea with urea phosphate to form a 36% N and 7.3% P product was ineffective in reducing N loss. Dicyandiamide did not reduce N loss from urea presumably because N was not leached from the sealed pots and denitrification was insignificant. Amendment of urea with 2% phenyl phosphorodiamidate reduced N loss significantly. However, band placement of urea at as 2-cm soil depth was more effective in reducing N loss than was amendment of broadcast urea with phenyl phosphorodiamidate.     

Control of gaseous nitrogen losses from urea applied to flooded rice soils

This paper reports field experiments designed to determine whether the two main processes responsible for nitrogen (N) loss from flooded rice (ammonia volatilization and denitrification) are independent or interdependent, and glasshouse studies which investigated the effect of soil characteristics on gaseous nitrogen loss.In the first field experiment ammonia (NH₃) loss from the floodwater was controlled using algicides, biocides, frequent pH adjustment, shade or cetyl alcohol, and the effect of these treatments on total N loss and denitrification was determined. Most treatments reduced NH₃ loss through their effects on algal growth and floodwater pH. Total gaseous N loss (54% to 35%) and NH₃ loss (20% to 1.2%) were affected similarly by individual treatments, indicating that the amount lost by denitrification was not substantially changed by any of the treatments.In a subsequent field experiment NH₃ and total N loss were again affected similarly by the treatments, but denitrification losses were very low. In control treatments with different rates of urea application, NH₃ and total N loss were each a constant proportion of the urea applied (NH₃ loss was 17% and total N loss was 24%). These results indicate that techniques which reduce NH₃ loss can be expected to reduce total gaseous N loss.The glasshouse experiment showed that gaseous N losses could be reduced by draining off the floodwater, and incorporating the urea into the 0–0.05 m soil layer before reflooding. Even with this method, losses varied widely (6–27%); losses were least from a cracking clay and greatest from a coarse sand which allowed the greatest mobility of the applied N. Incorporation of applied urea can therefore be expected to prevent losses more successfully from clay soils with high ammonium retention capacity.     

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 influence of soil properties on the effectiveness of phenylphosphorodiamidate (PPD) in reducing ammonia volatilization from surface applied urea

Urea can be an inefficient N source due to rapid hydrolysis by soil urease leading to NH₃ volatilization. The current study investigated the effect of the urease inhibitor phenylphosphorodiamidate (PPD) incorporated at two concentrations (0.5% and 1% w/w) within the fertilizer granule on NH₃ volatilization from surface applied urea. The daily rates of NH₃ loss from 20 soils of widely differing properties from Northern Ireland were measured over 14 days using ventilated enclosures under simulated spring conditions. Cumulative loss rates were calculated and fitted to a logistic model from which total NH₃ loss (Amax) and the time to maximum rate of loss (Tmax) were determined. Stepwise multiple linear regression analysis related the effectiveness of PPD in reducing NH₃ volatilization from urea to soil properties.The total cumulative loss of ammonia from unamended urea varied from 0.37 to 29.2% depending on soil type. Ammonia volatilization appeared to be greatest on a soil with a high pH (R² = 0.65), a low titratable acidity (TA) (R² = 0.63) and a soil that was drying out (R² = 0.50). Soil pH was negatively correlated with TA (r = –0.826, P < 0.001) suggesting that soils with a low TA may have received recent lime. Including cation exchange capacity (CEC) and % N as well as pH-KCl in the multiple linear regression equation explained 86% of the variance.The effectiveness of PPD in reducing Amax varied between 0% to 91% depending on soil type, the average over all 20 soils being 30 and 36% for 0.5% and 1% PPD respectively. The most important soil properties influencing the effectiveness of the urease inhibitor were soil pH-H₂O and TA accounting for 33% and 29% of the variance respectively. PPD was less effective on a soil with a high pH and low TA. These were the soil conditions that led to high NH₃ volatilization from unamended urea and may explain why PPD had limited success in reducing ammonia loss on these soils. Multiple linear regression analysis indicated that 75% of the variation in the % inhibition of NH₃ loss by PPD could be significantly accounted for by pH-H₂O, initial soil NO ₃ ^- -N concentration, % moisture content and % moisture loss.The delay in Tmax by PPD ranged from 0.19 to 7.93 days, the average over all 20 soils being 2.5 and 2.8 days for 0.5% and 1% PPD respectively. TA, % moisture content, urease activity and CEC were soil properties that significantly explained 83% of the variation in the % delay in Tmax by PPD in multiple linear regression analysis. However, none of these soil properties were significant on their own. As urea hydrolysis occurs rapidly in soil, delaying Tmax under field conditions would increase the chance of rain falling to move the urea below the soil surface and reduce NH₃ volatilization. A urease inhibitor should be more effective than PPD on soils with a high pH and low TA to be successful in reducing high NH₃ losses.     

Greenhouse evaluation of the effect of topsoil moisture and simulated rainfall on the volatilization of nitrogen from surfaceapplied urea,diammonium phosphate,and potassium nitrate

The effect of topsoil moisture content at the time of nitrogen fertilization and distribution of precipitation following N fertilization on volatile loss of surfaceapplied fertilizer N was studied in two greenhouse experiments using¹⁵N-labeled fertilizers. Loss of applied NO ₃ ^- -N was small compared with loss of urea-N and diammonium phosphate (DAP)-N; this suggests that NH₃ volatilization was the major pathway of N loss for urea and DAP. Loss of applied NO ₃ ^- -N averaged less than 6% of that applied regardless of initial topsoil moisture or amount of precipitation. Increased initial topsoil moisture content increased losses of urea-N greatly but losses of DAP-N only slightly. Increasing depths of precipitation, added five days after N fertilization, greatly decreased loss of urea-N but had no effect on the loss of DAP-N. Variations in moisture and precipitation treatments caused losses of urea-N to vary from 40 to 6% of that applied in a slightly acidic silty loam and from 26 to 11% in a calcareous clay. Moisture and precipitation treatments caused volatilization of DAP-N to vary from 20 to 10% in the silty loam and from 40 to 27% in the calcareous clay. In a second experiment, moisture and precipitation conditions affected N loss from urea as in the previous experiment. Addition of phenylphosphorodiamidate (PPDA), a known urease inhibitor, to urea at 20 g kg^–1 resulted in only a small reduction of N loss in the calcareous clay soil used.It was concluded that soil moisture at the time of N fertilization and precipitation following N fertilization can greatly affect volatile loss of fertilizer N. Since the effect of moisture on N loss is not the same for all N sources, moisture parameters are expected to affect the ranking of N sources by their susceptibility to N loss and their uptake by plants in field experiments. Results obtained suggest some management practices by which fertilizer N might be conserved. The great effect of moisture and precipitation on N loss in these studies underscores the need for detailed meteorological records for field sites of N trials.     

Effect of water depth on ammonia loss from lowland rice

This paper reports a study on the effects of water depth in modifying rates of ammonia emission and total nitrogen loss from flooded rice fields after fertilization with urea. Ammonia loss was determined by the mass balance micrometeorological method and total nitrogen loss by¹⁵N balance.Initially ammonia was lost at a faster rate from the shallow (0.05 m) than from the deep (0.14 m) floodwater; this was due to higher ammoniacal nitrogen concentrations and higher temperatures in the shallow water. Emission rates were more nearly comparable later in the experiment, but overall, 26% of the applied nitrogen was lost as ammonia from the shallow pond and only 18% from the deep pond.Even though changes in water depth markedly affected ammonia emission rates and the amounts of ammonia lost, they did not significantly affect total nitrogen loss. The results suggest that management practices based only on changes in water depth may not result in increased efficiency of fertilizer nitrogen for flooded rice.     

Influence of urea fertiliser formulation, urease inhibitor and season on ammonia loss from ryegrass

This paper reports the results of experiments to determine whether ammonia (NH₃) loss can be reduced and nitrogen (N) use efficiency improved by using two relatively new commercial urea formulations rather than granular urea and urea ammonium nitrate. Four nitrogen treatments were applied at a rate of 40 kg N ha^?1: granular urea, ‘Green Urea? 14’ [containing 45.8 % N as urea and ‘Agrotain^®’ (N-(n-butyl) thiophosphoric triamide) @ 5 L t^?1 of urea as a urease inhibitor], ‘Nhance’, a fine particle spray [containing 46 % N as urea, ‘Agrotain’ @ 1 L t^?1 of urea and gibberellic acid (applied at a rate of 10 g ha^?1)] and urea ammonium nitrate in solution (UAN) surface applied. Ammonia loss was determined in autumn and spring using a micrometeorological method. In autumn, use of the Green Urea and Nhance reduced NH₃ loss from the 30 % of applied N lost from the granular urea to 9 and 23 % respectively. Loss from all treatments in spring was very small (<2 % of applied N), because 4 mm of rain fell within 24 h of application onto an already wet site. The use of the Nhance and Green Urea instead of granular urea did not result in increased agronomic efficiency or recovery efficiency of the applied N, and this is most likely due to the presence of sufficient available N from both fertiliser application and the soil. A ¹⁵N study recovered 72.8 % of the applied N in the plants and soil, and showed that 30 % of the total N taken up by the plant was derived from the fertiliser, and 70 % from the soil.