Relationships between ammonia volatilization and nitrogen fertilizer application rate,intake and excretion of herbage nitrogen by cattle on grazed swards

Grazed pastures emit ammonia (NH₃) into the atmosphere; the size of the NH₃ loss appears to be related to nitrogen (N) application rate.The micrometeorological mass balance method was used to measure NH₃ volatilization from rotationally grazed swards on three plots in the autumn of 1989 and throughout the 1990 growing season. The aim of the research was to derive a mathematical relationship between NH₃ volatilization and N application rate, which would vary between soil type and weather conditions. In both years the plots received a total of 250, 400 or 550 kg N ha^–1 as calcium ammonium nitrate (CAN) split over 6 to 8 dressings. The number of grazing cycles ranged from 7 to 9 for the three N plots.In the last two grazing cycles of 1989, NH₃ losses were 3.8, 12.0 and 14.7 kg N ha^–1 for the 250N, 400N and 550N plots, which was equivalent to 5.3%, 13.9% and 14.4% of the amount of N excreted on the sward, respectively. In 1990, NH₃ losses were 9.1, 27.0 and 32.8 kg N ha^–1 for the 250N, 400N and 550N plots, which was equivalent to 3.3%, 6.9% and 6.9% of the N excreted, respectively. Differences in urine composition between the plots were relatively small. Rainfall and sward management affected the size of the NH₃ volatilization rate. Volatilization of NH₃ was related to N excretion and N application rate.A calculation procedure is given to enable the estimation of NH₃ volatilization from N application rate. Adjustments can be made for grazing efficiency, grazing selectivity, N retention in milk and liveweight gain, concentrate N intake and milking duration. Losses of NH₃ increase progressively with an increase in N application rate until herbage yield reaches a maximum at an application rate of about 500 kg N ha^–1 yr^–1.     

Nitrogen loss from different tillage systems and the effect on cereal grain yield

Grain yield, nitrogen (N) assimilation, ammonia (NH₃) volatilization, denitrification and fertilizer N distribution were examined in three commercially grown cereal crops; two were sown into conventionally tilled fields, while the third was direct drilled into an untilled field. The crops were top dressed with urea at establishment, tillering or ear initiation. Crop yield and N assimilation were measured in 16 m by 2.5 m plots receiving 0, 35, 70, 105, 140 or 175 kg N ha^–1. A mass balance micrometeorological technique was used to measure NH₃ volatilization, and other fertilizer N transformations and transfers were studied using¹⁵N labelled urea in microplots.On the conventionally tilled sites application of urea increased the grain yield of wheat from 3.9 to 5.5 t ha^–1, when averaged over the five application rates, three application times and two sites. There were no site or application time effects. However, on the direct drilled site, time of application had a significant effect on grain yield. When urea was applied at establishment, grain yield was not significantly increased and the mean yield (2.81 t ha^–1) was less than that obtained from treatments fertilized at tillering or ear initiation (4.09 and 4.0 t ha^–1, respectively). Much of the variation in grain yield at the no-till site could be ascribed to differences in NH₃ volatilization. At the no-till site, NH₃ losses were equivalent to 24, 12 and 1% of the N applied at establishment, tillering and ear initiation, respectively. Negligible volatilization of NH₃ occurred at the other sites. The surface soil at the no-till site had the highest urease activity and the soil was covered with alkaline ash resulting from stubble burning.Plant recovery of fertilizer N did not vary with application time on conventionally tilled sites (mean 62%). However, plant recovery of¹⁵N applied to the no-till site at establishment (35% of the applied N) was significantly less than that from plots where the application was delayed (45% at tillering and 55% at ear initiation, respectively). Leaching of N to below 300 mm depth was minimal (0 to 5% of the applied N). The calculated denitrification losses ranged from 1% to 14% of the applied N.The results show that the relative importance of NH₃ volatilization, leaching and denitrification varied with site and fertilization time. The importance of the various N loss mechanisms needs to be taken into account when N fertilization strategies are being developed.     

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.     

Dynamics of ammonia volatilization from simulated urine patches and aqueous urea applied to pasture I. Field experiments

Ammonia (NH₃) volatilization losses from simulated sheep urine patches in a perennial ryegrass (Lolium perenne L.)/white clover (Trifolium repens L.) pasture in New Zealand were measured in the field during the summer, autumn and winter periods. An enclosure technique was used with microplots (23 cm diameter) receiving either sheep urine or aqueous urea at rates equivalent to 500 kg N ha^–1 and monitored continuously until measured losses decreased to 0.5% per day. Mean volatilization losses for urine treated plots were 22.2% of the applied N in summer, 24.6% in autumn and 12.2% in winter. Corresponding losses for the urea treated plots were 17.9%, 28.9% and 8.5%. Differences between these two N sources were not significant although the seasonal differences were significant (P 0.05). Changes in NH₃ gas fluxes were found to be related to measured changes in soil pH and air temperature. Two repeated applications of urine or aqueous urea to the same microplot resulted in significantly greater subsequent volatilization losses averaging 29.6% from the second and 37.5% from the third application.Most of the applied N was accounted for as either soil mineral N (NH ₄ ⁺ + NO ₃ ^- + NO ₂ ^- ) or NH_3(g) . Urea hydrolysis was rapid and obeyed the first order kinetics during the 24 hours following application. Calculated half-lives of urea in urine and aqueous urea were significantly different and were 3.0 and 4.7 h respectively during the summer and 4.7 and 12.0 h during the autumn.Implications of the results obtained to practical field situation together with the efficacy of the enclosure technique for measuring volatilization losses are discussed.     

Nitrogen budget for fescue pastures fertilized with broiler litter in Major Land Resource Areas of the southeastern US

The southeast US produces a tremendous number of broiler chickens (Gallus gallus), which in turn produce massive quantities of litter (manure and bedding materials). In the Southeast, litter is most often disposed of via land application to pastures, however, the ultimate fate of much of the applied nitrogen (N) is not known. We have constructed N budgets for three sites across the southeastern U.S. in an effort to determine how much of the applied N is useful for plant production and how much is left to be absorbed by the environment. Study sites were located in the Coastal Plain (Alabama), Piedmont (Georgia), and Cumberland Plateau (Tennessee) Major Land Resource Areas (MLRA) of the southeastern US. Litter was applied in the Spring of two consecutive years at a rate to supply 70 kg of available N ha^–1. The total amount of N applied ranged from 103 to 252 kg N ha^–1 depending on site and year. Nitrogen fluxes monitored in this study were broiler litter N, ammonia (NH₃) volatilization, denitrification, plant uptake, and leaching. Plant uptake represented the largest flux of applied N, averaging 43% of applied N. Losses due to NH₃ volatilization and denitrification combined were only 6% of applied N on average. Loss of N due to NO₃-N leaching appeared to be significant only at the Coastal Plain site where NO₃-N concentrations in the groundwater peaked at 38 mg N l^–1. We believe the majority of excess N shown in these budgets is likely accounted for by leaching losses and soil accumulation. Regardless of these assumptions and low gaseous losses, it is apparent that on average, 57% of applied N is destined for a fate other than plant uptake. The results of this study indicate that land-application of broiler litter at currently recommended rates has the potential for negative impacts on the environment of the southeastern U.S. in the long-term.     

Slow-release urea fertilizers — effect on floodwater chemistry,ammonia volatilization and rice growth in an alkali soil

In experiments with transplanted rice (Oryza sativa L.) at the Central Soil Salinity Research Institute, Karnal, India, two methods of application of granular urea, wholly as basal dose U(W) or in splits U(S) were compared with deep, point placement (8 cm) of urea supergranules and broadcast application of two slow-release sources, sulphur-coated urea (SCU) and lac-coated urea (LCU). Comparisons were made in wet season 1984 and 1985 on the basis of ammoniacal N concentration and pH of floodwater, ammonia volatilization, rice yield and N uptake.In 1984 the highest peak concentrations of ammoniacal N (AN) in the floodwater, > 12g m^–3, and ammonia volatilization losses 54% of applied N were produced in U(W). Application of N in splits U(S) reduced peak AN levels 5g m^–3 and losses to 45.1%. LCU was ineffective in reducing peak AN levels ( 7.5g m^–3) or losses (43.6%). However SCU and USG were effective in reducing peak AN levels to < 2g m^–3 and N losses to 16.9 and 3.4% respectively. Total ammonia volatilization losses as well as the initial rate of loss correlated very well with the peak levels (second day) of AN, NH₃ (aq.) as well as equilibrium vapour pressure of NH₃. Floodwater pH was between 9.5 and 10.0.Split application of granular urea was generally more efficient in terms of yield and N recovery (41.4%, average of two years) as compared to whole application (29.5%). LCU was ineffective in improving grain yields or N recovery (30.9%). SCU was ineffective in improving grain yields but improved N recovery to 57.9%., USG increased grain yields only in first year by 19% over U(S) and improved N uptake to 60.5%. A negative linear relationship was established between N uptake by rice at harvest and AN levels in floodwater two days after fertilization which can be used as an index to evaluate fertilizers.     

Effect of urea management on yield and N use efficiency of lowland rice on an acid sulfate soil

Field experiments were conducted during 1988–1989 at two adjacent sites on an acid sulfate soil (Sulfic Tropaquept) in Thailand to determine the influence of urea fertilization practices on lowland rice yield and N use efficiency. Almost all the unhydrolyzed urea completely disappeared from the floodwater within 8 to 10 d following urea application. A maximum partial pressure of ammonia (pNH₃) value of 0.14 Pa and an elevation in floodwater pH to about 7.5 following urea application suggest that appreciable loss of NH₃ could occur from this soil if wind speeds were favorable. Grain yields and N uptake were significantly increased with applied N over the control and affected by urea fertilization practices (4.7–5.7 Mg ha^–1 in dry season and 3.0–4.1 Mg ha^–1 in wet season). In terms of both grain yield and N uptake, incorporation treatments of urea as well as urea broadcasting onto drained soil followed by flooding 2 d later were more effective than the treatments in which the same fertilizer was broadcast directly into the floodwater either shortly or 10 d after transplanting (DT). The¹⁵N balance studies conducted in the wet season showed that N losses could be reduced to 31% of applied N by broadcasting of urea onto drained soil and flooding 2 d later compared with 52% loss by broadcasting of urea into floodwater at 10 DT. Gaseous N loss via NH₃ volatilization was probably responsible for the poor efficiency of broadcast urea in this study.     

A comparison of methods of estimating ammonia volatilization in the field

Accurate estimation of the potential for NH₃ volatilization from urea-based fertilizers is an important step in attaining optimum N-use efficiency from these fertilizers. Published estimates of NH₃ volatilization losses from urea vary widely. Much of this variability may be due to the method of estimation and the degree of influence of the method on NH₃ loss. This study compared two field methods of estimating NH₃ volatilization in the field; a microplot-forced draft method, and a micrometeorogical method. Loss of NH₃ was estimated in three experiments following urea solution application to bare soil, and in two experiments following urea-ammonium nitrate solution application to wheat stubble residue. Both methods were found to be sensitive to soil and climatic variables influencing NH₃ volatilization. Cumulative N loss from the bare soil experiments ranged from 7 to 8 kg N ha^–1 for the microplot method and from 5 to 20 kg N ha^–1 for the micrometeorological method. Cumulative loss from wheat stubble residue ranged from 2 to 2.2 kg N ha^–1 for the microplot method and from 15 to 33 kg N ha^–1 for the micrometerological method. Loss of NH₃ was especially influenced by soil or residue water content and the influence of water content on the rate of urea hydrolysis. Maximum rates of loss were generally observed near midday, when water content at the soil surface was just beginning to decline and the surface temperature was rapidly rising. The microplot method was found to have a greater potential for affecting the environment and thus influencing NH₃ loss measurements than the micrometeorological method. Windspeed and mixing at the soil surface was influenced by the presence of the microplot cylinder and lid, especially in the wheat residue experiments. It is likely that the micrometeorological method, with its minimal influence on the field environment, more accurately reflects actual levels of ammonia loss. The primary advantage of the microplotforced draft method is its ability to easily compare relative NH₃ losses from different treatments.Contribution No. 87-300-J from the Kansas Agricultural Experiment Station. Part of a dissertation submitted by the senior author in partial fulfillment of the requirements for the Ph. D. Degree at Kansas State University. The research was supported in part by grants from Farmland Industries, Inc., and USDA-ARS.     

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.     

Calibration of a simple method for determining ammonia volatilization in the field – comparative measurements in Henan Province, China

The determination of ammonia volatilization with sufficient spatial and temporal resolution requires a simple and versatile in situ measurement technique, particularly in developing countries. Therefore, a simple chamber method for determining ammonia (NH₃) volatilization in the field (Dräger-Tube Method; DTM) was calibrated by comparison with simultaneous measurements with a micrometeorological Integrated Horizontal Flux (IHF) method. Five field experiments were conducted following urea fertilization on summer maize and winter wheat plots (1998–1999) at Fengqiu Experimental Station, Central China. The simplicity of the chamber method allowed for measurements to be carried out by trained farmers. The measurements with both methods yielded very similar patterns of NH₃ fluxes and similar differences between fertilization treatments. Cumulative NH₃ losses determined by the IHF method ranged from 14.6 to 47.9% and from 0.6 to 17.9% of urea-N applied for surface broadcast and incorporated fertilization, respectively. As expected, cumulated NH₃ losses were underestimated by the DTM as compared to the IHF by about one order of magnitude. A calibration equation was calculated by multiple linear regression which included NH₃ flux data as well as temperature and wind speed values. The calibration model yielded a modelling efficiency c ² of 0.86 resulting in an average estimation error of cumulative NH₃ losses of 17%. The equation was validated by comparison of IHF measurements and DTM fluxes not considered in the derivation of the calibration formula. The calibration approach can be used under similar meteorological and field conditions irrespective of the soil characteristics or type of N fertilizer applied.     

Studies on the relationship between slurry pH,volatilization processes and the influence of acidifying additives

The mutual influence of slurry pH and volatilization processes on one hand, and the possibility of N conservation by the use of acidifying additives on the other, were investigated in static incubation experiments. The influence of the NH₃ and CO₂ volatilizations on slurry pH was studied by selectively supporting one or both processes. The addition of Ca²⁺ to slurry was compared to that of K⁺ and H⁺. The effects of Cl^–, SO ₄ ^2– and NO ₃ ^– as corresponding anions of Ca²⁺ on slurry pH as well as NH₃ and N₂O emissions were tested. The slurry pH (7.4) increased during incubation. When CO₂ volatilization was suppressed, the pH increase was reduced, and NH₃ volatilization was cut down by 50%. Ca²⁺ additions hardly influenced the initial slurry pH, but reduced the pH increases and NH₃ losses. Proton addition, in contrast, decreased slurry pH but did not decrease the subsequent pH rise. K⁺ had no effect on slurry pH and N losses. As compared to CaCl₂, CaSO₄ showed less effect on slurry pH and N losses. Ca(NO₃)₂ was nearly as effective as CaCl₂ in preventing NH₃ volatilization, but caused denitrification losses and elevated N₂O production. Titration curves of the different slurry treatments were used to interpret the results of the incubation experiments. In a microplot field experiment the NH₃ volatilization and slurry pH after surface application of slurry was measured. The acidifying and N conserving effects of Ca²⁺ and H⁺ additions were confirmed.     

Ammonia volatilization from drill sown rice bays

Ammonia volatilization losses and other N transformations were studied in drill sown rice bays fertilized with urea at various times between permanent flooding (PF) and panicle initiation (PI). Ammonia loss was measured directly with flow chambers and indirectly through application of Freney et al.'s (1985) model. Both techniques indicated that ammonia volatilization was negligible from fields fertilized immediately before PF. Applying 100 kg urea-N ha^–1 to floodwater one day after flooding significantly increased floodwater ammoniacal-N and urea-N content, however the concentrations fell rapidly over the following five days. Fertilizer-N dissolved in the floodwater was in the urea rather than the ammoniacal-N form, indicating slow hydrolysis until it moved into the soil. Floodwater on plots receiving urea one day after PF frequently had more than double the NO₃-N concentration of plots fertilized before flooding.Applying up to 140 kg urea-N ha^–1 at PI increased floodwater ammoniacal-N concentrations from almost zero to over 27 g m^–3, but three days after fertilization there was less than 3 g m^–3 present. Fertilization also increased NH₄-N concentration in the top 40 mm of soil. Higher ammoniacal-N concentration at PI suggests higher urease activity. Floodwater pH at PI was low, with a mean daily maximum of 7.8 and this reduced ammonia loss to less than 1% of the applied N.The results indicate that volatilization from fields fertilized prior to PF is minimal because of the low floodwater pH and ammoniacal-N concentration, while low floodwater pH restricts volatilization from fields topdressed at PI.     

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.     

Nitrogen losses from fertilizers applied to maize, wheat and rice in the North China Plain

Ammonia volatilization, denitrification loss and total nitrogen (N) loss (unaccounted-for N) have been investigated from N fertilizer applied to a calcareous sandy loam fluvo-aquic soil at Fengqiu in the North China Plain. Ammonia volatilization was measured by the micrometeorological mass balance method, denitrification by the acetylene inhibition – soil core incubation technique, and total N loss by ¹⁵N-balance technique. Ammonia loss was an important pathway of N loss from N fertilizer applied to rice (30–39% of the applied N) and maize (11–48%), but less so for wheat (1–20%). The amounts of unaccounted-for fertilizer N were in the order of rice > maize > wheat. Deep placement greatly reduced ammonia volatilization and total N loss. Temperature, wind speed, and solar radiation (particular for rice), and source of N fertilizer also affect extent and pattern of ammonia loss. Denitrification (its major gas products are N₂ and N₂O) usually was not a significant pathway of N loss from N fertilizer applied to maize and wheat. The amount of N₂O emission (N₂O is an intermediate product from both nitrification and denitrification) was comparable to denitrification loss for maize and wheat, and it was not significant in the economy of fertilizer N in agronomical terms, but it is of great concern for the environment.     

Fruit yield of Valencia sweet orange fertilized with different N sources and the loss of applied N

Nutrient management recommendations are needed to increase nitrogen uptake efficiency, minimize nutrient losses and reduce adverse effects on the environment. A study of the effects of nitrogen fertilization on N losses and fruit yield of 6-yr-old Valencia sweet orange (Citrus sinensis (L.) Osb.) on Rangpur lime rootstock (C. limonia Osb.) grove was conducted in an Alfisol in Brazil from 1996 to 2001. Urea (UR) or ammonium nitrate (AN) fertilizers were surface-applied annually at rates of 20, 100, 180, and 260 kg N ha^–1 split into three applications from mid-spring to early fall. A semi-open trapping system, using H₃PO₄ + glycerol-soaked plastic foams, was used for selected treatments in the field to evaluate NH₃ volatilized from applied N fertilizers. Ammonia volatilization reached 26 to 44% of the N applied as UR at the highest rate of N used. Ammonia volatilization losses with AN were lower (4% of the N applied). On the other hand, AN resulted in greater nitrate leaching and greater soil acidification than UR. A marked effect of AN fertilizer on soil pH (CaCl₂) in the 0–20 cm depth layer was observed with a decrease of up to 1.7 pH units at the highest N rate. Acidification was followed by a decrease in exchangeable Ca and Mg; consequently, after 5 yr of fertilization with AN, soil base saturation dropped from 77% in the plots treated with 20 kg N ha^–1 per year, to 24% in those that received 260 kg N ha^–1 per year. The effect of N sources on fruit yield varied from year to year. In 2001, for a calculated N application rate of 150 kg ha^–1, the fertilizer efficiency index of UR was 75% of that of AN.     

Reducing N losses (NH3, N2o, N2) and immobilization from slurry through optimized application techniques

In model, pot and field trials the effect of C reduced slurries and different application techniques on N losses and N immobilization were investigated. The C reduced slurries were produced by mechanical separation. Ammonia losses from surface-applied and injected cattle slurry were measured under field conditions using a wind tunnel system. Injection of slurry was the most efficient way to reduce volatilization of ammonia. After 6 days the total loss from the injected slurry was only 9% of that from surface band application. Furthermore, additional losses of N may occur through denitrification, specially after injection of slurry which may create an anaerobic environment abundant in readily oxidizable C. Therefore denitrification measurements by the acetylene inhibition technique were conducted. Until 100 days after application the loss from the injected slurry was 7.3 kg compared to 4.5 kg N ha^–1 from surface band applied slurry. After injection, denitrification was only 4.1 kg N ha^–1 for C reduced compared to 6.5 kg N ha^–1 for normal slurry. In pot trials the ammonium-¹⁵N of normal slurry and C-reduced slurry was utilized by oats between 52 and 60%, the ammonium sulfate by 67%. The increased biomass C confirmed a greater immobilization of the NH₄-N of the normal slurry resulting in a lower initial efficiency.Dedicated to Prof. Dr. A. Amberger on his 75th birthday     

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 role ofAzolla in curbing ammonia volatilization from flooded rice systems

By the year 2020, an additional 300 million tons of rice are needed annually to meet the demands of a growing population. If our natural resource base is to be preserved, intensification strategies should rely on integrated nutrient management, making full use of biological nitrogen fixation. TheAzolla-Anabaena complex is amongst the most effective systems of fixing nitrogen. In this paper we present evidence from greenhouse studies on the potential ofAzolla to curb the volatilization of NH₃ following the application of urea to a mixedAzolla-rice culture, providing a new incentive for developing ways of integratingAzolla in intensive rice cultivation systems.The results of a series of short term greenhouse experiments show that a full cover ofAzolla can significantly reduce losses of applied urea-N from 45 and 50% to 20 and 13% for the 30 and 60 kg N ha^–1 treatments, respectively. About one-quarter of the applied N was tied up in theAzolla biomass. The applied N inhibitedAzolla growth as well as the amount of N fixed. Inoculation with smaller quantities ofAzolla allowing for more vigorousAzolla multiplication was equally effective in reducing NH₃ volatilization and doubled the amount of¹⁵N tied-up byAzolla. The reduction in NH₃ volatilization is largely related to the depression byAzolla of the floodwater pH, which in its absence may reach values between 9 and 10 as a result of algal activity.Early rice growth responded positively to urea as well as the large quantities of appliedAzolla and increased the yield potential of the crop. Smaller quantities ofAzolla alone were not effective in this regard. The conservation of fertilizer N byAzolla, particularly when it fully covered the water, was reflected in a synergistic effect on rice dry matter production, amounting to 9% at the 30 kg N rate and 16% at the 60 kg N rate. In all likelihood this interaction is attributable to the higher efficiency of the applied N. The benefits ofAzolla in conserving basal urea-N even in small quantities (200-500 kg fresh material ha^–1), outweighed competition for the applied N and may be as important as its BNF. The most promising integratedAzolla/rice management systems emerging from our studies should be given further attention under field conditions.     

The fate of urea nitrogen applied in a foliar spray to wheat at heading

Nitrogen losses from irrigated wheat (cv. Matong) grown on a heavy clay in the Goulburn-Murray Irrigation Region following foliar applications of urea at heading were investigated. Ammonia (NH₃) volatilization was determined by a micrometeorological method and total nitrogen (N) loss was determined by a¹⁵N balance technique. The effects of the foliar application on grain N concentration and grain yield were determined also.Little nitrogen was lost by NH₃ volatilization following the foliar application. The rate of NH₃ loss increased briefly from <11 g N ha^–1 hr^–1 to >19 g N ha^–1 hr^–1 following rainfalls of 3 and 2 mm which washed 34% of the applied N from the plant onto the soil and increased the pH of the surface soil. The pH effect was short lived and total NH₃ loss amounted to only 2.13 kg N ha^–1 or 4.3% of the applied N.The¹⁵N balance study also showed that little N was lost from the plant-soil system until rain had washed the fertilizer from the plant onto the soil. In the period 152 to 206 DAS, the soil component of the applied N decreased from 34% to 9%. This fraction then increased slightly to 12% of the applied N at harvest. At that time, 69% of the applied N was recovered in the plants indicating that 19% of the applied N had been lost from the plant-soil system. As there was no evidence for leaching of N, the difference between total N loss as measured by¹⁵N balance (19%) and NH₃ loss (4%) is considered to be loss by denitrification (15%).The fertilizer N assimilated by the plant was efficiently remobilised from the leaves and stems to the head; 78% of the fertilizer N assimilated by the plant tops had been translocated to the head by the time of harvest. Grain N concentration responded to the foliar N application. The fitted response function had significant linear (P = 0.004) and quadratic (P = 0.10) trends to N rate, whereas there was no significant effect on grain yield.     

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.