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.     

Calculated rates of soil acidification of intensively used grassland in the Netherlands

The major processes involved in acidification of soils under intensively managed grassland are the transformation and subsequent leaching of applied nitrogen (N), assimilation of excess cations in herbage and acidic atmospheric deposition. Carbonates from fertilizers and excess cations in purchased concentrates are the most important proton (H⁺) neutralizing agents applied to grassland. In this study, the effects of grazing, cutting and N application on the net proton loading from each of the main processes were calculated, using a simple model.On mown swards, simulated excess cation uptake by the sward released 4.5–9.3 kmol_c H⁺ ha^–1 yr^–1. The total proton loading on mown grassland decreased from about 8.0 to 5.3 kmol_c ha^–1 yr^–1 when fertilizer N input as CAN-27 increased from 0 to about 400 kg ha^–1 yr^–1. Contributions from atmospheric deposition ranged from 2.2 kmol_c ha^–1 yr^–1 when herbage yield exceeded 10 Mg ha^–1 yr^–1 to 3.0 kmol_c ha^–1 yr^–1 when herbage production was only 5.5 Mg ha^–1 yr^–1.On grazed swards, transformation of organically bound N from urine and dung to nitrate (NO ₃ ^- ) and the subsequent leaching of excess NO ₃ ^- was the main source of protons. Application of 400 kg N ha^–1 yr^–1 to grazed swards increased the proton loading from transformed N from 3.9 to 16.9 kmol_c ha^–1 yr^–1. The total proton loading on grazed swards exceeded that of mown swards when the input of fertilizer N exceeded 150 kg ha^–1 yr^–1.Underestimation of the amount of N immobilized in the soil biomass and lost by denitrification may have resulted in a slight overestimation of the amount of N lost by leaching and thereby also the simulated total proton loading.     

The combined effect of fertiliser nitrogen and phosphorus on herbage yield and changes in soil nutrients of a grass/clover and grass-only sward

The combined effect of reduced nitrogen (N) and phosphorus (P) application on the production of grass-only and grass/clover swards was studied in a five-year cutting experiment on a marine clay soil, established on newly sown swards. Furthermore, changes in soil N, P and carbon (C) were measured. Treatments included four P (0, 35, 70 and 105 kg P ha^–1 year^–1) and three N levels (0, 190 and 380 N kg ha^–1 year^–1) and two sward types (grass-only and grass/clover). Nitrogen was the main factor determining the yield and quality of the harvested herbage. On the grass-only swards, N application increased the DM yield with 28 or 22 kg DM kg N^–1, at 190 or 380 kg N ha^–1 year^–1, respectively. The average apparent N recovery was 0.78 kg kg^–1. On the grass/clover swards, N application of 190 ha^–1 year^–1 increased grass production at the cost of white clover, which decreased from 41 to 16%. Phosphorus application increased grass yields, but did not increase clover yields. A positive interaction between N and P applications was observed. However, the consequences of this interaction for the optimal N application were only minor, and of little practical relevance. Both the P-AL-value and total soil P showed a positive response to P application and a negative response to N application. Furthermore, the positive effect of P application decreased with increasing N application. The annual changes in P-AL-value and total soil P were closely related to the soil surface surplus, which in turn was determined by the level of N and P application and their interaction. The accumulation of soil N was similar on both sward types, but within the grass-only swards soil N was positively affected by N application. The accumulation of organic C was unaffected by N or P application, but was lower under grass/clover than under grass-only.     

Assessment of the nitrogen requirements of maize (Zea mays L.) in Southern Guinea Savanna agroecology of Nigeria

During the growing seasons (May to October) of 1987 and 1988 respectively five and four different rates of N were tested on maize (Zea mays L.) at 12 different field sites across the Southern Guinea Savanna of Nigeria. Nitrogen was applied through granular urea (size +14 mesh), ordinary prilled urea (–35 mesh) and calcium ammonium nitrate. Marked differences existed among experimental sites in maize grain yield response to N with Yelwa and Ta-Hoss in Plateau State having the highest response at 60 and 90 kg N ha^–1 respectively.During 1988, at five experimental sites the yield was maximized with 120 kg N ha^–1, while at three other sites the yield maximization occurred at 90 kg N ha^–1. During 1987, the corresponding number of sites was two and six with 120 and 90 kg N ha^–1 rates, respectively. Plant height and cob number exhibited a linear relationship with yield. Differences in yield in response to application of different N sources were non-significant.Contribution from the Nationally coordinated fertilizer use programme funded by Federal Government of Nigeria     

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.     

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.     

An assessment of N loss from agricultural fields to the environment in China

Using the 1997 IPCC Guidelines for National Greenhouse Gas Inventory Methodology, and statistical data from the China Agricultural Yearbook, we estimated that the direct N₂O emission from agricultural fields in China in 1990 was 0.282 Tg N. Based on micro-meteorological field measurement of NH₃ volatilization from agricultural fields in different regions and under different cropping systems, the total NH₃ volatilization from agricultural fields in China in 1990 was calculated to be 1.80 Tg N, which accounted for 11% of the applied synthetic fertilizer N. Ammonia volatilization from agricultural soil was related to the cropping system and the form of N fertilizer. Ammonia volatilization from paddy fields was higher than that from uplands, and NH₄HCO₃ had a higher potential of NH₃ volatilization than urea. N loss through leaching from uplands in north China accounted for 0.5–4.2% of the applied synthetic fertilizer N. In south China, the leaching of applied N and soil N from paddy fields ranged from 6.75 to 27.0 kg N ha^-1 yr^-1, while N runoff was between 2.45 and 19.0 kg N ha^-1 yr^-1.     

Influence of long-term changes in nitrogen flows on the environment: A case study of a city in Hokkaido, Japan

There is an urgent need to establish sustainable nutrient cycling. Changes in amounts of N flow and separation of production and consumption sectors are becoming a serious environmental problem. In this study, the yearly N in- and outflow of a city in northern Japan from 1912 to 2002 was investigated based on the statistics and inventory data. Based on the characteristics of the N flow, the period was divided into manure-based period (MBP, 1912–1950), transition period from manure- to chemical fertilizer-based period (TP, 1950–1970), and chemical fertilizer-based period (CBP, 1970–2002). The highest amount of N inflow (up to 350 Mg N y^–1) was observed at the end of the MBP, and the second peak (about 300 Mg N y^–1) at the beginning of the CBP. The N application rate on farmland increased from 68 kg N ha^–1 in 1912 to above 250 kg N ha^–1 in the 1950s, then decreased to 168 kg N ha^–1 in 2002. The farmland productivity increased from 30 kg N ha^–1 at the end of the 1950s to 90 kg N ha^–1 in 2002, due to improvement in crop varieties and management methods. In MBP surplus N in farmland and NH₃ volatilization accounted for 90% of the N outflow from the city, then in CBP, disposal N and surplus N in farmland became the main N outflow. All these outputs are considered to increase the N concentration in rivers and/or underground water. In the case of surplus N in farmland, it exceeded the amount of optimum N management (<50 kg N ha^–1; , Agricult. Ecosyst. Environ. 72: 35–52) during 1935–1970 and 1981–1997. In order to prevent degradation of the environment through artificially altered nutrient flow, we need to be aware of the environmental impact of the N flow and establish proper N management practices.     

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.     

Nitrogen budgets and environmental capacity in farm systems in a large-scale karst region, southern China

Field surplus nitrogen (N) and farm disposal N are major sources of water pollution in farming systems. These sources are estimated from N budgets in field and whole farms, which are associated with the production and consumption of food. This study was conducted to evaluate these two pollution sources in the steep mountainous karst region of Quibainong, Guangxi Province, southern China. The region is, characterized as an area of upland farms, due to the shallow soils and rapid water drainage through cracks in the limestone. Although field surplus N in 1960 was only 4.1 kg N ha^–1, current field surplus N ranged from 10.1 to 463 kg N ha^–1, with values above 50 kg N ha^–1 in farms along roads and less than 40 kg N ha^–1 in the farms away from roads. The results obtained in near-road farms were similar to those in a previous study of N budgets in China. There was a significant positive correlation between the field surplus N and N application rate, including when the previous data were incorporated. The proportion of manure to total N application decreased with increase of N application. Chemical fertilizer was applied in greater quantity in economically rich farms. Therefore, the increase of field surplus N in Quibainong may be caused by economic improvement. Although livestock and human excreta were stocked in manure barns, unused excreta N increased with the increase of N excreted. The unused excreta N also increased with the decrease of feed self-sufficiency, but was not related to N application rate. These facts indicate that livestock husbandry in Quibainong is related to economic status of farms, but independently of crop production.The N application rate of more than 160 kg N ha^–1 increased field surplus N to an extent greater than crop uptake N, and a N application rate of more than 185 kg N ha^–1 increased the potential nitrate-N concentration to more than 10 mg L^–1. Therefore, 160–185 kg N ha^–1 is suggested to be the environmental capacity to sustain optimal N cycling in Quibainong. The average value of excreta N produced on near-road farms in Quibainong was 171 kg N ha^–1. If excreta N was used evenly for crop cultivation without chemical fertilizer in whole fields, the optimal N cycling would be maintained.The survey conducted here using a questionnaire was effective in evaluating all kind of N flows in the farming systems.     

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.     

Effects of soil fumigation and N source on soil inorganic N and tomato growth

Soil fumigation, commonly used in vegetable production, may alter the rate of nitrification, affecting availability of N for crop use. The objective of this research was to examine effects of soil fumigation and N fertilizer source on tomato growth and soil NO₃–N and NH₄–N in field production. Experiments 1 and 2 included application of methyl bromide at 420 kg ha^-1 to a Norfolk sandy loam (fine loamy siliceous thermic Typic Kandiudult) in combination with preplant applications of calcium nitrate, ammonium nitrate, and ammonium sulfate at 144 kg N ha^-1. An additional fumigant, metam-sodium, was included in the second experiment at 703 L ha^-1 (268 kg sodium methyldithiocarbamate ha^-1). Experiment 3 included methyl bromide and metam-sodium, with ammonium sulfate as the sole source of N applied at 144 kg N ha^-1. In the first two studies, fumigants had little or no effect on soil NH₄–N or NO₃–N concentration. Tomato plants were larger and fruit yield was greater in fumigated plots, but there were few growth or yield responses to N source. In the third experiment, fumigants increased concentration of soil NO₃–N and NH₄–N at 16 days after fumigation (DAF), however, there was no effect on nitrification owing to fumigants. It appears that N source selection to overcome inhibition of nitrification is not necessary in plant production systems that involve fumigation     

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.     

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.     

Uptake of fertilizer and soil nitrogen by ryegrass swards during spring and mid-season

Double-labelled¹⁵N ammonium nitrate was used to determine the uptake of fertilizer and soil N by ryegrass swards during spring and mid-season. The effects of water stress (40% of mean rainfall v 25 mm irrigation per 25 mm soil water deficit) and the rate of application of N in the spring (40 v 130 kg ha^–1) on the recovery of 130 kg N ha^–1 applied in mid-season were also evaluated. Apparent recovery of fertilizer N (uptake of N in the fertilized plot minus that in the control expressed as a percentage of the N applied) was 95 and 79% for fertilizer N applied in the spring at rates of 40 and 130 kg ha^–1, respectively. Actual recovery of the fertilizer N assessed from the uptake of¹⁵N was only 31 and 48%, respectively. The uptake of soil N by the fertilized swards was substantially greater than that by the control. However, the increased uptake of soil N was always less than the amount of fertilizer N retained in or lost from the soil. Broadly similar patterns for the uptake of fertilizer and soil N were observed during mid-season. Uptake of N in mid-season was highest for swards which received 40 kg N ha^–1 in the spring and suffered minimal water stress during this period. Application of 130 kg N ha^–1 in spring reduced the uptake of N in mid-season to an extent similar to that arising from water stress. Only 1.8 to 4.2 kg ha^–1 (3 to 10%) of the N residual from fertilizer applied in the spring was recovered during mid-season. Laboratory incubation studies suggested that only a small part of the increased uptake of soil N by fertilized swards could be attributed to increased mineralisation of soil N induced by addition of fertilizer. It is considered that the increased uptake of soil N is partly real but mostly apparent, the latter arising from microbially mediated exchange of inorganic¹⁵N in the soil.     

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.     

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.     

A farm-level evaluation of nitrogen and phosphorus fertilizer use and planting density for pearl millet production in Niger

Mineral fertilizer use is increasing in West Africa though little information is available on yield response in farmers' fields. Farmers in this region plant at low density (average 5,000 pockets ha^–1, 3 plants pocket^–1), which can affect fertilizer use efficiency. A study was conducted with 20 farmers in Niger to assess the response of pearl millet [Pennisetum glaucum (L.) R. Br.] to phosphorus and nitrogen fertilizers under farm conditions. In each field, treatments included control, single superphosphate (SSP) only, SSP plus N (point placed near plant), and either SSP or partially acidulated phosphate rock (PAPR) plus N broadcast. N and P were applied at 30 kg N ha^–1 and 30 kg P₂O₅ ha^–1. Farmers were allowed to plant, weed, etc., as they wished and they planted at densities ranging from 2,000 to 12,000 pockets ha^–1. In the absence of fertilizer, increasing density from 2,000 to 7,000 pockets ha^–1 increased yield by 400%. A strong interaction was found between fertilizer use and density. Farmers planting at densities less than 3,500 pockets ha^–1 had average yields of 317 kg grain ha^–1 while those planting at densities higher than 6,500 pockets ha^–1 showed average yields of 977 grain ha^–1. Though phosphate alone increased yields significantly at all densities, little response to fertilizer N was found at densities below 6,000 pockets ha^–1. Significant residual responses in 1987 and 1988 were found to P applied in high-density plots in 1986. Depending on fertilizer and grain prices, analysis showed that fertilizer use must be be combined with high plant density (10,000 pockets ha^–1) or no economic benefit from fertilizer use will be realized.     

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     

A comparison of the performance of N simulation models in the prediction of Nmin on farmers' fields in the spring

The performance of three different models, which simulate changes in the inorganic N content of the soil, was evaluated in respect of their ability to predict N_min content in the spring under cereal crops. The models of British, Dutch and German origin, were tested using data from farmers' fields supplied by 70 farmers over two growing seasons in FRG. The models were run between harvest of the previous crop and spring of the following year, and predictions of N_min in the spring compared to soil measurements. The performance of the models was assessed by counting the number of cases in which predictions agreed within 10 or 20 kg (N) ha^–1 of the measurements. Predictions were less than ± 10 kg (N) ha^–1 of measured values in only 30–44% and 28–55% of cases in 1988 and 1989, respectively. Predictions were less than ± 20 kg (N) ha^–1 of measured values in 62–70% and 68–82% of cases in 1988 and 1989, respectively. Predictions in 1989 were better because the initial N_min content in the autumn was included in the model input. None of the models tested had been designed to use input data of the type available to farmers. It is concluded that, at present, the results are too variable for any of the models to be used with confidence as tools to aid in N fertilizer recommendations.