Efficiency of some modified urea fertilisers for wetland rice grown on a permeable soil

Split broadcast applications of prilled urea, deep point-placed urea supergranules (USG), and broadcast sulfur-coated urea (SCU) were compared as nitrogen sources for wetland rice (Oryza sativa L.) in two field experiments on a sandy soil (Typic Ustipsamment) with a high percolation rate (approx. 110 mm/day) in the Punjab, India. The USG was consistently less effective than the split urea and averaged 1 ton ha^–1 less rice yield at the highest nitrogen rate (116 kg N ha^–1). SCU produced the highest grain yields in both experiments; it averaged 1.7 ton ha^–1 more than did the split urea at the highest N rate.The fertilisers were then compared in field microplots; percolation was permitted or prevented so that the cause of the poor performance of USG could be elucidated. USG gave higher grain yield and N uptake in microplots that were not leached than in those that were leached. In leached microplots, the grain yields were higher from prilled urea than from USG treatments provided the placement pattern of the USG matched that of the field plots. Yields were not higher from treatments in which the USG were more closely spaced. In microplots in which leaching was prevented, the broadcast prilled urea was less effective than the deep-placed USG, which gave yields approximately 60% greater than those from split urea and the same as those from SCU. Broadcast prilled urea in undrained microplots caused high levels of ammonium (40 ppm) to develop in the floodwater where high pH (8.9) and high alkalinity (4.9 meq l^–1) may have led to extensive ammonia volatilisation. The use of USG and SCU in undrained microplots reduced floodwater ammonium levels to less than 3 ppm.Urea and ammonium leaching losses measured in fallow soil columns in the laboratory were much greater from USG than from prilled urea. Leaching losses from SCU were negligible. The data suggest that SCU is the preferred N source for rice soils having a high percolation rate and that USG is a poor alternative to split applications of prilled urea.     

Detectability of15N-depleted fertilizer N in soil and plant tissue during a corn-rye crop rotation

Use of¹⁵N-depleted fertilizer materials have been primarily limited to fertilizer recovery studies of short duration. The objective of this study was to determine if¹⁵N-depleted fertilizer N could be satisfactorily used as a tracer of residual fertilizer N in plant tissue and various soil N fractions through a corn (Zea mays L.) -winter rye (Secale cereale L.) crop rotation. Nitrogen as¹⁵N-depleted (NH₄)₂SO₄ was applied at five rates (0, 84, 168, 252, and 336 kg N ha^–1) to corn. Immediately following corn harvest a winter rye cover crop treatment was initiated. Residual fertilizer N was easily detected in the soil NO ₃ ^- -N fraction following corn harvest (140-d after application). Low levels of exchangeable NH ₄ ⁺ -N (<2.5 mg kg^–1) did not permit accurate isotope-ratio analysis. Fertilizer-derived N recovered in the soil total N fraction following corn harvest was detectable in the 0 to 30-cm depth at each N rate and in the 30 to 60 and 60 to 90-cm depths at the 336 kg ha^–1 N rate. Atom %¹⁵N concentrations in the nonexchangeable NH ₄ ⁺ -N fraction did not differ from the control at each N rate. Nitrogen recovery by the winter rye cover crop reduced residual soil NO ₃ ^- -N levels below the 10 kg ha^–1 level needed for accurate isotope-ratio analysis. Atom %¹⁵N concentrations in the soil total N fraction (approximately one yr after application) were indistinguishable from the control plots below the 168, 252, and 336 kg ha^–1 N rate at the 0 to 30, 30 to 60, and 60 to 90-cm depths, respectively. Recovery of residual fertilizer N by the winter rye cover crop was verified by measuring significant decreases in atom %¹⁵N concentrations in rye tissue with increasing N rates. The greatest limitation to the use of¹⁵N-depleted fertilizer N as a tracer of residual fertilizer N in a corn-rye crop rotation appears to be its detectibility from native soil N in the total N pool.Research partially supported by grants from the National Fertilizer and Environmental Research Center/TVA and the Virginia Division of Soil and Water Conservation.     

Nitrogen use efficiency,floodwater properties,and nitrogen-15 balance in transplanted lowland rice as affected by liquid urea band placement

Alternative N fertilizer management practices are needed to increase productivity and N use efficiency in lowland rice (Oryza sativa L.). In 1986 dry season, a field study using¹⁵N-labeled urea evaluated the effect of time and method of fertilizer N application on grain yield and N use efficiency. Conventional fertilizer application was compared with band placement of liquid urea and point placement of urea supergranules (USG). Grain yields were significantly higher with either band or point placement than with broadcast and incorporation or surface application. Partial pressure of NH₃ (NH₃) was significantly reduced when N was deep-placed.¹⁵N balance data show that fertilizer N applied basally and incorporated gave a total¹⁵N recovery of 52% and crop (grain + straw) recovery of 30%. Band placement of liquid urea N resulted in 82–90% total and 57–65% crop¹⁵N recovery. USG point placement gave 94% total and 70% crop¹⁵N recovery. Deep placement of second N application gave only slightly higher (98%)¹⁵N recovery compared with broadcast application (89%).     

Time and mode of nitrogen fertilizer application to tropical wetland rice

In experiments with transplanted rice (Oryza sativa L.) at the International Rice Research Institute, Philippines, two methods of split application of urea and ammonium sulfate were compared with deep, point placement (10 cm) of urea supergranules and broadcast application of a slow-release fertilizer sulfur-coated urea (SCU). Comparisons were made in the wet and dry seasons and were based on rice yield and N uptake. Urea- and ammonium-N concentrations and pH of the floodwater were measured to aid interpretation of the results.Split applications of urea were generally less efficient than ammonium sulfate. The split in which the initial fertilizer dose was broadcast and incorporated into the soil before transplanting was more effective than the split in which the fertilizer was broadcast directly into the floodwater 21 days after transplanting. Both split applications were inferior to the urea supergranules and SCU, in terms of both yield and N uptake efficiency; average apparent N recoveries ranged from 30% for the delayed split urea to 80% for the urea supergranule.Broadcast applications of urea and ammonium sulfate produced high floodwater concentrations of urea- and ammonium-N, which fell to zero within 4–5 days. Floodwater pH was as high as 9.3 and fluctuated diurnally due to heavy algal growth. Ammonia volatilization and algal immobilization of N in the floodwater were probably responsible for the poor efficiency of the split applications; the supergranules and SCU on the other hand produced low floodwater N concentrations and were efficiently used by the rice crop.     

Effect of encapsulated calcium carbide and urea application methods on denitrification and N loss from flooded rice

Poor N fertilizer use efficiency by flooded rice is caused by gaseous losses of N. Improved fertilizer management and use of nitrification inhibitors may reduce N losses. A microplot study using¹⁵N-labelled urea was conducted to investigate the effects of fertilizer application method (urea broadcast, incorporated, deep-placed) and nitrification inhibitor [encapsulated calcium carbide (ECC)] treatments on emission of N₂+N₂0 and total loss of applied N on a grey clay near Griffith, NSW, Australia. Both incorporation and deep placement of urea decreased N₂+N₂O emission compared to urea broadcast into the floodwater. Addition of ECC significantly (P < 0.05) reduced emission of N₂+N₂0 from incorporated or deep-placed urea and resulted in increased exchangeable ammonium concentrations in the soil in both treatments. Fifty percent of the applied N was lost when urea was broadcast into the floodwater. Total N loss from the applied N was significantly (P < 0.05) reduced when urea was either incorporated or deep placed. In the presence of ECC the losses were reduced further and the lowest loss (34.2% of the applied N) was noted when urea was deep-placed with ECC.     

Determination of dinitrogen emission and retention in floodwater and porewater of a lowland rice field fertilized with15N-urea

This paper reports a study on the distribution of dinitrogen between the atmosphere, floodwater and porewater of the soil in a flooded rice field after addition of¹⁵N-labelled urea into the floodwater.Microplots (0.086 m²) were established in a rice field near Griffith, N.S.W., and labelled urea (80 kg N ha^–1 containing 79.25 atoms %¹⁵N) was added to the floodwater when the rice was at the panicle initiation stage. Emission of nitrous oxide and dinitrogen was measured directly during the day and overnight, using a cover collection method and gas chromatographic and mass spectrometric analytical methods. Ammonia volatilization was calculated with a bulk aerodynamic method from measurements of wind speed and floodwater pH, temperature and ammoniacal nitrogen concentration. Seven days after urea application the¹⁵N₂ content of the floodwater and soil porewater was determined and total fertilizer nitrogen loss was calculated from an isotopic balance.Throughout the experimental period gas fluxes were low; nitrous oxide, ammonia and dinitrogen flux densities were less than 5, 170 and 720 g N ha^–1 d^–1, respectively. The greatest dinitrogen flux density was observed two days after urea addition and this declined to ~ 100 g ha^–1 d^–1 after seven days.The data indicate that, of the urea nitrogen added, 0.02% was lost to the atmosphere as nitrous oxide, 0.9% was lost by ammonia volatilization, and 3.6% was lost as dinitrogen gas during the 7 days of measurement. At the end of this period 0.028% and 0.002% of the added nitrogen was retained as dinitrogen gas in the floodwater and soil porewater respectively. Recovery of the¹⁵N applied as nitrogen gases, plant uptake, and soil and floodwater constituents totaled about 94% of the nitrogen added.     

Initial and residual effects of nitrogen fertilizers on grain yield of a maize/bean intercrop grown on a Humic Nitosol and the fate and efficiency of the applied nitrogen

Initial and residual effects of nitrogen (N) fertilizers on grain yield of a maize/bean intercrop grown on a deep, well-drained Humic Nitosol (66% clay, 3% organic carbon) were evaluated. Enriched (¹⁵N) N fertilizer was used to study the fate of applied N in two seasons: using urea (banded) at 50 kg N ha^–1 in one season, and¹⁵N-enriched urea (banded), calcium ammonium nitrate (CAN, banded), and urea supergranules (USG, point placement) were applied in the other season (different field) at 100 kg N ha^–1. Nitrogen fertilizer significantly (P = 0.05) increased equivalent maize grain yield in each season of application with no significant differences between N sources, i.e., urea, CAN, and USG. Profitmaximizing rates ranged from 75 to 97 kg N ha^–1 and value: cost ratios ranged from 3.0 to 4.8. Urea gave the highest value: cost ratio in each season. Most (lowest measurement 81%) of the applied N was accounted for by analyzing the soil (to 150 cm depth) and plant material. Measurements for urea, CAN, and USG were not significantly different. The high N measurements suggest low losses of applied N fertilizer under the conditions of the study. Maize plant recovery ranged from 35 to 55%; most of this N (51–65%) was in the grain. Bean plant recovery ranged from 8 to 20%. About 34–43% of the applied N fertilizer remained in the soil, and most of it (about 70%) was within the top soil layer (0–30 cm). However, there were no significant equivalent maize grain increases in seasons following N application indicating no beneficial residual effect of the applied 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.     

Fate and efficiency of nitrogen fertilizers applied to wetland rice. II. Punjab,India

Two modified urea products (urea supergranules [USG] and sulfur-coated urea [SCU]) were compared with conventional urea and ammonium sulfate as sources of nitrogen (N), applied at 58 kg N ha^–1 and 116 kg N ha^–1, for lowland rice grown in an alkaline soil of low organic matter and light texture (Typic Ustipsamment) having a water percolation rate of 109 mm day^–1. The SCU and USG were applied at transplanting, and the whole dose of nitrogen was¹⁵N-labeled; the SCU was prepared in the laboratory and was not completely representative of commercial SCU. The SCU was broadcast and incorporated, whereas the USG was point-placed at a depth of 7–8 cm. The urea and ammonium sulfate applications were split: two-thirds was broadcast and incorporated at transplanting, and one-third was broadcast at panicle initiation. All fertilizers except the last one-third of the urea and ammonium sulfate were labeled with¹⁵N so that a fertilizer-N balance at flowering and maturity stages of the crop could be constructed and the magnitude of N loss assessed.At all harvests and N rates, rice recovered more¹⁵N from SCU than from the other sources. At maturity, the crop recovered 38 to 42% of the¹⁵N from SCU and only 23 to 31% of the¹⁵N from the conventional fertilizers, urea and ammonium sulfate, whose recovery rates were not significantly different. In contrast, less than 9% of the USG-N was utilized. Fertilizer nitrogen uptake was directly related to the yield response from the different sources. Most of the fertilizer N was taken up by the time the plants were flowering although recovery did increase up to maturity in some treatments.Analysis of the soil plus roots revealed that less than 1% of the added¹⁵N was in the mineral form. Between 20 and 30% of the¹⁵N applied as urea, SCU, and ammonium sulfate was recovered in the soil plus roots, mainly in the 0–15 cm soil layer. Only 16% of the¹⁵N applied as USG was recovered in the soil, and this¹⁵N was distributed throughout the soil profile to a depth of 70 cm, which was the lowest depth of sampling.Calculations of the¹⁵N balance showed that 46 to 50% of the urea and ammonium sulfate was unaccounted for and considered lost from the system. Only 27 to 38% of the¹⁵N applied as SCU was not recovered at maturity, but 78% of the USG application was unaccounted for. The extensive losses and poor plant recovery of USG at this site are discussed in relation to the high percolation rate, which is atypical of many ricegrowing areas.     

Response of dryland wheat to fertilizer nitrogen in relation to stored water,rainfall and residual farm yard manure

Yield response of dryland wheat to fertilizer N application in relation to components of seasonal water (available soil moisture and rainfall) and residual farm yard manure (FYM) was studied for five years (1983–84 to 1987–88) on a maize-wheat sequence on sandy loam soils in Hoshiarpur district of Punjab, India. Four rates of N viz. 0, 40, 60 and 80 kg ha^–1 in wheat were superimposed on two residual FYM treatments viz. no FYM (F₀) and 15 t ha^–1 (F₁₅) to preceding maize. FYM application to maize increased the residual NO₃-N content by 19–30 kg ha^–1 in the 180 cm soil profile. For a given moisture distribution, F₁₅ increased attainable yields. Over the years, F₁₅ increased wheat yield by 230 to 520 kg ha^–1. Response to fertilizer N was lower in FYM amended plots than in unamended plots. Available soil moisture at wheat seeding and amount and distribution of rainfall during the vegetative and the reproductive phases of crop development affected N use efficiency by wheat. Available soil moisture at seeding alone accounted for 50% variation in yield. The residual effect of FYM on wheat yield could be accounted for by considering NO₃-N in 180 cm soil profile at seeding. The NO₃-N and available soil moisture at wheat seeding along with split rainfall for two main phases of crop development and fertilizer N accounted for 96% variation in wheat yield across years and FYM treatments.     

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

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

Impact of organic residue management on nitrogen use efficiency in an annual rice cropping sequence of lowland Central Thailand

Field experiments were conducted in Central Thailand under a rice–fallow–rice cropping sequence during consecutive dry and wet seasons of 1998 to determine the impact of residue management on fertilizer nitrogen (N) use. Treatments consisted of a combination of broadcast urea (70 kg N ha^–1) with rice straw (C/N 67) and rice hull ash (C/N 76), which were incorporated into the puddled soil 1 week before transplanting at a rate of 5 Mg ha^–1. Nitrogen-15 balance data showed that the dry season rice recovered 10 to 20% of fertilizer N at maturity. Of the applied N, 27 to 36% remained in the soil. Loss of N (unaccounted for) from the soil–plant system ranged from 47 to 54% of applied N. The availability of the residue fertilizer N to a subsequent rice crop was only less than 3% of the initial applied N. During both season fallows NO₃-N remained the dominant form of mineral-N (NO₃+NH₄) in the aerobic soil. In the dry season grain yield response to N application was significant (P=0.05). Organic material sources did not significantly change grain yield and N accumulation in rice. In terms of grain yields and N uptake at maturity, there was no significant residual effect of fertilizer N on the subsequent rice crop. The combined use of organic residues with urea did not improve N use efficiency, reduced N losses nor produced higher yields compared to urea alone. These results suggested that mechanisms such as N loss through gaseous N emissions may account for the low fertilizer N use efficiency from this rice cropping system. Splitting fertilizer N application should be considered on the fertilizer N use from the organic residue amendment.     

Fate and efficiency of nitrogen fertilizers applied to wetland rice. I. The Philippines

Urea is the main form of fertilizer nitrogen applied to wetland rice. As part of an effort to evaluate the efficiency of nitrogen fertilizers, conventional urea and modified urea products such as sulfur-coated urea (SCU), urea supergranules (USG), and sulfur-coated urea supergranules (SCUSG) were compared with ammonium sulfate on an Aquic Tropudalf at the experimental farm of the International Rice Research Institute (IRRI) in the Philippines. The sulfur-coated materials were prepared in the laboratory and were not completely representative of commercial SCU. Two experiments were conducted in the wet season (1978, 1979) and one in the dry season (1979). All fertilizers were labeled with 5% or 10% excess¹⁵N so that the fertilizer-N balance at two or three sampling times during the growing season could be constructed and the magnitude of N loss assessed. The SCU, USG, and SCUSG were applied at transplanting, and the whole dose of nitrogen was¹⁵N-labeled. The urea and ammonium sulfate applications were split: two-thirds was broadcast and incorporated at transplanting, and one-third was broadcast at panicle initiation; only the initial dose was¹⁵N-labeled.Deep-point placement (10 cm) of urea supergranules (USG) between the rice hills consistently provided the highest plant recovery of¹⁵N in all experiments and at all harvest times; recoveries ranged from 48% to 75% with an average of approximately 58% at maturity. Among the fertilizers broadcast and incorporated before transplanting, average plant recoveries of¹⁵N were only approximately 34% and 26% from urea and ammonium sulfate, respectively. Plant recovery of¹⁵N from the broadcast and incorporated SCU (37%) was far inferior to that from USG. Sulfur coating of supergranules did not improve plant recovery over USG alone although sulfur coating delayed the plant uptake of¹⁵N from the USG.The¹⁵N not accounted for in the plant and soil was presumed lost. Loss of N from urea and ammonium sulfate was high (63%) in the dry season. Coating with sulfur gave a slight improvement, and deep placement of USG and SCUSG greatly reduced the losses. Losses of N were substantially lower in the wet season than in the dry season for broadcast and incorporated urea, SCU, and ammonium sulfate (9%–30%), whereas losses from deep-placed urea remained more or less the same as in the dry season. Net immobilization of¹⁵N from the broadcast fertilizers in the wet season ranged from 49% to 53% in the first experiment and from 16% to 32% in the second experiment, presumably because of aquatic weeds and green algae; immobilization was proportionally less at higher rates of fertilizer application. Deep placement reduced the extent of¹⁵N immobilization in the soil plus roots to less than 21% in all experiments.     

Urea briquette applicator for transplanted rice

A plunger-type, completely hand-operated applicator prototype, made of polyvinyl chloride (PVC), for deep placement of urea briquettes (UB), i.e., pillow-shaped urea supergranules with edges, in line transplanted rice has been developed for use by small-scale rice farmers. The field evaluation of the applicator was conducted in the Philippines during the 1989 dry season. The applicator consistently placed UB at proper depth (7 to 8 cm), which resulted in low concentrations of urea N (<7 ppm) in about 4 cm of floodwater 1 day after placement. These findings indicated that the prototype worked properly. Average work output of the applicator was 0.20 ha workday^–1 and may increase with practice. The yields of irrigated transplanted rice in the field trials show that agronomic efficiencies of hand-placed UB and applicator-placed UB were equal and were superior to those of split-applied prilled urea.     

A microplot study of the fate of15N-labelled ammonium nitrate and urea applied at two rates to ryegrass in spring

Confined microplots were used to study the fate of¹⁵N-labelled ammonium nitrate and urea when applied to ryegrass in spring at 3 lowland sites (S1, S2 and S3). Urea and differentially and doubly labelled ammonium nitrate were applied at 50 and 100 kg N ha^–1. The % utilization of the¹⁵N-labelled fertilizer was measured in 3 cuts of herbage and in soil to a depth of 15 cm (soil_0–15).Over all rates, forms and sites, the % utilization values for cuts 1, 2, 3 and soil_0–15 were 52.4, 5.3, 2.4 and 16.0% respectively. The % utilization of¹⁵N in herbage varied little as the rate of application increased but the % utilization in the soil_0–15 decreased as the rate of application increased. The total % utilization values in herbage plus soil_0–15 indicated that losses of N increased from 12 to 25 kg N ha^–1 as the rate of N application was increased from 50 to 100 kg N ha^–1.The total % utilization values in herbage plus soil_0–15 over both rates of fertilizer N application were 84.1, 80.8 and 81.0% for urea compared with 74.9, 72.5 and 74.4% for all ammonium nitrate forms at S1, S2 and S3 respectively. Within ammonium nitrate forms, the total % utilization values in herbage plus soil_0–15 over both rates and all sites were 76.7, 69.4 and 75.7% for¹⁵NH₄NO₃, NH₄ ¹⁵NO₃ and¹⁵NH₄ ¹⁵NO₃ respectively. The utilization of the nitrate moiety of ammonium nitrate was lower than the utilization of the ammonium moiety.The distribution of labelled fertilizer between herbage and soil_0–15 varied with soil type. As the total utilization of labelled fertilizer was similar at all sites the cumulative losses due to denitrification and downward movement appeared to account for approximately equal amounts of N at each site.     

Fate of broadcast urea in a flooded soil when treated with N-(n-butyl) thiophosphoric triamide,a urease inhibitor

The compound N-(n-butyl) thiophosphoric triamide (NBPT) was found to be a more effective ureas inhibitor than phenyl phosphorodiamidate (PPDA) in flooded soils when compared at concentrations of from 0.5 to 5% of the weight of urea. It allowed essentially no ammoniacal-N to acumulate in the floodwater when added at 0.5% of the weight of urea. The fate of urea was also determined in a flooded, unplanted soil with NBPT used as an inhibitor at a rate of 2% by weight of urea. At 41 days, fertilizer-N loss without the inhibitor was 73.4%, whereas with NBPT, 34.7% of the fertilizer was lost, presumably all by denitrification. With NBPT, urea hydrolysis was not inhibited below a 1 cm depth in the soil and most of the N (35.0%) accumulated as exchangeable NH ₄ ⁺ -N. Except for 15.0% of the fertilized accumulated as organic-N on the soil surface layer, immobilized N accounted for only an additional 7.0% in the soil at 22 days. Although the N saved from NH₃ volatilization loss obviously is eligible for denitrification losses, denitrification apparently was not enhanced to an appreciable extent by use of the inhibitor in that total losses were 15.7% at 22 days.     

Light fraction organic N, ammonium, nitrate and total N in a thin Black Chernozemic soil under bromegrass after 27 annual applications of different N rates

Anadequate supply of N for a crop depends among others on the amounts of N thataremineralized from the soil organic matter plus the supply of ammonium andnitrateN already present in the soil. The objective of this study was to determine thebehaviour of light fraction organic N (LFN), NH₄-N, NO₃-Nand total N (TN) in soil in response to different rates of fertilizer Napplication. The 0–5, 5–10, 10–15 and 15–30cm layers of a thin Black Chernozemic soil under bromegrass(Bromus inermis Leyss) at Crossfield, Alberta, Canada,weresampled after 27 annual applications of ammonium nitrate at rates of 0, 56,112,168, 224 and 336 kg N ha^–1. The concentration andmass of TN and LFN in the soil, and the proportion of LFN mass within the TNmass usually increased with N rates up to 224 kg Nha^–1. The increase in TN mass and LFN mass per unit ofNadded was generally maximum at 56 kg N ha^–1 anddeclined with further increases in the rate of N application. The percentchangein response to N application was much greater for the LFN mass than for the TNmass for all the N rates and all soil depths that were sampled. Mineral N intheform of NH₄-N and NO₃-N did not accumulate in the soil at 112 kg N ha^–1 rates, whereas theiraccumulation increased markedly with rates of 168 kg Nha^–1. In conclusion, long-term annual fertilization at 112 kg N ha^–1 to bromegrass resulted insubstantial increase in the TN and LFN in soil, with no accumulation ofNH₄-N and NO₃-N down the depth. The implication of thesefindings is that grasslands for hay can be managed by appropriate Nfertilization rates to increase the level of organic N in soil.     

Significance of timing and method of N fertilizer application for the N-use efficiency in flooded tropical rice

N-use efficiency in flooded tropical rice is usually low. Fertilizer N losses result mainly from losses of volatile NH₃ after broadcast application of urea into floodwater between transplanting and early tillering which is a common practice of farmers. Losses appear predominantly during the first week after urea application. With broadcast and incorporation of N into soil at transplanting losses may be reduced but are still substantial. Deep placement of urea supergranules (USG) has not been adopted by farmers because it is very laborious. A new application technique, namely injection of dissolved urea into the upper soil layer, was developed by which fertilizer N losses were effectively minimized while at the same time allowing flexible timing of application independent of crop stage and water management. It provides N-use efficiency equal to that achieved by USG point placement but is less labor-intensive.     

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.     

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

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