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.     

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.     

Movement and distribution of fertilizer nitrogen as affected by depth of placement in wetland rice

Experiments were conducted to monitor the movement and distribution of ammonium-N after placement of urea and ammonium sulfate supergranules at 5, 7.5, 10, and 15 cm. By varying depths of fertilizer placement, it is possible to determine the appropriate depth for placement machines. There were no significant differences in grain yields with nitrogen placed 5 and 15 cm deep. However, grain yields were significantly higher with deep placement of nitrogen than with split application of the fertilizer. The lower yields with split-applied nitrogen were due to higher nitrogen losses from the floodwater. The floodwater with split application had 78–98µg N ml^–1 and that with deep-placed nitrogen had a negligible nitrogen concentration.Movement of NH ₄ ⁺ -N in the soil was traced for various depths after fertilizer nitrogen application. The general movement after deep-placement of the ammonium sulfate supergranules was downward > lateral > upward from the placement site. Downward movement was prevalent in the dry season: fertilizer placed at 5–7.5 cm produced a peak of NH ₄ ⁺ -N concentration at 8–12 cm soil depth; with placement at 15 cm, the fertilizer moved to 12–20 cm soil depth. Fertilizer placed at 10 cm tended to be stable. In the wet season, deep-placed N fertilizer was fairly stable and downward movement was minimal.A substantially greater percentage of plant N was derived from¹⁵N-depleted fertilizer when deep-placed in the reduced soil layer than that applied in split doses. The percent N recovery with different placement depths, however, did not vary from each other. The results suggest that nitrogen placement at a 5-cm soil depth is adequate for high rice yields in a clayey soil with good water control. In farmers' fields where soil and water conditions are often less than ideal, however, it is desirable to place nitrogen fertilizer at greater depths and minimize NH ₄ ⁺ -N concentration in floodwater.     

Fate of Fertilizer15N applied as urea and ammonium sulphate in opium poppy (Papaver somniferum L.) grown under greenhouse conditions

Laboratory incubation and greenhouse experiments were conducted to investigate the comparative effectiveness of urea and ammonium sulphate in opium poppy (Papaver somniferum L.) using¹⁵N dilution techniques. Fertilizer treatments were control (no N), 600 mg N pot^–1 and 1200 mg N pot^–1 (12 kg oven dry soil) applied as aqueous solution of urea or ammonium sulphate. Fertilizer rates, under laboratory incubation study were similar to that under greenhouse conditions. A fertilizer¹⁵N balance sheet reveals that N recovery by plants was 28–39% with urea and 35–45% with ammonium sulphate. Total recovery of¹⁵N in soil-plant system was 77–82% in urea. The corresponding estimates for ammonium sulphate were 89–91%. Consequently the unaccounted fertilizer N was higher under urea (18–23%) as compared to that in ammonium sulphate (9–11%). The soil pH increased from 8.2 to 9.4 with urea whereas in ammonium sulphate treated soil pH decreased to 7.3 during 30 days after fertilizer application. The rate of NH₃ volatilization, measured under laboratory conditions, was higher with urea as compared to the same level of ammonium sulphate. The changes in pH of soil followed the identical trend both under laboratory and greenhouse conditions.     

Effect of phenyl phosphorodiamidate on the fate of urea applied to wetland rice fields

The effect of phenyl phosphorodiamidate (PPD) on floodwater properties, N uptake,¹⁵N recovery, and grain yield of wetland rice (Oryza sativa L.) was evaluated in a series of field studies conducted at Muñoz and Los Baños, Philippines. Prilled urea and PPD-amended urea were applied to soil and incorporated immediately prior to transplanting or applied to floodwater after transplanting. Urea was also deep-placed or added in a coated form in two studies.The addition of PPD with urea retarded urea hydrolysis by 1–3 days, depending on the time and method of application. Significant reductions in the concentration of ammoniacal-N in floodwater resulted when PPD-amended urea was applied between 18 and 26 days after transplanting (DT). In contrast, PPD did not appreciably affect the concentration of ammoniacal-N in floodwater when applied with urea either immediately before or after transplanting of the seedlings.Plant N uptake and grain yield were not significantly affected by the addition of PPD with urea in three of the four experiments conducted, even though PPD substantially reduced the concentration of ammoniacal-N in the floodwater in several treatments in these studies. The¹⁵N balance studies conducted at both field locations showed PPD to increase total¹⁵N recovery by between 10% and 14% of the¹⁵N applied, 14 days after the application of urea. No further loss of¹⁵N occurred between the initial sampling (40 DT) and grain harvest at Los Baños. An increase in¹⁵N recovery occurred at grain harvest at Muñoz because¹⁵N-labeled urea was applied at 50 DT in the study. PPD increased the amount of¹⁵N in the plant and nonexchangeable soil N fraction at all harvests at Los Baños. In contrast, at Muñoz, PPD increased the quantity of¹⁵N in the KCL-extractable pool 14 days after urea was applied. Reasons for the discrepancies in results between experiments and the overall failure of PPD to increase grain yield are discussed.     

Fate of nitrogen applied in different fertilizers to the surface of a calcareous soil in Syria

¹⁵N-labelled ammonium sulphate or¹⁵N-labelled urea were each applied in solutionat a rate of 30 kg N ha^-1 to the surface of 20soil cores (52 mm internal diameter × 100 mm deep)located on a field experiment at the ICARDA station,Tel Hadya, Syria. Recovery of ¹⁵N-label in theammonium, nitrate, organic and/or urea-N pools in thesoil was measured on days 0, 1, 2, 5 and 13 afterapplication. Total recovery of ¹⁵N was initially100%, but by day 13 after application it had declinedto 51% with urea and 73% with ammonium sulphate.Ammonium nitrate labelled either as ammonium or asnitrate was also applied to the soil surface of 8other cores at the same time. ¹⁵N recovery in thefour soil N pools was measured only on day 12 afterapplication. Total recovery of ¹⁵N-label was 75%with labelled ammonium and 57% with labelled nitrate.Volatilization of ammonia from this calcareous soil(pH 8.1) is one probable mechanism of N loss fromammonium and urea fertilizers: with nitrate bothleaching beyond the base of the core (i.e. 100 mm) and denitrification were responsible for Nlosses. These large losses of N immediately afterapplication have implications for fertilizermanagement practices.     

Fate of nitrogen fertilizer applied to lowland rice on a Sulfic Tropaquept

A field experiment was conducted on an acid sulfate soil in Thailand to determine the effect of N fertilization practices on the fate of fertilizer-N and yield of lowland rice (Oryza sativa L.). A delayed broadcast application of ammonium phosphate sulfate (16-20-0) or urea was compared with basal incorporation of urea, deep placement of urea as urea supergranules (USG), and amendment of urea with a urease inhibitor. Deep placement of urea as USG significantly reduced floodwater urea- and ammoniacal-N concentrations following N application but did not reduce N loss, as determined from an¹⁵N balance, in this experiment where runoff loss was prevented. The urease inhibitor, phenyl phosphorodiamidate (PPD), had little effect on floodwater urea- and ammoniacal-N, and it did not reduce N loss. The floodwater pH never exceeded 4.5 in the 7 days following the first N applications, and application of 16-20-0 reduced floodwater pH by 0.1 to 0.3 units below the no-N control. The low floodwater pH indicated that ammonia volatilization was unimportant for all the N fertilization practices. Floodwater ammoniacal-N concentrations following application of urea or 16-20-0 were greater on this Sulfic Tropaquept than on an Andaqueptic Haplaquoll with near neutral pH and alkaline floodwater. The prolonged, high floodwater N concentrations on this Sulfic Tropaquept suggested that runoff loss of applied N might be a potentially serious problem when heavy rainfall or poor water control follow N fertilization. The unaccounted-for¹⁵N in the¹⁵N balances, which presumably represented gaseous N losses, ranged from 20 to 26% of the applied N and was unaffected by urea fertilization practice. Grain yield and N uptake were significantly increased with applied N, but grain yield was not significantly affected by urea fertilization practice. Yield was significantly lower (P = 0.05) for 16-20-0 than for urea; however, this difference in yield might be due to later application of P and hence delayed availability of P in the 16-20-0 treatment.     

Influence of source, time and method of application, and simulated rainfall on recovery of nitrogen fertilizers applied to bromegrass

Four field experiments were conducted in central Alberta to determine influence of the N source, time and method of application and simulated rainfall on the recovery of¹⁵N-labelled fertilizers applied to meadow bromegrass (Bromus biebersteinii Roem and Shultz. cv. Regar) in plants (topgrowth plus roots) and in soil. The first experiment compared two N sources (urea and ammonium nitrate (A.N.)) and six times of application (early fall, late fall, early winter, early spring, late spring and spring-summer split) where N fertilizers were surface-broadcast. Urea gave lower N recovery than A.N., regardless of time of application (on the average by 16.4% in plants and by 18.3% in plants plus soil). For urea, early spring application gave higher N recovery than the other times of application, especially at the Eckville site. For A.N., spring applications gave higher N recovery than fall or early winter applications but N recovery was only slightly greater with early spring than late spring application. The second experiment evaluated methods of N placement (surface-broadcasting and banding). The N recovery in plants increased with subsurface band placement over surface-broadcast by 20.2% for fall application and by 15.5% for spring application. The other two experiments investigated the effect of amount (0, 5, 10, 20 and 40 mm) of simulated rainfall and interval (0, 1, 2, 4, 8, and 16 d) between surface urea application and simulated rainfall on N recovery. Simulated rainfall of 10 mm immediately after surface urea application on moist soil increased the N recovery in plants by 8.1–10.7% compared to no simulated rainfall. Delaying simulated rainfall (20 mm) by 4 d after surface urea application decreased the N recovery in plants by 8.7–15.2%. In conclusion, the N recovery improved greatly when urea was placed below the soil surface or with simulated rainfall immediately after surface urea application.     

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.     

Effect of lime nitrogen on the efficiency of urea and other ammonium nitrogen fertilizers

Five pot experiments were conducted with wheat and rice in a net house to study the effect of lime nitrogen (LN, contains about 55% calcium cyanamide) amendment rates on the efficiency of urea, the recovery urea-¹⁵N, the efficiency of the three nitrogen fertilizers(NF), on the efficiency of urea in the three soils, and on NO ₃ ^- -N leaching from a flooded soil. A rate of LN-N of 5–8% of applied fertilizer N increased the recovery of labeled urea-N by 9.42%. The effect of LN on the efficiency of NF was urea > ammonium sulfate > ammonium chloride. Under flooded conditions, LN decreased NO ₃ ^- formation and leaching.Responses of several crops to LN amended fertilizers were also studied in field experiments. At equal NPK applications, the efficiency of basal applications to rice, wheat, corn, potatoes, soybean, peanut, grapes, peaches, melon and watermelon were bette r with LN than without. Efficiency with a basal fertilizer for rice or wheat with LN were the same as with the same fertilizer without LN applied in split applications.     

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.     

Effect of the timing and rate of nitrogen fertilization on the growth and recovery of fertilizer nitrogen within the potato (Solanum tuberosum L.) crop

The effect of the timing of N fertilizer application on the uptake and partitioning of N within the crop and the yield of tubers has been studied in two experiments. In 1985 either none, 8 or 12 g N m^–2 was applied and in 1986 none, 12 or 18 g N m^–2. Fertilizer N was applied either at planting, around the time of tuber initiation or half at planting and the remainder in four foliar sprays of urea during tuber bulking.¹⁵N-labelled fertilizer was applied to measure the recovery of fertilizer N in the crops.There was an apparent pre-emergence loss of nitrate from the soil when N was applied at planting in 1986, thereby reducing the efficiency of fertilizer use. Applying the N at tuber initiation delayed and reduced the accumulation of N in the canopy compared with crops receiving all their fertilizer at planting. Foliar sprays of urea slightly increased both tuber yields and tuber N contents when compared to a single application at planting. The proportion of the fertilizer N recovered in the crop was little affected by the rate of N application, but a greater proportion of foliar-applied N was recovered than N broadcast at planting, due partly to pre-emergence losses of nitrate in 1986. It is suggested that late applications of N was foliar sprays can be of benefit to crops with a long growing season and reduce environmental losses of N.     

Fate of nitrogen (15N) from oxamide and urea applied to turf grass: A lysimeter study

Slow release N fertilizers are receiving increasing attention for use on turf grass, but their fate in the plant-soil system is still poorly understood. We aimed to quantify the uptake and recovery of N by a mixture of grasses when applied as either urea or oxamide in different diameter granules using a tracer technique (¹⁵N). The effects of the N source on soil biomass, root density and amount of readily available organic C in soil were also evaluated.In a first experiment oxamide in 4–5 mm diameter granules was compared with urea. The initial N absorption, 40 days after fertilization (d.a.f.), was higher for urea (23.5%) than for oxamide (12.1%), but after 64 days absorption efficiencies were about the same (11%) for both fertilizers. Fertilizer-derived N lost by leaching was much greater from the urea-fertilized soil (1.57 g), compared with losses from oxamide-fertilized soil (0.05 g). The total residual fertilizer N remaining in the system at the end of the experiment was 26.7% of applied urea N and 39.6% of applied oxamide N. Cumulated absorption efficiencies, calculated after dismantling the lysimeters, were 43.1% for urea and 54.8% for oxamide (roots included). A priming effect caused by a larger uptake of soil N because of the better root development was found in the oxamide-treated lysimeter. Fertilization with oxamide also caused an increase in the amount of soil microbial biomass.In a second experiment, the efficiencies and fertilizer N uptake rates from oxamide applied at two different granule sizes (1–2 mm and 5–10 mm) were evaluated. The amount of soil N taken up by the grass was linearly related to root density (r = 0.92).     

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.     

Turnover of15N ammonium sulfate with dicyandiamide under aerobic and anaerobic soil conditions

The influence of the nitrification inhibitor dicyandiamide (DCD) on the turnover of¹⁵N-labelled ammonium sulfate (AS) was investigated in two soils under aerobic and waterlogged conditions. Nitrification of ammonium sulfate was markedly inhibited by addition of DCD in both soils. Up to 45% of the supplied N was transformed into a non-extractable N form, which only slowly released nitrogen over 147 or 264 days. This immobilization was higher in the presence of DCD than without DCD. In all aerobic experiments, the recovery was 100% ± max. 2.4%, indicating that no gaseous losses of N occurred.If aerobic preincubation of 28 or 42 days was followed by water-logging with H₂O or a solution of glucose, considerable N losses occurred only in presence of the carbohydrate. DCD retarded nitrification and thus reduced losses by denitrification from 61 to 15%.DCD application resulted in an increased immobilization of labelled N into the non-exchangeable soil N fraction. This amounted to more than 50% of the applied N, compared to 39% without DCD.The late Dr. Klaus Vilsmeier, a very dedicated and talented young scientist, died before he was able to finish completely the revised version of this article. We will always keep him in our minds and kindly remember his kind personality as well as his sense of humour and justice. Prof. Dr. Heiner Goldbach on behalf of all members of the department.     

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.     

Nitrogen-15 and sulfur-35 balances for fertilizers applied to transplanted rainfed lowland rice

A field experiment was conducted on a poorly-drained Aeric Paleaquult in northeastern Thailand to determine the effect of N and S fertilizers on yield of rainfed lowland rice (Oryza sativa L.) and to determine the fate of applied¹⁵N- and³⁵S-labeled fertilizers. Rice yield and N uptake increased with applied N but not with applied S in either sulfate or elemental S (ES) form. Rice yield was statistically greater for deep placement of urea as urea supergranules (USG) than for all other N fertilizer treatments that included prilled urea (PU), urea amended with a urease inhibitor (phenyl phosphorodiamidate), and ammonium phosphate sulfate (16% N, 8.6% P).The applied¹⁵N-labeled urea (37 kg N ha^–1) not recovered in the soil/plant system at crop maturity was 85% for basal incorporation, 53% for broadcast at 12 days after transplanting (DT), 27% for broadcast at 5–7 days before panicle initiation (DBPI), and 49% for broadcast at panicle initiation (PI). The basal incorporated S (30 kg ha^–1) not recovered in the soil/plant system at crop maturity was 37% for sulfate applied as single superphosphate (SSP) and 34% for ES applied as granulated triple superphosphate fortified with S (S/GTSP). Some basal incorporated¹⁵N and³⁵S and some broadcast¹⁵N at PI was lost by runoff. Heavy rainfall at 3–4 days after basal N incorporation and at 1 day after PI resulted in water flow from rice fields at higher elevation and total inundation of the 0.15-m-high¹⁵N and³⁵S microplot borders. Unrecovered¹⁵N was only 14% for 75 kg urea-N ha^–1 deep placed as USG at transplanting. This low N loss from USG indicated that leaching was not a major N loss mechanism and that deep placement was relatively effective in preventing runoff loss.In order to assess the susceptibility of fertilizer-S to runoff loss, a subsequent field experiment was conducted to monitor³⁵S activity in floodwater for 42 days after basal incorporation of SSP and S/GTSP. Maximum³⁵S recoveries in the floodwater were 19% for SSP after 7 days and 7% for S/GTSP after 1 day. Recovery of³⁵S in floodwater after 14 days was 12% for SSP and 3% for S/GTSP.This research suggests that on poorly drained soils with a low sorption capacity, a sizeable fraction of the fertilizer S and N remains in the floodwater following application. Runoff could then be an important mechanism of nutrient loss in areas with high probability for inundation following intense rainfall.     

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.     

1. The chemistry and biology of flooded soils in relation to the nitrogen economy in rice fields

Response of lowland rice to sources and methods of nitrogen fertilizer application were summarized for more than 100 experiments. In about 2/3 of the experiments, the yield increase per kg of fertilizer N was judged to be relatively poor with best split applications of urea. Based on frequency distribution, sulfur coated urea and urea briquets or urea supergranules deep placed more often produced satisfactory yield increases than best split urea, but even with these sources/methods the yield increases were judged to be relatively poor in about 1/2 of the experiments. There is an enormous potential to increase rice production with no further increases in inputs of fertilizer N if we could learn to match the best method/source of fertilizer with the soil-crop management complex.About 60% of the yields with no fertilizer N were in the range of 2 to 4 t/ha. Based on the average yield response to urea, this is equivalent to about 100 kg of urea N. It would appear worthwhile to study ways to improve utilization of soil nitrogen since it is already in place on the land and apparently in fairly abundant amounts in many soils.About 50 experiments with¹⁵N fertilizers were summarized. In almost all cases, the uptake of tagged fertilizer was less than the net increase in N in the above ground matter. In about 2/3 of the experiments, the addition of fertilizer N increased soil N uptake more than 20% and in 1/3 of the experiments the uptake of soil N was increased more than 40%. These results lead to much uncertainty about practical interpretation and use of¹⁵N data.     

Management of nitrogen by fertigation of potato in Lebanon

Reports on soil and groundwater contamination with nitrates in Lebanon and other developing countries could be related to the mismanagement of water and fertilizer inputs. The objective of this work was to determine the N requirements and N-use efficiency of a main-crop potato in Lebanon, irrigated by a drip system, compared to the farmer's practice of macro-sprinkler. In the drip irrigation, fertilizers input was as soil application at the time of sowing or added continuously with the irrigation water (fertigation). Nitrogen-fertilizer recovery was determined using ¹⁵N-labeled ammonium sulfate. Fertigation with continuous N feeding based on actual N demands and available sources allowed for 55% N recovery. For a total N uptake of 197 kg ha^–1 per season in the lower N rate, the crop removed 66 kg N ha^–1 from fertilizers. The spring potato crop in this treatment covered 44.8% of its N need from the soil and 21.8% from irrigation water. Higher N input increased not only N derived from fertilizers, but also residual soil N. Buildup of N in the soil with the traditional potato fertilization practice reached 200 kg N ha^–1 per season. With increasing indications of deteriorating groundwater quality, we monitored the nitrate leaching in these two watering regimes using soil solution extractors (tensionics). Nitrate leaching increased significantly with the macro-sprinkler technique. But N remained within the root zone with the drip irrigation. The crop response to applied N requires a revision of the current fertilizer recommendation in semi-arid regions, with an improved management of fertilizer and water inputs using fertigation to enhance N recovery.