Deep Placement Of Urea Supergranules Research Articles

Modelling the diffusion and exchange of ammoniacal nitrogen following deep placement of urea supergranules in wetland rice cultivation

Deep placement of urea in the form of supergranules is often recommended in transplanted rice cultivation as a means of avoiding nitrogen loss in the environment. As ammoniacal nitrogen produced by urea hydrolysis just after deep application is chemically stable in the anaerobic layer of the paddy soil, it is relevant to study the diffusion–exchange phenomenon affecting this ammoniacal-N. More precisely, numerical calculus was called on to solve the nonlinear diffusion–exchange equation, admitting spherical symmetry in this context. The adsorption isotherm for ammoniacal-N intervening in this equation was a Freundlich isotherm. Experiments performed in the laboratory (untransplanted situation) and in the field (transplanted situation) enabled the monitoring of the time course of ammoniacal-N concentration in the central position. The tortuosity factor was evaluated at 0.80 in the paddy soil. In the field, the diffusion–exchange phase was clearly separated from the ammonia assimilation phase. The limit between the two was evaluated in six replicates. The first-order mechanism of ammonia disappearance is interpreted as the indication of a tight physiological regulation at the carbon–nitrogen interface acting within roots pushed to their limit in growth and assimilative capacity. This limit is here reported at 12 mM ammonia, attained 39 d after point application. This delay is enhanced when the tortuosity factor is reduced. Similar modelling in the context of cylindrical symmetry can be an option to study new urea fertilizers in the form of urea rods.

Agronomic effectiveness of urea deep placement technology for upland maize production

Effective fertilizer management is critical for sustainable maize production. Field trials were conducted in six locations in northern Ghana during the 2016 and 2017 farming seasons to evaluate the agronomic effectiveness and economic viability of urea deep placement technology (UDP), which involves deep placement of urea supergranules (USG), for maize production. This fertilization strategy was compared with four other maize fertilization strategies: (1) microdosing (MD); (2) farmer practice (FP), which involves surface broadcasting of granular fertilizer; (3) modified farmer practice (MFP), where granular fertilizer was incorporated into the soil; and (4) NPK-only, where no supplemental N was added. Averaged across all six locations and both years, UDP produced 18% more grain yield than MFP, 38% more than MD, 155% more than FP, and 270% more than NPK-only. The greatest apparent N recovery efficiency of 73% occurred in MD, followed by UDP (68%), NPK-only (63%), MFP (61%) and FP (35%), in that order. However, agronomic efficiency followed the order: UDP > MFP = MD > NPK-only = FP. The greatest gross profit margin of 49% was obtained with UDP, followed by MFP (41%) and MD (33%). Farmer practice resulted in a break-even level (3%) whereas NPK-only resulted in a loss (< − 35%). From the combined results, we conclude that, barring other external factors such as drought, flooding, pests, diseases, etc., UDP could be one of the most efficient fertilizer management strategies for sustainable maize production. Further studies should evaluate optimum timing of USG application for increased productivity and profitability.

Theoretical aspects on local alkalinization following hydrolysis of deep-placed urea in paddy soils under transplanting

Urea broadcasting in rice cultivation often leads to ammonia volatilization because the hydrolysis reaction of urea quickly gives ammonium, ammonia and bicarbonate. Particularly ammonia (NH3) is subject to emanation which can be avoided if urea is deep placed. This no loss optimistic scenario is realized by strict positioning of urea supergranules in the anaerobic layer of the paddy soil. In this study, we wanted to verify if the development of alkalinity in the site of placement could be predicted from the chemical equilibria in soil solution, neglecting the effect(s) of the soil matrix. Calculation of pH related to ammoniacal N concentration was performed considering the main acid-basic equilibria taking place within the soil solution. Data were compared to localized pH and ammoniacal N values obtained in two paddy fields following deep placement of urea supergranules. The pH never trespassed 9.25 which is the pKa of ammonia, in theoretical terms as well in the localized point placement sites in the fields. The linear relationship between log of ammoniacal N concentration and pH seems to indicate a strong influence of the cation exchange capacity so far not taken into account by the model. Urea supergranules are compared to mini-bombs of NH3; to prevent any leak to the floodwater (then to the atmosphere) they have to be buried with precision.© 2019 International Formulae Group. All rights reserved.Keywords: Point placement, urea supergranule, generated alkalinity, wetland soil, soil solution, pH

Study of nitrogen use efficiency and yield of rice influenced by deep placement of nitrogen fertilizers

Continuous increase of nitrogen fertilizer use adversely affecting the soil health as well as the environment. It necessitates the systematic study of the impact of nitrogen (N) fertilizer use-reduction on crop yield. In this article, the effects of deep placement of nitrogen fertilizers on nitrogen use efficiency (NUE) and yield of rice variety “BRRI dhan46” were investigated and compared for three N formulations: urea super granule (USG), NPK briquettes and prilled urea (PU). The experiment was conducted in randomized complete block design (RCBD) with eight N treatments replicated three times. The treatments were applied by varying doses (0, 52, 78 and 104 kg ha-1) of nitrogen fertilizers in a known identical soil and weather condition. Applications of PU, USG and NPK briquettes exhibited significant differences on yield attributes of rice. Results revealed maximum grain yield of 6.391 t ha-1 (54% increases over control) for the dose USG 104 kg ha-1 which is statistically similar to the dose USG 78 kg ha-1 and NPK briquettes 104 kg ha-1. The deep placement of USG and NPK briquettes enhanced both the recovery of applied N and N use efficiency compared to broadcast application of prilled urea. However, USG 78 kg ha-1 showed maximum apparent N recovery, N use efficiency, gross margin and marginal benefit-cost ratio. Thus, an application of 78 kg ha-1 USG can be recommended for profitable cultivation of rice variety BRRI dhan46.&#x0D; SAARC J. Agri., 17(1): 93-103 (2019)

Nitrogen use efficiency and yield of BRRI dhan49 as influenced by different forms of N fertilizers under AWD condition

An experiment was conducted at the Soil Science Field Laboratory of Bangladesh Agricultural University, Mymensingh during Aman season of 2016 to evaluate the effects of deep placement of N fertilizers in the form of urea super granule (USG) and NPK briquettes in comparison with prilled urea (PU) on nitrogen use efficiency and yield of BRRI dhan49. The soil was silt loam in texture having pH 6.27, organic matter content 1.95% and total N 0.136%. The experiment was laid out in a Randomized Complete Block Design (RCBD) with eight treatments and three replications. The treatments include T1 (Control), T2 (PU, 104 kg N ha-1), T3 (USG, 104 kg N ha- 1), T4 (USG 78 kg N ha-1), T5 (USG, 52 kg N ha-1), T6 (NPK briquettes, 104 kg N ha-1), T7 (NPK briquettes, 78 kg N ha-1) and T8 (NPK briquettes, 52 kg N ha-1). All the treatments except T6, T7 and T8 received 16 kg P and 42 kg K ha-1 from TSP and MoP, respectively. In T6, T7 and T8 treatments, P and K were supplied from NPK briquettes. The PU was applied in three equal splits. The USG and NPK briquettes were deep placed 10 DAT and the briquettes were placed at 8-10 cm depth between four hills at alternate rows. Application of PU, USG and NPK briquette under alternate wetting and drying (AWD) condition exerted significant influence on yield attributes as well as grain and straw yields of BRRI dhan49. The maximum grain yield of 6311 kg ha-1 (100% increases over control) and straw yield of 6956 kg ha-1 was recorded in T3 (USG, 104 kg N ha-1). The second highest grain yield of 5865 kg ha-1 produced by T2 (PU, 104 kg N ha-1) was statistically similar with T4 (USG 78 kg N ha-1) and T6 (NPK briquettes, 104 kg N ha-1). The lowest grain yield (3155 kg ha-1) and straw yield (3908 kg ha-1) were recorded in T1 (Control). The deep placement of USG and NPK briquettes enhanced both the recovery of applied N and N use efficiency in comparison with broadcast application of prilled urea. The T5 (USG, 52 kg N ha-1) showed maximum apparent N recovery, and N use efficiency. Next to T5, the T4 depicted the position in respect of N recovery and N use efficiency. So the application of 78 kg N ha-1 in the form of USG can be recommended for the production of BRRI dhan49.&#x0D; Res. Agric., Livest. Fish.6(1): 27-33, April 2019

Response of Some Landrace Rice (&lt;i&gt;Oryza sativa&lt;/i&gt; L.) Cultivars to Applied Nitrogen in Lowland Ecosystem

An experiment was conducted to determine the effects of nitrogen (N) fertilization on growth and yield of some landrace rice (Oryza sativa L.) cultivars in lowland ecosystem. Three popular landrace rice cultivars: Rajashail, Kutiagni and Sadamota were treated with five times of N application viz. (i) two split applications of prilled urea (PU) at 10 days after transplanting (DAT) and before panicle initiation (PI) stage, (ii) deep placement of urea super granule (DPUSG) at 10 DAT, (iii) DPUSG before PI stage, (iv) PU application before PI stage and (v) control. The results showed that the landrace rice cultivar Kutiagni had the highest number of tillers under PU, while Rajashail gave the highest number of tillers under DPUSG applied before PI stage. Further, Kutiagni produced higher amount of dry matter under two splits of PU, while Rajashail produced higher amount of dry matter under DPUSG applied before PI stage. However, application of PU before PI stage significantly increased leaf area index in all the three landraces. The varieties responded to applied N and produced the highest yield in Kutiagni. The DPUSG at 10 DAT increased straw yield but failed to increase grain yield even over control. The DPUSG before PI stage significantly increased rice yield and economic return. The yield of rice in DPUSG applied before PI stage was comparable to two splits of PU and top dressing of PU before PI stage. Compared to control (2.93 t ha-1), yield increase was 26% in Kutiagni (3.70 t ha-1) under DPUSG before PI stage. These results suggest that Kutiagni was more responsive to added N and this cultivar could be cultivated with DPUSG at PI stage.The Agriculturists 2018; 16(1) 01-12

A comparative appraisal of urea super granule application in raised bed and prilled urea application in conventional planting for transplanted boro rice (Oryza sativa)

Utilization of urea super granule (USG) with raised bed cultivation system for transplanted boro (winter, irrigated) rice production is a major concern now days. A field experiment was conducted in the chuadanga district of Bangladesh to compare the two cultivation methods: deep placement of USG on raised bed with boro rice, and prilled urea (PU) broadcasting in conventional planting. Results showed that USG in raised bed planting increased grain yields of transplanted boro rice by up to 18.18% over PU in conventional planting. Deep placement of USG in raised bed planting increased the number of panicle m-2, number of grains panicle-1 and 1000-grains weight of boro rice than the PU in conventional planting. Better plant growth was observed by deep placement of USG in raised bed planting compared to PU in conventional planting. Sterility percentage and weed infestation were lower on USG in raised bed planting compared to the PU in conventional planting methods. Forty seven percent irrigation water and application time could be saved by USG in raised bed planting than PU in conventional planting. Deep placement of USG in bed saved N fertilizer consumption over conventional planting. Water use efficiency for grain and biomass production was higher with deep placement of USG in bed planting than the PU broadcasting in conventional planting methods. Similarly, agronomic efficiency of N fertilizer by USG in bed planting was significantly higher than the PU broadcasting in conventional planting. This study concluded that deep placement of USG in raised bed planting for transplanted boro rice is a new approach to achieve fertilizer and water use efficiency as well as higher yield and less water input compared to existing agronomic practices in Bangladesh.

Is the source–sink relationship in transplanted rice receiving deep-placed urea supergranules dependent upon the geometry of transplanting?

Deep placement of urea supergranules in wetland rice (Oryza sativa L.) or correct urea band application enables to protect nitrogen (N) from various loss mechanisms, but recovering of fertilizer N by plants depends upon geometric and agronomic factors. The objective of this study was to characterize the diffusion of ammoniacal N from the two urea sources, point or line application, in a typical paddy soil. A model of ammonia diffusion was developed for the two geometries. The relation between the N application rate and the transplanting geometry was studied in two fields using probes attached to urea supergranule of different mass (2 and 4 g). The transplanting pattern was adapted for obtaining 58 or 116 kg N ha−1 in the 4 g application. The ammoniacal nitrogen concentration was compared to the diffusion model prediction. The values of the diffusion coefficient were found to be 1.160 and 1.107 cm2 d−1. Ammonia disappearance below 10 mmol L−1 did not follow the same kinetics in the two treatments corresponding to 4 g application. Relative to the 2 g treatment, root ammonia uptake in the 4 g treatment was delayed and slowed in the classical geometry of 20 cm × 20 cm (116 kg N ha−1) when it was mainly delayed in the geometry provided with 58 kg N ha−1. This manipulation of the source–sink relationship enables to foresee possibilities for the development of new fertilizers adapted to wetland rice cultivation.

Effects of PU, USG and NPK briquette on nitrogen use efficiency and yield of BR22 rice under reduced water condition

An experiment was conducted at the Soil Science Field Laboratory of Bangladesh Agricultural University, Mymensingh during the aman season of 2012 to investigate the effects of prilled urea (PU), urea super granule (USG) and NPK briquette on NH4- N concentration in field water, yield and nitrogen (N) use efficiency (NUE) of BR22 rice under reduced water conditions. The experiment was laid out in a randomized complete block design with three replications. There were altogether eight treatment combinations viz. T1: Control (No N fertilizer), T2: 52 kg N ha-1 from USG, T3: 104 kg N ha-1 from USG, T4: 78 kg N ha-1 from PU, T5: 120 kg N ha-1 from PU, T6: 51 kg N ha-1 from NPK briquette, T7: 78 kg N ha-1 from USG and T8: 78 kg N ha-1 from NPK briquette. Water samples were collected from rice field for seven consecutive days after deep placement of USG and the first split application of PU and the samples were analyzed for NH4-N. The Highest concentration of NH4-N in water was observed at the second day of PU application followed by gradual decrease with time. The yield contributing characters like plant height, panicle length, number of effective tillers hill-1 and grains panicle-1 were significantly influenced by different treatments. The highest grain yield of 3.93 t ha-1 was recorded from 104 kg N ha-1 as USG (T3) and the lowest value of 2.12 t ha-1 was obtained from control. The N use efficiency was increased when the N was applied as USG. The overall results revealed that application of USG and NPK briquette may be practised for obtaining better yields in addition to increasing the efficiency of N fertilizer. DOI: http://dx.doi.org/10.3329/jbau.v11i2.19897 J. Bangladesh Agril. Univ. 11(2): 215-220, 2013

The kinetics of ammonia disappearance from deep-placed urea supergranules (USG) in transplanted rice: the effects of split USG application and PK fertiliser

Deep placement of urea supergranules (USG) in transplanted rice cultivation can protect nitrogen from losses. In the 20 × 20 cm plant geometry, the 4 g urea application corresponds to 116 kg N ha−1. However, the ammoniacal nitrogen concentration at the placement sites increases as the mass of the USG increases, and with the 4 g USG it takes too long for the ammonia concentration to decrease to 10 mM, the concentration at which root uptake commences. Therefore, split applications (two applications of 2 g USG) were studied regarding the change of ammoniacal nitrogen concentration at the point of application. The first application was always at transplanting, at a depth of 10 cm; the second application was made 41 days after transplanting (DAT) (tillering stage) or 62 DAT (panicle initiation). These times correspond to the beginning or the end of ammonia disappearance (plant uptake) from the site of placement, respectively. Ammonia disappearance from the placement site is faster for the second application, and it appears that the rice roots took up ammonia at a higher concentration: 20 mM for the second application versus 10 mM for the first application. The kinetics of ammonia disappearance from a single application of 2 g granules at transplanting was also influenced by the PK application rate. The rate of ammonia disappearance, the number of tillers per hill, and grain yield all increased with PK rate. The constant which characterizes the first order rate of ammonia disappearance below 10 mM also increased with PK level, proportionally to the number of tillers.

Nitrogen deep-placement technologies for productivity, profitability, and environmental quality of rainfed lowland rice systems

The recovery of applied nitrogen by rice is low due to several loss processes operating in the ricefields. Split application of fertilizer suggested for increasing nitrogen-use efficiency is often not practical in rainfed lowland rice due to adverse soil–water situations. Hence, the entire required amount of N has to be applied in one single application when the water regime is favorable. A single broadcast application, however, increases N loss. Deep placement of urea supergranules (USG) has been proven to improve N fertilizer efficiency. The placement technology is best suited to conditions where the predominant N loss mechanism is ammonia volatilization rather than leaching or denitrification. Deep placement of USG thus has greater benefit over surface split application on soils with moderate to heavy texture, low permeability and percolation rate, and high cation exchange capacity and pH. Environments and management factors conducive to high ammonia volatilization potential would benefit most from deep-placement technology. Improved N recovery and efficiency of USG has been well-documented for lowland rice, but its market availability and methods to achieve placement pose problems. The technology has very limited adoption because USG is not commercially available or manufactured in most countries, and labor requirement is high with hand placement. Manual application creates more difficulties in handling the granules, besides taking 36–42 more hours per hectare, than 2 split broadcast applications of prilled urea. Applicators developed so far have not worked satisfactorily under standing water conditions and in direct-seeded rice conditions due to hardness of the soil. Hence, it is necessary to develop a suitable applicator to overcome these difficulties. Alternatively, for direct-seeded rice, N-fertilizers can be subsoil-banded near seedrows. The placement technology, if adopted by the farmers of the potential lowland areas in eastern India, is expected to give an additional production of 5.6 million tons of rice.

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.

Effects of nitrogen fertilizer and pesticide management on floodwater ecology in a wetland ricefield

This paper is the first in a series reporting a study on the effects of N fertilizer, green manure, and pesticide applications on the floodwater ecology of a tropical ricefield. Algal, zooplankton, and mollusc popu- lations were monitored during a crop cycle in plots sub- jected to various methods of N management and differ- ent pesticide regimes. This paper presents the experimen- tal design, general features of the statistical methods for data analysis, and the dynamics of algal growth in the floodwater. Transient green algal blooms and increases in floodwater dissolved O2 concentrations were observed after the broadcast application of N fertilizer 7 days after transplanting. These effects were less pronounced after the second N application 55 days after transplanting. Photosynthetic activity in the floodwater of planted plots decreased as the crop season progressed. Mucilaginous colonial blue-green algae proliferated in the zero-N con- trol and unplanted plots, but were inhibited by the broad- cast N fertilizer and green manure incorporation. Deep placement of urea supergranules reduced the negative im- pact of N fertilizer on blue-green algal populations. There was limited evidence that the application of pesticides promoted the development of blue-green algal popula- tions in the absence of readily available N.

Nitrogen management and fertilizer use efficiency for lowland rice in Pakistan

Abstract With a view to develop simpler techniques for improving the efficiency of nitrogen fertilizer use for lowland rice, field studies were conducted in 1986 and 1987. During 1986, dry soil incorporation of urea was compared with puddled soil application of urea and calcium ammonium nitrate (CAN) and deep placement of urea super granule (USG). In 1987, the application of urea in puddled soil without standing water (soil saturation) was compared with the application in standing water and deep placement of USG. The results revealed that deep placement of USG and dry soil incorporation of urea were comparable in their effect on the increase of the number of productive tillers, straw and paddy yield, and agronomic efficiency. Consequently N utilization and recovery efficiency were higher with these methods of fertilizer application than with the application of urea and CAN in puddled soil. Application of urea fertilizer under soil saturation conditions and deep placement of USG were found to be comparable in the production of tillers, straw and grain yield and both gave similar agronomic and N recovery efficiencies to the application of urea on the Miranpur soil under standing water conditions.

Deep placement of urea supergranules in transplanted rice: Principles and practices

Rice is the most important food crop in the developing countries of Asia, where population densities are very high and overall dietary levels are not adequate. In south and southeast Asia, rainfed and irrigated transplanted rice occupies nearly two–thirds of the rice-growing area and produces more than 80% of the paddy rice. In these areas, prilled urea (PU) conventionally applied by farmers is very inefficiently used by transplanted rice largely because of serious losses (up to 60% of applied N) via NH3 volatilization, denitrification, leaching, and/or runoff. In order to minimize N loss, especially loss due to denitrification, historically the Japanese have used different ways of deep placing fertilizer N. In 1975, IFDC proposed use of supergranules of urea (USG) in place of mudballs containing urea fertilizer to achieve the same agronomic benefits as achieved through the Japanese concept of deeppoint placement of fertilizer N in transplanted rice.

Fertilizer–use efficiency of amended urea materials in flooded rice

summaryIn a field experiment on a typic hapludoll in 1983 and 1984, deep placement of urea supergranules at 40 and 80 kg N/ha proved to be the best N source, of five tested, for grain production, but at 120 kg N/ha it was similar to neem-cake-coated urea. The results showed that deep placement of urea supergranules can save fertilizer use by 60% compared with prilled urea to obtain the same yield. Shellac-coated urea and dicyandiamide-coated urea was more effective than prilled urea in 1984. Differences in dry-matter production and grain yield were directly related to N uptake by the plants. On average, apparent recovery of applied N increased from 35% for prilled urea to 55, 52·5,46·5 and 37·5% for urea supergranules, neem-cake-coated urea, shellac-coated urea and dicyandiamide-coated urea, respectively.

Nitrogen‐15 Balance and Residual Effects of Urea‐N in Wetland Rice Fields as Affected by Deep Placement Techniques

AbstractThe 15N balance at harvest, the residual effects of applied N fertilizer, and the distribution of N by depth in a wetland rice (Oryza sativa L.) soil (Alfisol) were determined in field experiments using 15N‐labeled urea. Nitrogen‐15 balance data show that with point, deep placement of urea supergranules (USG), only 4 to 5% of the applied N was unaccounted for after harvest of both the dry‐season and wet‐season crops. In contrast, when prilled urea was basally broadcast and incorporated and topdressed at panicle initiation (PI), 11 to 35% of the applied N was not recovered at harvest, indicating substantial loss of fertilizer N, especially when poor incorporation resulted in high floodwater mineral‐N levels. Uniform deep placement also resulted in high recovery of 15N (92–107%), confirming that the high total 15N recovery obtained with point deep‐placement of USG was largely a function of depth of placement. Band application of urea gave complete 15N recovery (102%) in the dry season and low recovery (61%) in the wet. Nitrogen‐15 balance data showed that 22 to 56% of the applied urea‐N still remained in the soil after the final harvest. The uptake by the wet season crop of the residual labeled soil N from the dry season experiment was about 5% of the initial applied N (87 kg N/ha) for all treatments. Placement method appeared to influence the actual recovery of residual soil 15N; the percentage recovery from the USG treatment was highest and that from the split urea treatment lowest. In neither the dry season nor the wet season was 15N enrichment significant in soil below 30 cm. Generally, the distribution of the residual 15N in soil reflected the placement pattern of urea.

The effect of deep placement and surface application of nitrogen fertilizers at different light intensities on growth and yield of wetland rice

Lowland rice (RD 3) was cultivated in containers of clay soil submerged with 5 cm water under controlled conditions in the phytotron. Deep placement of urea supergranules 5 cm in the soil significantly enhanced both plant growth and fertilizer efficiency when the plants were cultivated under high light intensity (70 Wm−2). At the highest urea level grain yield increased 119% above the control level, while growth and fertilizer efficiency was not as high when deep placement of calcium nitrate was used.

Distribution Patterns of Ammonium Nitrogen and 15N Uptake by Rice after Deep Placement of Urea Supergranules in Wetland Soil

AbstractIn‐situ distribution patterns of ammonium nitrogen (NH+4‐N) and 15N uptake by wetland rice (Oryza sativa L.) were studied on the farm of the International Rice Research Institute (IRRI) during the 1979 dry season. Individual 2‐g urea supergranules were placed at 5‐, 10‐, and 15‐cm soil depths in wetland rice plots transplanted at spacings of 20 by 20 cm and 10 by 40 cm. The ammonium (NH+4) concentration gradients near placement sites showed that the NH+4 movement was slow; in the transplanted plots, NH+4 concentrations decreased with time largely due to plant uptake. Distribution patterns of NH+4 were influenced by the depth of supergranule placement and the transplanting geometry. In general, the greater the distance from the point of placement of supergranule to the plant, the slower was the depletion of NH+4‐N at the placement site. During the dry season, the NH+4 almost completely disappeared during the 40 days after transplanting, whereas during the wet season, NH+4 persisted for up to 40 days after transplanting.The 15N uptake patterns were s‐shaped. During the first 20‐day period after transplanting, deep‐placed N was presumably physically or spatially unavailable, and negligible plant uptake occurred. After 20 days, the 15N uptake was retarded by placement of supergranules at lower depths and away from the rice hills. However, at final harvest more 15N was taken up from urea placed at the 15‐cm depth than from the 5‐cm depth. A marked lower recovery of split‐applied urea‐N (25 to 34%) than that of deep‐placed urea‐N (50 to 61%) was observed.

Deep placement: A method of nitrogen fertilizer application compatible with algal nitrogen fixation in wetland rice soils

The effect of different methods of nitrogen fertilizer application on the algal flora and biological nitrogen fixation (Acetylene-reducing activity) in a wetland rice soil was studied in pot and field experiments. Broadcast application of urea inhibited nitrogen fixation and favored the growth of green algae. In contrast, deep placement of urea supergranules (1–2 g urea granules) did not suppress the growth of N2-fixing blue-green algae and permitted acetylene-reducing activity on the soil surface to continue virtually uninhibited.