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The determination of urea in soil extracts and related samples--a review.

Introduction

Urea is the most widely used nitrogen fertiliser in the world, accounting for 46% of world nitrogen consumption (Watson 2000). Urea-based fertiliser is distributed to soils as dry granules or dissolved in solution, often concurrently with irrigation water. Urea may also be applied as part of excreted animal wastes. Once in the soil, urea may undergo a range of chemical and physical transformations and transfers. Bacterial urease facilitates the conversion of urea to ammonium (N[H.sub.4.sup.+]). Microorganisms oxidise ammonium to nitrate (N[O.sub.3.sup.-]) and, under anaerobic conditions, reduce nitrate to gaseous nitrogen oxides and molecular nitrogen. The nitrogen supplied as urea may also be taken up by plants and, subsequently, by herbivorous animals. Cycling of nitrogen through plant and animal systems results in a suite of further chemical reactions and transfers, thus incorporating the nitrogen into a range of organic compounds, potentially leading to the regeneration of urea. In addition, the nitrogen supplied as urea may be transferred to the atmosphere through ammonia volatilisation or carried with soil and surface water (including leaching to subsoils via vertical drainage or transported within lateral overland and subsurface flows).

An understanding of these nitrogen transformations in farmlands and wetlands is crucial to establish and maintain optimum fertility and for environmental restoration and care (Bremner 1982; Helyar and Price 1999; Ma et al. 1999). A greater awareness of the processes that cause nitrogen loss will increase the efficiency of fertiliser application and therefore improve farm profitability (Weier 1994; Strong and Mason 1999). The efficiency of urea fertilisers can be increased through the use of inhibitors or slow release systems, which include chemical additives that modify the environment of the granule microsite or inhibit the activity of urease or nitrification (Watson 2000; Sartain et al. 2004). Analytical methods that allow quantification of nitrogenous chemical species significant to soil and related matrices should form important tools for future investigation and enhanced understanding of these processes, in this paper, we review the determination of urea in soil and fertiliser samples, with a focus on the relatively recent adoption of flow-injection and microplate procedures that allow low-cost, high-throughput analyses. We aim to provide both a historical account of the methodologies and a judicious examination of current approaches to allow the analyst to make more informed choices, particularly in light of the increased use of urease inhibitors in agriculture.

Extraction of urea from soil samples

In 1967, Keeney and Bremner noted that although there had been many investigations of urea transformations in soils, there was no critical evaluation of the methodology for the extraction and determination of urea. Extraction had been performed with water, 1 M potassium sulfate, or saturated calcium sulfate or calcium hydroxide solutions. They developed a procedure that involved extraction with 2 M potassium chloride, which had previously been used for the determination of exchangeable ammonium, nitrate, and nitrite. However, the recovery of urea was incomplete, due to hydrolysis by soil urease during extraction.

During the development of a spectrophotometric determination of urea with diacetyl monoxime, Douglas and Bremner (1970a) showed that addition of 5 [micro]g/g of silver sulfate, mercuric chloride, or phenylmercuric acetate (Fig. 1, [I]) completely inhibited the enzymatic hydrolysis of urea. At this concentration, both silver sulfate and mercuric chloride impaired the colour-forming reaction, and therefore 5 mg/L of phenylmercuric acetate was added to the 2 M potassium chloride solution for the extraction (10mL solution per g soil). These conditions continued to be used to the present day (Sullivan and Havlin 1991; Greenan et al. 1995; Ma et al. 1999), but other salt solutions have also been employed (Schulz 1975; Onken and Sunderman 1977). As potassium and chloride ions interfered with the early spectrophotometric methods for the determination of ammonium and nitrate, Onken and Sunderman (1977) prepared a soil extract for the determination of urea, ammonium, nitrate, and nitrite with a solution of sodium sulfate and phenylmercuric acetate (5 mg/L).

[FIGURE 1 OMITTED]

Samples can be stored over long periods under suitable conditions; extracts obtained with the previously described solution of potassium chloride and phenylmercuric acetate can be stored for at least 3 weeks (Douglas and Bremner 1970a). No significant change in either the ammonium or nitrate levels in field-moist soil samples were found over 349 days when stored in polyethylene bags at -15[degrees]C, even with samples where nitrogen compounds had recently been applied. In contrast, storage in a refrigerator at 5[degrees]C resulted in significant changes after only 1 or 2 weeks (Mattos Junior et al. 1995).

Determination of urea

Research on the determination of urea has predominantly focussed on clinical applications, using chemical systems that incorporate urease, which catalyses the hydrolysis of urea to ammonia and carbon dioxide (Taylor and Vadgama 1992). A variety of detection methods are available: colour-forming reactions, including the Berthelot or Nesseler's method; indicator dyes; coupling with other enzyme-catalysed reactions; and electrochemical techniques (Kaplan 1987; Taylor and Vadgama 1992). These chemistries have been adapted for use in a variety of commercially available discrete analysers, particularly for point-of-care applications (Francis et al. 2002).

Urease-based procedures have also been proposed for the determination of urea in soil (Table 1), fertiliser (Table 2), and surface or irrigation water samples (Jansen et al. 1985; Cosano et al. 1989). The current AOAC International Official Method for urea in fertilisers was finalised in 1960 and involves initial precipitation of phosphates with barium hydroxide, reaction with urease, and acid-base titration (AOAC 2000a). A method for the determination of urea and methyleneureas in fertilisers, involving HPLC with refractive index detection, was included in 1984 (AOAC 2000b). These methods are not appropriate for the determination of unreacted urea in liquid urea-formaldehyde controlled-release formulations, but this has been addressed with another HPLC procedure (Hojjatie et al. 2004).

Enzyme-based determinations of urea in soil are impaired by the use of urease inhibitors, such as N-(n-butyl) thiophosphoric triamide [II] or phenylphosphorodiamidate [III], to reduce the loss of nitrogen by ammonia volatilisation after the addition of urea fertilisers (Bremner and Mulvaney 1978; Christianson and Baethgen 1994; Bremner 1995; Watson 2000). Furthermore, as described in the previous section, phenylmercuric acetate is often added to plant and soil extracts before analysis to reduce the breakdown of urea by endogenous urease (Douglas and Bremner 1970a; Kyllingsbaek 1975). According to Wilding and Blanton (1982), the determination of urea in fertilisers is impaired when ammonium ion is the predominant source of nitrogen, but many procedures that incorporate urease have been shown to be suitable for urea-based fertilisers (Table 2).

Of the non-enzymatic procedures examined for soil and fertiliser samples, the spectrophotometric determination of urea with acidic diacetyl monoxime (Fig. 2, [IV]) and thiosemicarbazide [V] is the most dominant. The two other main procedures, in terms of number of papers published, involve reactions with 4-dimethylaminobenzaldehyde (Erlich's reagent [VI]) and hypobromite. Erlich's reagent produces red to purple chromophores with many different compounds, but it is best known as a test for pyrroles (Butler and Walsh 1982). This reagent also gives a stable yellow product with urea (Watt and Crisp 1954), which has been applied to the determination of this analyte in soil (Rotini et al. 1970; Schulz 1975) and fertiliser (Singh and Saksena 1979) samples. Although 4-dimethylaminobenzaldehyde is only slightly soluble in water, it readily dissolves in strong acid, but the colour-forming reaction with urea is inhibited under these conditions, and consequently, the reagent is normally dissolved in 95% ethanol before addition of acid (Hoseney and Finney 1964). However, it has been argued that this method lacks the sensitivity required for studies on urea transformations in soils (Douglas and Bremner 1970a). Onken and Sunderman (1977) found it preferable to use 4-dimethylaminobenzaldehyde for urea concentrations >100 mg/L, and a diacetyi monoxime/thiosemicarbazide procedure for less concentrated solutions.

[FIGURE 2 OMITTED]

Another technique, chiefly promoted by Halasz et al. (1973a, 1973b, 1974a, 1974b; Szorad-Rusz and Halasz 1977), involved the oxidation of urea with alkaline hypobromite. This reagent had been used in the early 20th Century to determine urea with manometric apparati for clinical applications (Taylor and Vadgama 1992), but was applied to ammonium and urea in fertilisers using photometric (Halasz et al. 1974a) or thermometric (Halasz et al. 1974b) detection. Schilbach and Kirmse (1978) exploited the same reaction with platinum electrodes for both reagent generation and detection. Vil'dt et al. (1981) employed differential spectrophotometry of the excess reagent at 330nm and nitrate at 300nm to determine the urea/calcium nitrate ratio in fertilisers. Chatterjee and Sanyal (1972) employed this reaction for the determination of biuret in commercial urea samples after enzyme-catalysed hydrolysis of urea. Nevertheless, urea, ammonia, and many other compounds react with hypobromite, so selective applications are limited. Furthermore, in the clinical laboratory, the nitrogen recovery was found to be incomplete (Martinek 1969; Taylor and Vadgama 1992). More recently, this chemistry was used for the determination of urea in urine (Hu et al. 1994a) and haemodialysate (Lewis et al. 2002), based on the accompanying chemiluminescence, but this approach is not suitable for the determination of urea in environmental samples due to interferences that include humic acids (Hu et al. 1994b).

Douglas and Bremner (1970a, 1970b) adapted the spectrophotometric determination of urea with acidic diacetyl monoxime in the presence of thiosemicarbazide to the determination of urea in soil samples. At the time, the reaction was widely used in clinical laboratories, due in part to the commercially available automated procedure using segmented-flow instrumentation (Taylor and Vadgama 1992). The method for soils involved extracting the sample with 2 M potassium chloride solution containing a urease inhibitor, heating each aliquot with diacetyl monoxime and thiosemicarbazide in solutions containing sulfuric and phosphoric acid, and measurement at 527nm (Douglas and Bremner 1970a). Nitrite has been found to interfere if it is more than 5 times the concentration of urea but can be removed with sulfamic acid. Onken and Sunderman (1977) adopted this and other spectrophotometric procedures for the determination of ammonium, nitrite, nitrate, and urea in soil samples extracted with a sodium sulfate solution containing phenylmercuric acetate.

The use of phosphoric acid in the original procedure sometimes led to wide batch variations due to impurities. Mulvaney and Bremner (1979a) modified the reagent and acid concentrations, and reduced the heating time to 30 min at 85[degrees]C, which both removed the influence of the impurities and increased the sensitivity and precision. Praveen and Aggarwal (1989) also examined this problem and removed the phosphoric acid from the procedure. They claimed that their procedure had double the linear calibration range. However, Douglas and Bremner (1970b) had previously found that a mixture of sulfuric and phosphoric acids gave better colour stability than the use of sulfuric acid alone. Nagel and Weller (1989) have adapted the procedure of Douglas and Bremner (1970a) to the determination of urea in wine.

A recent report on the efforts of the Controlled Release Fertilizer Task Force (established by the Association of American Plant Food Control Officials in 1994) described their proposal for the analysis and characterisation of slow-release fertilisers (Sartain et al. 2004). The procedure, based on increasingly aggressive extractions to isolate nutrients that would become available over time, has been applied to commercially available materials such as sulfur-coated urea, isobutylidene diurea, and urea-formaldehyde polymers. During this procedure, the concentration of urea in the leachate solutions was established with the spectrophotometric method described by Mulvaney and Bremner (1979a).

Low-cost, high-throughput analyses

Although the determination of urea using diacetyl monoxime is an attractive method for samples that may contain urease inhibitors, performing large numbers of analyses with many of the available manual methods can be time-consuming and labour-intensive. To address these issues, Anton et al. (1990) proposed a stopped-flow procedure for the determination of urea in serum, which reduced the required reaction time to 3 min with a reaction temperature of only 50[degrees]C. Subsequently, flow-injection analysis (FIA) (Sullivan and Havlin 1991; Lachat Instruments 2001) and microplate (Greenan et al. 1995; Carlile and Dickinson 1997) procedures have been introduced.

In the monograph entitled Determination of Organic Compounds in Soils, Sediments and Sludges, Crompton (2000) describes FIA as a useful technique, particularly for routine automated analyses at [micro]g/g levels. Nevertheless, application of this technique for soil and sediment matrices is very limited, but includes the determination of total phosphorus, organic carbon, and nitrogen in soils, and total organic carbon and sulfur in sediments. Sullivan and Havlin (1991) reported a flow injection procedure for the determination of urea in 2 M potassium chloride soil extracts. In this procedure, diacetylmonoxime, thiosemicarbazide, and the acid reagent were mixed on-line and combined with the sample carrier prior to the heating bath (95[degrees]C) and detector. A number of previous manual methods were examined and the reagent conditions were optimised. Using their flow-injection procedure, the limit of detection was 10[micro]g urea-N/L (4 x [10.sup.-7] M) and 60 samples could be analysed per hour using a commercially available flow-injection system from Lachat. This instrument manufacturer later prepared a standard method for brackish and seawater samples (Lachat Instruments 2001). In this case, one less manifold line was used; diacetyl monoxime and thiosemicarbazide solutions were pre-mixed off-line (daily). The sample throughput was quoted at 30 injections per hour and the limit of detection was 3 [micro]g urea-N/L ([10.sup.-7] M).

Unlike almost all of the methods for the determination of urea in soil based on the procedures by Douglas and Bremner, both FIA procedures included the addition of ferric chloride hexahydrate to the acid reagent. This additive is believed to assist the formation of the chromogen by in 2 ways: (i) the destruction of hydroxylamine produced during the hydrolysis of diacetyl monoxime to diacetyl, and (ii) catalysis of the reaction, allowing less acid to be used (Taylor and Vadgama 1992). Sullivan and Havlin (1991) observed a 5-fold increase in absorbance in the presence of as little as 0.08 mM iron(III). This additive is commonly used in diacetyl monoxime procedures developed for the determination of urea in clinical or marine samples (Rahmatullah and Boyde 1980; Price and Harrison 1987; Taylor and Vadgama 1992).

An alternative approach that has recently been presented is to scale down existing manual procedures for use with 96-well microplates. The lower volume of solutions in these assays reduces the risk to laboratory personnel and minimises the production of waste. Greenan et al. (1995) examined the use of microplates for the spectrophotometric determination of urea in soil extracts. Reagents and samples were inserted into the wells using micropipettes and the plate was then inserted into a laboratory oven at a temperature of 87[degrees]C for 55 min. After cooling for 20 min, the absorbance of each well was measured with a microplate reader. The accuracy, precision, and sensitivity of this procedure were similar to a conventional method. The spectrophotometric procedure of Mulvaney and Bremner (1979b) was adapted for microscale assays of growing media extracts by Carlile and Dickinson (1997). In this case, the plate was sealed with polythene film and half-submerged in a water bath at 85[degrees]C for 30 min. In our laboratory, we have used a microplate procedure that was based on the reagent conditions for the flow analysis procedure in QuikChem Method 31-206-00-1-A (Lachat Instruments 2001).

Although the analysis time for the microplate procedures was significantly longer than for flow-injection, a standard microplate can hold up to 88 samples (plus 8 standards), which enables a much greater overall throughput. Furthermore, the simultaneous analysis of samples eliminates the problems associated with colour instability. Multi-channel pipettes can be used to dispense reagents to increase the efficiency of the manual sample preparation. Microplate methods have also been developed for the determination of urea in human blood serum (Lin et al. 2000) and seawater (Baudinet and Galgani 1991). The method proposed by Lin et al. (2000) incorporates the use of urease-positive Helicobacter pylori in place of purified urease and a pH indicator dye, BromoCresol Purple, to provide a colour change. Baudinet and Galgani (1991) examined microplate procedures for ammonia, nitrate, and urea in estuarine and coastal waters. For the determination of urea they used the diacetyl monoxime/thiosemicarbazide method presented by Rahmatullah and Boyde (1980), which provided a detection limit of around 1 [micro]M. However, Aminot (1996) has questioned the suitability of microplate procedures for the low concentrations generally encountered in marine work.

An elusive chromophore

Although numerous procedures for the determination of urea that incorporate diacetyl monoxime have emerged, the mechanism of the reaction and the nature of the chromophore are very rarely mentioned in these papers. In the absence of thiosemicarbazide, the reaction of unsubstituted urea with diacetyl monoxime produces a yellow compound with an absorbance maximum at 480nm. Patel (1978) reviewed the previous work and presented an extensive series of experiments to establish the nature of this species. it was concluded that the open-chain diureide (Fig. 3, [VII]) postulated by Lugosi et al. (1972) was indeed the protochromogen. Butler and co-workers (Butler and Hussain 1981; Butler et al. 1981; Butler and Walsh 1982) argued that the diureide is a byproduct and discounted other previous proposals for the key species (such as [IX] and [X]) with an investigation that provided evidence for a 'skipped diene' [XI], which had been postulated by Ueda et al. (1968). Butler and Hussain (1981) suggested a mechanism for the generation of this intermediate and its subsequent oxidation to form a coloured carbonium ion [XII], which has since been cited in reviews on the determination of urea (Taylor and Vadgama 1992; Francis et al. 2002).

[FIGURE 3 OMITTED]

Contemporary methodology based on the reaction between urea and diacetyl monoxime normally includes the addition of thiosemicarbazide, which results in a bathochromic shift to 525-530nm. Khramov and Galaev (1969) suggested that the species responsible might be the 1,2,4-triazine [XIII]. Patel (1978) rejected the idea of triazine formation under the acidic conditions employed. It was noted that the cyclisation of diacetyl monosemicarbazone to the corresponding triazine required refluxing in 2M sodium hydroxide. Patel interpreted a series of experiments as evidence that the chromogen is the monoureide, monosemicarbazone derivative [VIII] of diacetyl. Yuki et al. (1981) isolated triazines [XIV] from the neutralised reaction mixture of diacetylmonoxime-thiosemicarbazone and urea or its ethyl and p-tolyl derivatives. The products immediately formed the characteristic red hue in acidic media (which they attributed to the corresponding aromatised cation [XV]). A shoulder band in the spectrum was attributed to the skipped-diene [XI] or its corresponding ethyl and p-tolyl derivatives.

Concluding remarks

Although the true chromophore and mechanism of the reaction between urea and diacetyl monoxime in the presence of thiosemicarbazide are subjects of some debate, the analytical application of this reaction is well characterised and is a suitable option for samples that may contain urease inhibitors. Furthermore, the time-consuming nature of this approach has been overcome by the development of microplate and flow injection analysis procedures. In contrast, contemporary procedures for the determination of urea in fertilisers are largely based on the selective hydrolysis of urea with urease coupled with flow injection analysis methodology.
Table 1. Methodology for the determination
of urea in soil and growing media

Reference Comments

 Spectrophotometry--
 4-dimethylaminobenzaldehyde

Watt and Crisp (1954); Hydrazine was found to interfere in this
 Rotini et al. (1970) reaction. Soil and vegetable samples
Schulz (1975) Extraction with 1% Ka1
 [(S[O.sub.4]).sub.2] solution
Onken and Sunderman (1977) Procedure developed by Watt and
 Crisp (1954)

 Spectrophotometry--diacetyl
 monoximelthiosemicarbazide

Douglas and Bremner (1970a, Extraction with potassium chloride
 1970b); Bremner (1982) solution containing phenylmercuric
 acetate
Douglas and Bremner (1971) Above method used for the evaluation
 of urease inhibitors in soils
Kyllingsbaek (1975) Activated charcoal and exchange resin
 column used to remove interferents
 from plant extracts
Onken and Sunderman (1977) Extraction with sodium sulfate solution
 containing phenylmercuric acetate
Douglas et al. (1978) Automated system (BRAUN-Syste-Matik
 apparatus)
Mulvaney and Bremner Original method modified due to
 (1979a); Bremner (1982) impurities in phosphoric acid
Elliot (1986, 1988) Method of Douglas and Bremner (1970a)
 used to determine urea in soil-less
 potting media
Gorelik et al. (1986) Modified reagent (dimethyl glyoxime)
 used
Praveen and Aggarwal (1989) Phosphoric acid replaced by sulfuric
 acid. Increased linear calibration
 range
Sullivan and Havlin (1991) Flow injection analysis. Sensitivity
 can be improved by increasing
 residence time in the on-line
 heating coil
Greenan et al. (1995) Microplate version of procedure by
 Mulvaney and Bremner (1979a).
 Heating and cooling time re-optimised
Carlile and Dickinson Microplate version of procedure by
 (1997) Mulvaney and Bremner (1979b) for
 use with growing media

 Urease-based

Keeney and Bremner (1967); The ammonium ion released is separated
 Bremner (1982) by steam distillation and determined
 by titration of the distillate with
 standard acid. An extraction and a
 direct distillation were demonstrated
Drews and Geyer (1971) Derivatisation with Nessler's reagent.
 Intended for use after fertilisation
 of garden soil with urea
Panda and Patnaik (1985) Steam distillation. Commercial split
 pigeon pea used as a source of urease
Abshahi et al. (1988) HPLC with three columns: guard, cation
 exchange and urease. Post-column
 derivatisation with o-phthalaldehyde
Papaefstathiou and Ammonia separated by pervaporation and
 Luque de Castro (1997) detected with an ion-selective
 electrode. Solid soil samples used

Reference Reaction Detection
 temperature limit or
 and time range
 (mg N/L)

 Spectrophotometry--
 4-dimethylaminobenzaldehyde

Watt and Crisp (1954); Room temp. 50-240
 Rotini et al. (1970)
Schulz (1975) Room temp. 9-254
Onken and Sunderman (1977) Room temp., 10 min 100-2500

 Spectrophotometry--diacetyl
 monoximelthiosemicarbazide

Douglas and Bremner (1970a, 120[degrees]C, 30 min 0.5-9
 19706); Bremner (1982)
Douglas and Bremner (1971) 120[degrees]C, 30 min 0.5-9
Kyllingsbaek (1975) 80[degrees]C, 35 min --
Onken and Sunderman (1977) 120[degrees]C, 30 min --
Douglas et al. (1978) 90[degrees]C, 20 min 2-6
Mulvaney and Bremner 85[degrees]C, 35 min --
 (1979a); Bremner (1982)
Elliot (1986, 1988) 120[degrees]C, 30 min --
Gorelik et al. (1986) ? 1
Praveen and Aggarwal (1989) 98[degrees]C, 45 min --
Sullivan and Havlin (1991) 95[degrees]C, 130s 0.01
Greenan et al. (1995) 87[degrees]C, 55 min 1-25
Carlile and Dickinson 85[degrees]C, 30 min --
 (1997)

 Urease-based

Keeney and Bremner (1967); 30[degrees]C, 1-2 h --
 Bremner (1982)
Drews and Geyer (1971) 3 h ?
Panda and Patnaik (1985) 120 min --
Abshahi et al. (1988) 40[degrees]C --
Papaefstathiou and 40[degrees]C, 60 min 22 (mg/kg)
 Luque de Castro (1997)

?, Papers published in languages other than English
and the required information was not included in the
English language abstract.

Table 2. Methodology for the determination of urea in fertilisers

Reference Comments

 Direct spectrophotometry

Singh and Saksena (1979) 4-Dimethylaminobenzaldehyde reaction.
 Comparison with Kjeldahl and urease
 methods
Wilding and Blanton (1982) Diacetyl monoxime/thiosemicarbazide
 reaction for traces of urea in
 ammonium nitrate fertilisers

 Hypobromite

Halasz et al. (1974a) Absorbance of reagent monitored at
 330 nm. No discrimination between
 urea and ammonium nitrogen
Halasz et al. (19746); Szorad- Thermometric detection. An estimate
 Rusz and Halasz (1977) can be made in the presence of
 ammonium ion, total nitrogen can
 be calculated
Schilbach and Kirmse (1978) Determination of urea and ammonia.
 Platinum electrodes for reagent
 generation and detection
Vil'dt et al. (1981) Differential spectrophotometry.
 Urea, calcium, nitrate and
 their concentration ratio

 Urease-based

AOAC (2000a) Finalised in 1960. Official Method
 959.03. Precipitation of phosphates
 with Ba[(OH).sub.2]. Acid-base
 titration with methyl purple
 indicator
Tagami et al. (1974) Initial decomposition rates were
 measured by recording the change
 in conductivity over time
Marecek et al. (1975) Titration by the formaldehyde method.
 Phosphate removed as iron salt
Falls et al. (1976) Titration with sulfuric acid. End-
 point detected with pH meter
Barbera and Canepa (1977) Ammonia gas-sensing electrode. Urea-
 formaldehyde ammonium fertilisers.
 Condensation products did not
 interfere
Anigbogu et al. (1983) UV absorbance (194 nm) of ammonia gas
 measured after the addition of NaOH
Mizobuchi et al. (1984) Chromatographic separation and
 derivatisation with fluorescamine
 for fluorescence detection. Also
 applied to ammonia
Jansen et al. (1985) HPLC with enzyme reactor and
 o-phthalaldehyde derivatisation for
 fluorescence detection. Simultaneous
 determination of ammonia
Uchiyama et al. (1985) Ion-chromatography coupled with
 an immobilised enzymes and
 conductometry. Simultaneous
 determination of urea, [Na.sup.+],
 [K.sup.+], and [N[H.sub.4].sup.+]
Menconi (1985) Ion-chromatography with conductometry.
 Same system (without urease) can be
 used to determine ammonium and
 nitrate ions
Cosano et al. (1989) FIA with enzyme reactor and
 derivatisation with Nessler's
 reagent. Simultaneous determination
 of urea and ammonia in irrigation
 waters
Xie et al. (1990) Enzyme-based fibre optic biosensor
 incorporating pH indicator
Junior et al. (1997) FIA with immobilised Canavalia
 brasiliensis as a urease source.
 Membrane separation of ammonia
 and potentiometric detection
Lachat Instruments (1999) FIA. Derivatisation of ammonia with
 photometric method at elevated
 temperature

 HPLC

AOAC (2000b) Official Method 983.01, finalised in
 1984. Urea and water-soluble
 methylene ureas in urea-formaldehyde
 fertilisers. Refractive index
 detection
Hojjatie et al. (2004) Unreacted urea in water-soluble
 urea-formaldehyde fertilisers.
 UV absorbance detection

Reference Reaction Detection
 temperature limit
 or range
 (mg N/L)

 Direct spectrophotometry

Singh and Saksena (1979) 25[degrees]C, 15 min --
Wilding and Blanton (1982) 75[degrees]C, 20 min --

 Hypobromite

Halasz et al. (1974a) Room temp. --
Halasz et al. (19746); Szorad- -- --
 Rusz and Halasz (1977)
Schilbach and Kirmse (1978) 80[degrees]C ?
 or room temp.
Vil'dt et al. (1981) ? ?

 Urease-based

AOAC (2000a) -- Not stated
Tagami et al. (1974) 55[degrees]C ?
Marecek et al. (1975) ? ?
Falls et al. (1976) 20-25[degrees]C, 60 min --
Barbera and Canepa (1977) 25[degrees]C 120 min --
Anigbogu et al. (1983) -- 1
Mizobuchi et al. (1984) 37[degrees]C, 30 min 0.1
Jansen et al. (1985) Room temp., 0.5
 3 min
 analysis time
Uchiyama et al. (1985) Room temp. 0.3-30
Menconi (1985) 40[degrees]C, 30 min --
Cosano et al. (1989) -- 1-6
Xie et al. (1990) -- 7-224
Junior et al. (1997) 25[degrees]C 7-70
Lachat Instruments (1999) 60[degrees]C 75-600

 HPLC

AOAC (2000b) -- --
Hojjatie et al. (2004) -- 25-200

?, Papers published in languages other than English
and the required information was not included in the
English language abstract.


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Manuscript received 25 March 2004, accepted 31 May 2004

David F. Lambert (A), John E. Sherwood (A), and Paul S. Francis (B,C)

(A) School of Ecology and Environment, Deakin University, Warrnambool, Vic. 3280, Australia.

(B) School of Biological and Chemical Sciences, Deakin University, Geelong, Vic. 3217, Australia.

(C) Corresponding author. Email: psf@deakin.edu.au
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Publication:Australian Journal of Soil Research
Date:Dec 1, 2004
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