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Soil Science Society of America Journal 64:1083-1089 (2000)
© 2000 Soil Science Society of America

DIVISION S-8-NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

Direct-Diffusion Methods for Inorganic-Nitrogen Analysis of Soil

S.A. Khana, R.L. Mulvaneya and R.G. Hoeftb

a Dep. of Natural Resources and Environ. Sci., Urbana, IL USA
b Dep. of Crop Sci., Univ. of Illinois, 1102 S. Goodwin Ave., Urbana, IL 61801 USA

mulvaney{at}uiuc.edu


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 
Determinations of exchangeable NH4, NO3, and NO2 in soil normally require the preparation of an extract, which is usually obtained with a neutral salt solution such as 2 M KCl. In cases where analytical speed and convenience are key factors, as in routine soil testing, a more practical approach would be to carry out the determination directly on the soil sample itself. Studies were conducted to ascertain whether this approach could be employed using Mason-jar diffusion methods previously developed for inorganic-N analysis of soil extracts. Direct diffusions were performed with MgO to recover NH4–N, or with MgO and Devarda's alloy to recover (NH4 + NO3 + NO2)–N, from 1- to 5-g samples of a wide variety of air-dried soils, following treatment of the soil with sufficient 2 M KCl to give a 1:10 ratio of soil to solution. Up to 4 mg of inorganic N was recovered quantitatively in 18 h to 6 d at room temperature, or in 1.75 to 5.5 h with gentle heating on a hot plate (45–50°C). Analyses by direct diffusion were in close agreement with results obtained by diffusion of soil extracts, and recoveries ranged from 97 to 102% when direct diffusions were performed on samples that had been treated with NH4–N or NO3–N. Besides being quantitative, direct diffusions were found to have potential for N-isotope analysis. In diffusing 15N-treated samples, isotope-ratio analyses of NH4–N or (NH4 + NO3 + NO2)–N were accurate to within 3%. The latter finding virtually precludes interference by labile organic N or nonexchangeable NH4–N; however, interference did lead to an increase in inorganic-N analyses when diffusion was prolonged beyond the recommended period.

Abbreviations: ARA–MS, automated Rittenberg apparatus–mass spectrometry • CEC, cation-exchange capacity • CV, coefficient of variation • SD, standard deviation


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 REFERENCES
 
THE DETERMINATION OF INORGANIC FORMS of soil N usually involves extraction of the soil with a neutral salt solution such as 2 M KCl. For many research applications, the time and effort required for extraction are more than justified by the fact that the extracts can be easily stored for later analysis. This advantage does not apply to routine soil testing, in which case speed and convenience are at least as important as analytical accuracy and precision.

Keeney and Bremner (1966) described steam-distillation methods for direct determination of exchangeable NH4, NO3, and NO2 in soils, in which the analysis is performed on the soil sample itself, without prior extraction. These methods have been used successfully with a wide variety of soils (Keeney and Bremner, 1966; du Preez et al., 1987); however, interference can arise from soil organic N, and substantially higher values have been observed in measuring NH4–N in organic and paddy soils by direct distillation than by extraction–distillation (Robinson, 1967; Sahrawat and Ponnamperuma, 1978). Moreover, liberation of CO2 during distillation of calcareous or anaerobic soils has been found to cause negative error in titrimetric determination of NH4–N or NO3–N (Clausen et al., 1981; Sahrawat, 1982), owing to formation of H2CO3 in the distillate.

Recent work in our laboratory has led to simple diffusion methods for inorganic-N analysis of soil extracts, whereby diffusion is carried out in a 473-mL (1-pint) wide-mouth Mason jar (Mulvaney, 1996; Mulvaney et al., 1997a), and can be completed within a few hours by gentle heating on a hot plate (Khan et al., 1997). Besides being much less expensive and considerably more convenient than steam distillation, Mason-jar diffusion methods are highly accurate and precise. Moreover, since diffusions are performed at a much lower temperature than are distillations, there is little risk of interference from hydrolysis of labile organic N (Mulvaney and Khan, 1999), nor does interference arise from liberation of CO2.

The purpose of the work reported here was to ascertain whether Mason-jar diffusion methods for determination of inorganic N in soil extracts can be applied directly to the soil sample itself. The direct methods were evaluated for analytical accuracy and precision from recovery of N and 15N added to a wide variety of soils as NH4 or NO3, and by comparison to inorganic-N analyses by extraction–diffusion.


   Materials and methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 REFERENCES
 
Soils
The soils used (Table 1) were surface (0–15 cm) samples of 12 soils selected to obtain a wide range in properties. The Bejucal soil (very fine, mixed, acid Entic Pelludert) used was obtained from Trinidad, and the Fargo soil (fine, smectitic, frigid Typic Epiaquert) was collected in North Dakota. The remaining soils were from Illinois, including an uncropped sandy soil under coniferous vegetation (Bloomfield [sandy, mixed, mesic Psammentic Hapludalf]), an upland forest soil (Xenia [fine-silty, mixed, mesic Aquic Hapludalf]), a waterlogged organic soil (Houghton [euic, mesic Typic Haplosaprist]), and seven cultivated soils used for corn (Zea mays L.) and soybean (Glycine max L. Merr.) production. Before use, each sample was air-dried, and unless otherwise specified, the soil was then crushed to pass through a 2-mm screen.


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  		Table 1 Selected properties of soils

 
In the analyses reported in Table 1, pH was determined with a microelectrode (soil to water ratio, 1:1), organic C by the method of Mebius (1960), total N by the regular Kjeldahl method using a block digester (Bremner, 1996), texture as described by Kilmer and Alexander (1949), nonexchangeable NH4 by digestion with HF–HCl following a KOBr pretreatment (Mulvaney, 1996), and cation-exchange capacity (CEC) by NH4 saturation at pH 7 (Sumner and Miller, 1996). The organic C, total N, and nonexchangeable NH4 analyses were performed on <0.15-mm soil; the other analyses reported were performed on <2-mm soil. All analyses were performed in triplicate.

Extraction–Diffusion
Soil extracts were obtained by shaking (192 rev min-1) 20 g of soil (<2-mm mesh size) with 200 mL of 2 M KCl in a 250-mL polyethylene bottle for 1 h, and filtering the resulting suspension through Whatman no. 42 filter paper under vacuum (Mulvaney, 1996). Diffusions to determine NH4–N and (NH4 + NO3 + NO2)–N were performed on 10 mL of extract, using the accelerated Mason-jar technique described by Khan et al. (1997). In this technique, the sample is treated with MgO (0.2 g), with or without Devarda's alloy (0.2 g), and the NH3 liberated in 1.75 h at 45 to 50°C on a hot plate is collected in 5 mL of H3BO3–indicator solution (40 g H3BO3 L-1). A Metrohm Model 678 EP/KF Processor, equipped with a Model 665 Dosimat (Metrohm, Herisau, Switzerland) and a microelectrode (Model MI-414S, Microelectrodes, Inc., Bedford, NH), was employed for titrations.

Direct Diffusion
Mason-jar methods of determining NH4–N and (NH4 + NO3 + NO2)–N in soil extracts were performed at 25°C with or without orbital shaking (Mulvaney et al., 1997a), or at 45 to 50°C with heating on a hot plate (Khan et al., 1997), but instead of soil extract, 1, 2, or 5 g of soil (<2-mm mesh size unless otherwise specified) was placed in the jar and treated with sufficient 2 M KCl to obtain a 1:10 ratio of soil to solution. Unless otherwise noted, these diffusions were performed for the periods specified in Table 2 . The automatic titrator specified previously was used for all titrations.


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  		Table 2 Diffusion periods for quantitative analysis of up to 4 mg of inorganic N

 
Evaluation of Direct-Diffusion Methods
To evaluate direct diffusion for quantitative determination of inorganic N in a wide variety of soils, a comparison was made with extraction–diffusion for analysis of NH4–N and (NH4 + NO3 + NO2)–N, using 1-g samples of the 12 soils listed in Table 1. Analytical accuracy was evaluated by checking the completeness of recovery when 1-g samples of soil were treated with 1 mL of deionized water containing 300 µg of N as (NH4)2SO4 or KNO3, immediately following the addition of 10 mL of 2 M KCl for direct diffusion. In the latter investigation, diffusions were carried out with heating on a hot plate (45–50°C) for 1.75 h, and percentage recovery was calculated after correction for determinations without added N. The analyses in both experiments were performed in triplicate.

A study was also conducted to check that direct diffusions were quantitative when employed to estimate NH4–N in soil containing up to 4 g of exchangeable NH4–N kg-1, through comparison to analyses by extraction–diffusion. To effect NH4 saturation of CEC, 20-g samples of soil were treated with 20 mL of deionized water containing 5.6 g of N as NH4Cl. Excess NH4Cl was removed by filtering the resulting slurry through Whatman no. 42 filter paper under vacuum, after which the residual soil was leached twice with 100 mL of 20.6 M (95%) ethanol, air-dried, and crushed to pass through a 2-mm screen. Unless otherwise specified, direct diffusions were performed on 1 g of NH4–saturated soil, and analyses by extraction–diffusion were performed on 1 mL of 2 M KCl soil extract. In each case, there were six replicate analyses, and the diffusions were done at 45 to 50°C on a hot plate for 1.75 h.

To check whether direct diffusions are affected by soil aggregate size, a comparison was made by carrying out analyses for exchangeable NH4–N on samples of soil that had been crushed to pass through a 5.5-, 3.5-, or 2.0-mm screen, relative to analyses of <2-mm material by extraction–diffusion. All diffusions were performed in triplicate, for 1.75 h at 45 to 50°C on a hot plate.

Studies were also conducted to ascertain whether direct diffusions can be performed successfully on 2 or 5 g of soil, so as to facilitate analyses of soil having a low concentration of inorganic N. To check the need for additional MgO in neutralizing soil acidity when analyses for NH4–N are performed on 5 g of soil, triplicate diffusions were carried out for 5.5 h at 45 to 50°C using 0.2 to 1 g of MgO, from a neutral soil (Houghton) and a highly acidic soil (Bejucal). In all cases, the sample was treated with 50 mL of 2 M KCl, and the pH of the resulting suspension was measured with the microelectrode specified previously. The analyses were evaluated by comparison to data obtained with 1 g of soil.

A study was also done to check whether there is a need to increase the amount of Devarda's alloy when diffusions are performed using 5 g of soil. Triplicate samples of an organic soil (Houghton) were diffused with MgO and Devarda's alloy, either for 6 d at 25°C or for 5.5 h with heating on a hot plate, after treatment with 50 mL of 2 M KCl containing 0 or 300 µg of N as KNO3. In carrying out these diffusions, a fixed quantity of MgO was used (0.6 g) with different additions of Devarda's alloy (up to 1.8 g in 0.3-g increments). Recovery of NO3–N was calculated from the difference in (NH4 + NO3 + NO2)–N recovered with and without addition of KNO3.

To verify the accuracy of direct diffusion when analyses are performed on 2- or 5-g samples of soil, a comparison was made to inorganic-N analyses with 1 g of soil, using four soils that ranged widely in physicochemical properties (Bejucal, Drummer, Harpster, and Houghton). In all cases, the sample was treated with a sufficient volume of 2 M KCl to obtain a 1:10 ratio of soil to solution, followed by addition of MgO and Devarda's alloy to recover (NH4 + NO3 + NO2)–N when diffusion was carried out with heating on a hot plate (45–50°C) for 1.75 (1 g), 4 (2 g), or 5.5 (5 g) h. With 1 or 2 g of soil, samples were treated with 0.2 g of MgO and 0.3 g of Devarda's alloy, whereas 0.6 g of MgO and 0.9 g of Devarda's alloy were used when analyses were performed on 5 g of soil.

The use of direct diffusion for isotopic analysis of soil inorganic N was investigated by carrying out diffusions (four replicates) for 1.75 h at 45 to 50°C after addition of MgO (0.2 g) to recover NH4–N, or MgO and Devarda's alloy (0.2 g of each) to recover (NH4 + NO3 + NO2)–N, from 1-g samples of soil treated with 10 mL of 2 M KCl containing 0 or 300 µg of N as 15N-labeled (NH4)2SO4 (1.435 atom% 15N) or KNO3 (1.598 atom% 15N). Following quantitative determinations by titration, isotope-ratio analyses were performed on each sample by automated Rittenberg apparatus–mass spectrometry (ARA–MS), using an automated system described in detail by Mulvaney et al. (1990), Mulvaney and Liu (1991), and Mulvaney et al. (1997b). The accuracy of these analyses was evaluated relative to an expected value (E) calculated by the equation

(1)
where A is atom % 15N and N is the micrograms of N in the form(s) under analysis originating from the sample (S) or the labeled N added (L).

To determine the effect of prolonging the period of diffusion on inorganic N analyses by direct diffusion, determinations of (NH4 + NO3 + NO2)–N were performed on 1-g samples of soil (three replicates) after addition of 10 mL of 2 M KCl. Diffusion was done with heating on a hot plate (45–50°C), for 1.75 h (the period specified in Table 2) or for an additional 1, 2, or 4 h.


   Results and discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
REFERENCES
 
The determination of inorganic N in soil is inherently more difficult when the analysis is performed directly on the soil sample itself rather than on an extract, because the soil solid phase serves as a source of numerous interferences. The most obvious interference is turbidity, which precludes direct analyses by colorimetry. The high organic-N content of surface soils is also a potential problem, because even if hydrolysis is very limited, inorganic N may be overestimated to a considerable extent. The latter interference is promoted by strong alkalinity, so MgO is used in direct steamdistillation methods of determining inorganic N in soils (Keeney and Bremner, 1966), based on work by Bremner and Shaw (1955) showing that this reagent, which buffers pH at approximately 10.5, can be employed to carry out diffusions at room temperature without effecting hydrolysis of amino sugars and other alkali–labile organic N compounds. The same reagent is used in Mason-jar diffusion methods for inorganic-N analysis of soil extracts (Khan et al., 1997; Mulvaney et al., 1997a), in which case NH3 is liberated, transported, and collected without the use of steam, and without the high temperatures inherent to steam distillation. Besides reducing the risk of interference by labile organic N, the mild conditions adopted for diffusion effectively prevent the liberation of acidic or alkaline constituents in the sample that would vitiate quantitative determinations by titration. So effective is the Mason-jar diffusion approach in reducing interferences that inorganic N and 15N analyses can even be performed on samples having an exceptionally high content of organic N, such as urine, manure extract, or septic effluent (Mulvaney and Khan, 1999).

To ascertain whether Mason-jar diffusion methods can be used for direct determination of soil inorganic N, a comparison was made of direct analyses by diffusion at 25°C with or without orbital shaking or at 45 to 50°C on a hot plate, relative to analyses of 2 M KCl soil extracts. Regardless of the diffusion technique employed or the soil under analysis, data obtained by direct diffusion were in close agreement with those by extraction–diffusion, and analytical precision was comparable (Table 3) . Of particular interest is that the direct approach did not underestimate inorganic N due to liberation of CO2 from a calcareous soil (Harpster), nor did underestimation occur with two soils having a substantial capacity for fixation of NH4 (Bejucal and Fargo). Moreover, direct analyses were unaffected by the high organic-matter content of a Histosol (Houghton), indicating very little, if any, hydrolysis of labile organic N.


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  		Table 3 Comparison of extraction–diffusion and direct diffusion for inorganic-N analysis of soils.{dagger}

 
Table 4 shows the results of recovery tests to evaluate analytical accuracy and precision when direct diffusions were performed for 1.75 h with heating on a hot plate. To prevent NH4 fixation by 2:1 clay minerals that would reduce recovery of NH4–N, addition of mineral N followed treatment of the soil with 2 M KCl. Recovery of added NH4–N or NO3–N was quantitative with each soil studied. The coefficient of variation was usually <1%. Table 4 provides further evidence that direct diffusions are free of interference, as NH4 fixation would have led to low recoveries with the Fargo soil, and hydrolysis of soil organic N would have led to high recoveries with the Houghton soil.


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  		Table 4 Recovery by direct diffusion of NH4–N and NO3–N added to soil.{dagger}

 
An important advantage of Mason-jar methods for inorganic-N analysis of soil extracts is their generous analytical capacity (up to 4 mg N), which virtually eliminates the need to repeat analyses, even with extracts from highly fertilized soil. To ascertain whether the same advantage applies to direct diffusions, analyses for exchangeable NH4–N were performed on NH4–saturated samples of soil, and also on 2 M KCl extracts from these samples. The results (Table 5) reveal excellent agreement between analyses by the two methods, and comparable precision. If, as is indicated by Tables 3 and 4, no interference occurred from hydrolysis of soil organic N, then the data in Table 5 demonstrate that the KCl treatment employed (10 mL of a 2 M solution) was completely effective in displacing up to 3.7 mg of exchangeable NH4–N per g of soil, in which case the ratio of K to NH4 would have exceeded 75:1. Moreover, these data preclude the possibility that liberation of NH4–N was impeded by the presence of soil, as direct diffusions were performed for the same period employed in analysis of soil extracts (1.75 h).


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  		Table 5 Comparison of extraction-diffusion and direct diffusion for determination of exchangeable NH4–N in NH4–treated soil.{dagger}

 
In keeping with standard laboratory procedures, the analyses reported in Tables 3 to 5 were performed on soil that had been crushed to pass through a 2-mm screen. Considering the need for speed and convenience in the use of direct diffusion for inorganic-N analysis, a study was conducted to evaluate the effect of larger aggregate size on direct analyses, as compared to analyses of extracts obtained with 2-mm soil. Three well structured soils were used in this study, including two silty clay loams and one silty clay. The results (Table 6) indicate no need for fine grinding of soil when analyses are to be carried out by direct diffusion, presumably because soil aggregates quickly form a slurry when treated with sufficient 2 M KCl to obtain a 1:10 ratio of soil to solution.


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  		Table 6 Effect of soil aggregate size on analyses for exchangeable NH4–N by direct diffusion.{dagger}

 
When direct diffusions are performed on 1 g of soil using 0.0025 M H2SO4 for titrimetric determination of the diffused N, as was done in collecting the data reported in Tables 3 to 6, the lowest N concentration that can be detected will be approximately 5 mg N kg-1. If desired, sensitivity can be increased through use of a more dilute acid. The sharpness of the endpoint will be reduced, so titrations should be done using an automatic titrator, as described by Mulvaney (1996), Mulvaney et al. (1997a), and Khan et al. (1997).

Alternatively, sensitivity would be increased if direct diffusions could be performed on more than 1 g of soil. A potential problem arises with this approach, however, in that soil buffering capacity must be overcome with sufficient alkalinity to ensure complete liberation of NH3. Studies showed no need to increase the addition of MgO (beyond 0.2 g, as recommended for diffusion of soil extracts) in carrying out direct diffusions to recover exchangeable NH4–N from 2 g of soil, although a longer diffusion period was required, owing to the need for 20 mL of 2 M KCl to achieve a 1:10 ratio of soil to solution. Table 7 summarizes the results obtained in a study to evaluate the need for additional MgO when direct diffusions were performed using 5 g of soil and 50 mL of 2 M KCl. Regardless of the amount of MgO added to a neutral soil (Houghton), no difficulty was encountered in maintaining a pH above 10, and NH4–N analyses were in excellent agreement with the value obtained using a 1-g sample of soil (Table 3). In the case of a highly acidic tropical soil (Bejucal), 0.2 g of MgO was ineffective in raising the pH above 8, and serious underestimation occurred in recovering exchangeable NH4–N. The latter problem was eliminated by treating the sample with 0.6 g of MgO, and this practice is recommended whenever direct diffusions are performed on 5 g of soil.


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  		Table 7 Effects of different amounts of MgO in determining exchangeable NH4–N in 5 g of soil.{dagger}

 
Besides increasing the addition of MgO, a larger amount of Devarda's alloy must be employed to ensure quantitative recovery of NO3–N or NO2–N from 5 g of soil. This is illustrated by Table 8 , which shows the results of recovery tests involving direct diffusions with or without heating on a hot plate. An organic soil (Houghton) was used in these tests, based on previous work showing the need for a substantial increase in the addition of Devarda's alloy to recover NO3–N from a complex organic matrix (Mulvaney and Khan, 1999), which was attributed to the presence of reducible organic functional groups or to metal chelation. The results in Table 8 show 1.5 g of Devarda's alloy to be adequate when diffusions are performed at room temperature, whereas 0.9 g suffices when a hot plate is used, owing to an increase in the alloy's reactivity.


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  		Table 8 Effect of adding different amounts of Devarda's alloy in direct diffusion on recovery of NO3–N added to 5 g of Houghton soil.{dagger}

 
Further evidence for the accuracy of direct diffusion is provided by Table 9 , which compares the results obtained when analyses for (NH4 + NO3 + NO2)–N were performed using 1, 2, or 5 g of soil. When expressed per kilogram of soil, mean values were unaffected by an increase in sample size, whereas analytical precision improved. The latter finding can be attributed to a higher N content, which reduced the variability of titrations.


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  		Table 9 Comparison of inorganic-N analyses by direct diffusion for different sample sizes.{dagger}

 
Although direct-diffusion methods were developed primarily for quantitative determinations of inorganic N, these methods also allow isotope-ratio analysis of the diffused NH3–N and thereby have potential value for use in 15N-tracer investigations. To evaluate the accuracy and precision of 15N analyses by direct diffusion, replicate diffusions were performed from samples of soil that had been treated with a known amount of 15N-labeled (NH4)2SO4 or KNO3 after addition of 2 M KCl. The results are reported by Table 10 , which shows that the coefficient of variation (CV) for four replicate isotope-ratio analyses never exceeded 1% and was usually < 0.5%. The analyses were always accurate to within 3%, as compared to an expected value calculated by an isotope–dilution equation from the addition of labeled N and the sample's content of unlabeled NH4–N or (NH4 + NO3 + NO2)–N. Of particular interest is the complete absence of negative errors, which provides further evidence that, when performed as described, direct diffusions are not subject to interference by labile organic N.


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  		Table 10 Accuracy and precision of direct diffusion for 15N analysis of soil inorganic N.{dagger}

 
In our work, isotope-ratio analyses were performed with an automated Rittenberg system (Mulvaney et al., 1990, 1997b; Mulvaney and Liu, 1991), which requires at least 50 µg of NH4–N when samples are processed using the methanol treatment described by Mulvaney et al. (1997a). The direct-diffusion techniques described also may be employed for manual Rittenberg analyses, although a larger amount of NH4–N would probably be required. If necessary, samples having a low-N content can be treated with a known amount of unenriched (NH4)2SO4 after titration, in which case the isotopic composition of the diffused N would be determined by an isotope–dilution calculation (Hauck, 1982).

Care should be taken to ensure that the periods in Table 2 are not exceeded, particularly when heating is employed to accelerate diffusion, owing to the presence of a large quantity of organic N in the soil solid phase. This is illustrated by Fig. 1 , which shows that prolonging the period for direct diffusion on a hot plate led to an increase in analyses for (NH4 + NO3 + NO2)–N. This finding can probably be attributed to mineralization of soil organic N, since the increase was considerably larger with the Drummer, Fargo, and Houghton soils than with the Bloomfield soil, which has a much lower content of organic matter (Table 1).



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  		Fig. 1 Effect of prolonging the diffusion period on direct analysis of inorganic N in soil. Accelerated diffusions were performed in triplicate on 1 g of soil after addition of 10 mL of 2 M KCl, for 1.75 h (the recommended period) or for an additional 1, 2, or 4 h. The standard deviation for replicate analyses ranged from 0.22 to 1.59 mg N kg-1

 
In view of the longer diffusion period required, sequential diffusions should not be performed from a single sample to determine (NO3 + NO2)–N after recovering NH4–N, as may be done in the analysis of inorganic N in soil extracts (Khan et al., 1997; Mulvaney et al., 1997a). For the same reason, no attempt was made to develop direct-diffusion methods that use an acidified filter disk for automated 15N analysis of soil inorganic N (Khan et al., 1998).

If present, NO2–N may be eliminated by treating the sample with 1 mL of a 0.2 M solution of sulfamic acid and swirling the jar to mix the contents, before the addition of MgO and Devarda's alloy to recover (NH4 + NO3)–N (Khan et al., 1997; Mulvaney et al., 1997a). When this treatment is employed, NO3–N is determined as the difference between diffusions with and without Devarda's alloy; otherwise, the difference represents (NO3 + NO2)–N. The quantity of NO2–N is obtained from the difference between determinations of (NH4 + NO3 + NO2)–N and (NH4 + NO3)–N.

To ensure reliable results when the Mason-jar diffusion units are reused, thorough cleaning of all components is necessary, and this is particularly important when diffusions are being done for 15N analysis. The recommended cleaning procedures, and numerous other aspects concerning Mason-jar diffusion methodology, have been described in detail by Khan et al. (1997) and Mulvaney et al. (1997a).

Although all of our work was done using air-dried soil, the direct-diffusion approach can also be employed for inorganic-N analysis of field-moist soil. This will be a valuable option for reducing the risk of artifacts, which may result from chemical or microbial transformations during sample storage or air-drying (Mulvaney, 1996). Since direct diffusions do not require finely ground soil, analyses could be performed to determine the actual concentration of inorganic N at the time of sampling, and subsequently the results could be adjusted for soil moisture content.


   ACKNOWLEDGMENTS
 
We thank Dr. G. Gouveia for providing the Bejucal soil used in our work. Partial support for this research was obtained through a grant from the Fertilizer Research and Education Council.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 
This study was a part of Project ILLU-65-0371, Illinois Agric. Exp. Stn.

Received for publication August 26, 1999.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 




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The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome