Soil Science Society of America Journal 63:1667-1669 (1999)
© 1999 Soil Science Society of America
DIVISION S-2-SOIL CHEMISTRY
Measurement of fixed ammonium and nitrogen isotope ratios using dry combustion
B.C. Lianga,
A.F. Mackenzieb and
E.G. Gregorichc
a Semiarid Prairie Agric. Research Centre, Agriculture and Agri-Food Canada, P.O. Box 1030, Swift Current, SK, Canada, S9H 3X2
b Dep. of Natural Resource Sci., Macdonald Campus of McGill University, 21,111 Lakeshore Road, Ste. Anne de Bellevue, QC, Canada, H9X 3V9
c Eastern Cereal and Oilseed Research Centre, Agriculture and Agri-Food Canada, Ottawa, ON, Canada, K1A 0C6
liangb{at}em.agr.ca
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ABSTRACT
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Clay-fixed NH4–N can represent up to 18 percent of the total soil N for Canadian agricultural soils. A modification of the Silva and Bremner's method for determining clay-fixed NH4 was developed. The Silva and Bremner procedure incorporates dissolution of mineral residues by a HF–HCl digestion in order to release bound NH4, following removal of exchangeable NH4 and organic N. Steam distillation is used to recover NH4 from the soil digest. The procedure presented in this paper substitutes combustion in an automated elemental analyzer for the HF–HCl digestion step. Combustion with an elemental analyzer follows removal of exchangeable NH4 with 0.5 M KCl and organic N with wet digestion using KOBr. The elimination of the HF–HCl digestion and steam distillation steps shortens the time required for analysis and avoids the use of strong reagents. The procedure is also suitable for 15N studies when combined with a mass spectrometer. The two techniques gave virtually the same results for 17 soils with the slope of the regression of 1.03 and a coefficient of determination of 0.97. Simultaneous measurements of fixed NH4 and N isotope ratios using continuous flow mass spectrometry can be carried out.
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INTRODUCTION
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CLAY-FIXED NH4 at concentrations of a few to several hundred mg N kg-1 have been reported for Canadian soils (Sowden et al., 1978), which indicates that this form of N is an important component of N in soil. A number of methods have been developed and used to determine clay-fixed NH4 (Young and Aldag, 1982), but procedures are complex and usually involve two stages: (i) removal of organic N compounds from the soil, followed by (ii) extraction of fixed NH4 using acid dissolution of the mineral particles and subsequent quantitative determination of NH4.
The most widely accepted method for determining fixed NH4 is that of Silva and Bremner (1966). In this method organic N and exchangeable NH4 are removed by oxidation with potassium hypobromide (KOBr). The mineral residue is dissolved by HF–HCl digestion and the liberated NH4 is recovered by steam distillation with KOH. This procedure is laborious and time-consuming. Additional steps are also required before N isotopic ratios of fixed NH4 can be determined (Hauck, 1982). The procedure could be improved through the use of automated elemental analyzer of N. Combination of an elemental analyzer with a mass spectrometer should permit simultaneous measurements of N and its isotopic composition. The objective of this study was to evaluate whether an elemental analyzer could be used as a replacement for the acid digestion step in the Silva and Bremner (1966) method for the simultaneous measurement of fixed NH4 and its isotopic ratio.
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Materials and Methods
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Soil Sample Characteristics
Soil samples in the surface plow layer (0–20 cm) were collected from research farms in the provinces of Ontario and Quebec, and from farm fields in the province of Saskatchewan (Jalil et al. 1996). The soils varied in cropping history and management, soil pH, organic C and N content, and texture (Table 1) . The soil samples were air-dried, passed through a 2-mm sieve, and ground to pass through a 0.25-mm sieve prior to analysis.
Measurement of Fixed Ammonium by Two Methods
For each analysis of fixed NH4 approximately 1 g of soil was weighed in a tall beaker, treated with KOBr, washed with 0.5 M KCl, and centrifuged in a polyethelene centrifuge tube to remove soil mineral N (i.e., NO3 and exchangeable NH+4) and organic N (Silva and Bremner, 1966). After the removal of mineral and organic N, fixed NH4 was determined in two ways: (i) The soil was subjected to the HF–HCl digestion, distilled through steam distillation with KOH, and the amount of fixed NH4 was then determined volumetrically (Silva and Bremner, 1966). Nitrogen isotopic ratios of the distillate were determined using a RoboPrep tracer mass spectrometer (Europa Scientific, Crew, UK). (ii) The KOBr-treated residue was air-dried, and 
50 mg of soil sample was weighed in a capsule, and the total N and N ratios determined using continuous flow mass spectrometry on an elemental analyzer in-line with a mass spectrometer. All analyses were replicated at least three times.
Evaluation of Release of Fixed Ammonium
To verify the complete release of the fixed NH4 from clay minerals after combustion, soil samples were combusted in a furnace for 45 min. at 1030°C, the same operating temperature used for the elemental C and N analyses. The sample was then subjected to the HF–HCl digestion, followed by steam distillation and analysis of NH4. Four soils, Fox loamy sand and Chicot sandy clay loam (both in the Typic Hapludalf family), Brandon loam (Typic Halpludoll), and Ste. Rosalie clay (Typic Humaquept), were used in order to provide a wide range in clay content.
Incubation Procedures with Ammonium Sulfate
Nine different soils were incubated with NH4SO4 to measure the degree of exchange with fixed ammonium. The incubation procedures were as described by Duxbury et al. (1991). Briefly, 50 g (oven-dry basis, passed through a 2-mm sieve) of air-dried soil was mixed thoroughly with a 20 mg N kg-1 of 69.2% 15N enrichment as NH4SO4 and 200 mg C kg-1 as glucose, both of which were added to the soil in solution. The soil moisture content at the beginning of the incubation for each soil was 80% of water holding capacity. The incubation was carried out in a 1-L air-tight plastic container at 23°C in the dark. The containers were open briefly to replenish the O2 once a week. After 7 wk, soil samples were air-dried, and passed through a 0.25-mm sieve for analyses of fixed NH4 and N isotopic ratios.
Statistical analyses
Statistical analysis was performed using procedures of the Statistical Analysis System (SAS Inst., 1990).
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Results and Discussion
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Effectiveness of Elemental Analysis in Removing Fixed Ammonium
No measurable amount of N remained in subsamples of the four soils selected to test the recovery of clay-fixed NH4 using the elemental analyzer. It was therefore concluded that use of the elemental analyzer results in 100% recovery of fixed NH4 in the mineral fraction of the soil.
Comparison of Methods
The amounts of fixed NH4 determined using the elemental analyzer were correlated with those determined using the Silva and Bremner method (Fig. 1)
. The intercept was not statistically different from zero, and the slope was near 1. Thus, the modified procedure produces the same results as the original for the range of values tested.
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Fig. 1 Comparison of the combustion method with the digestion–distillation method for the determination of fixed ammonium
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The amount of fixed NH4 varied from 0 to 350 mg N kg-1 (Table 1), or from 0 to 18% of the soil total N. These values are consistent with other reports in the same regions in Canada (Drury et al., 1989; Chen and MacKenzie, 1992; Liang and MacKenzie, 1994). Nitrogen isotope ratios of fixed NH4 determined either by the method of Silva and Bremner or by dry combustion after removal of inorganic and organic N were virtually the same, with a correlation of 0.99 (Table 2)
. The precision of isotopic N analysis by elemental analysis was better than that determined by the Silva and Bremner method, as shown by the smaller coefficients of variation obtained with the dry combustion method. The mean value of the coefficient of variations for the dry combustion method was 2.0%, and for the Silva and Bremner method the mean value was 6.4%.
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Table 2 Nitrogen-15 enrichment of fixed ammonium as determined by oxidation of organic and mineral N with KOBr, followed by either dry combustion or HF–HCl digestion, and the percentage of added 15N recovered in fixed ammonium
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Although the dry combustion method still requires the wet oxidation procedure for the removal of organic N, which uses KOBr followed by extraction of inorganic N by KCl, the determination of fixed NH4 can be done directly by combustion on an automated elemental analyzer. The elimination of the HF–HCl digestion and steam distillation not only avoids the use of strong reagents, but it also shortens the time required for analysis considerably. This procedure allows simultaneous measurements of fixed NH4 and N isotopic ratios through the use of elemental analysis combined with continuous flow mass spectrometry.SAS Institute 1990
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ACKNOWLEDGMENTS
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We thank C.A. Campbell for providing some of the soil samples. Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada through a collaborative research grant.
Received for publication May 8, 1998.
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REFERENCES
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