Conventional steam‐distillation techniques for concentrating inorganic N in soil extracts prior to 15 N analysis are time‐consuming and labor‐intensive procedures. Diffusion procedures have been proposed as alternatives to steam distillation, but incomplete recovery and isotope dilution have been reported. We evaluated a procedure for diffusing low masses of 15 NH 4 ‐N from 2 M KCl solution to determine the effects of NH 4 and 15 N concentrations on recovery and isotope‐ratio analysis of diffused N. A series of solutions were prepared containing from 0.5 to 2.5 mg N L −1 at 1.1461 atom % 15 N. Forty‐milliliter aliquots were placed in plastic containers, equipped with acidified glass‐fiber disks, and allowed to diffuse for either 6 d at room temperature or 5 d at 55 °C. Nitrogen recovery in glassfiber disks averaged 100.5% of the added N. Quantitative recovery of added N was also obtained in a second study in which 30‐mL aliquots of standard solutions containing from 25 to 100 µg N at 15 N concentrations varying from 0.1030 to 1.1461 atom % 15 N were diffused for 10 d at 55 °C. Significant dilution of 15 N concentration for low‐mass N samples occurred. Differences in 15 N concentrations between diffused and nondiffused samples were attributed to background N at a 15 N concentration of natural abundance. Close agreement was obtained between diffused and nondiffused samples when an isotope‐dilution equation was used to calculate the atom % 15 N concentration of the sample N. These studies indicate that the diffusion method is suitable for preparing low‐mass N samples for 15 N analysis if appropriate blanks are determined and used to correct for background N.
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Kelley et al. (1991) studied this question.