Evaluation of a Diffusion Method for Preparing Low‐Nitrogen Samples for Nitrogen‐15 Analysis

Ammonia diffusion techniques have been advocated to eliminate crosscontamination problems associated with distillation methods for concentrating low-N-content samples for N analysis. Two potential problems with diffusion are nonquantitative N recovery and isotopic fractionation. These are most likely to have significant effects for low-N-content samples. We evaluated the ability of a diffusion method to accurately prepare low-N (<1000 ug N), high-volume (55 mL) simulated acidic digest samples and quantified sinks for N in the diffusion system. This diffusion system trapped N on an acid-impregnated glass fiber disk suspended above the sample solution. The N/ N isotopic ratio of N recovered on the disk declined linearly as the quantity of N in the original sample solution decreased below 500 ug N. Nitrogen-15 values adjusted for contamination detected in reagent blanks still differed significantly from nondiffused reagent blanks. Significant sinks of N other than the acidified disk existed despite efforts to minimize them. The method produced unexplained changes in isotopic composition of the sample. These results emphasize the importance of thoroughly evaluating any NH3 diffusion technique with representative sample types in each laboratory before adoption of the technique for routine use. In addition, we recommend standard curves as a more robust method for adjusting N values for errors associated with diffusion. T USE OF DIFFUSION TECHNIQUES to prepare samples for determination of N concentration has recently been encouraged (O'Deen and Porter, 1979; Turner and Bergersen, 1980; MacKown et al., 1987; Brooks et al., 1989; Kelley et al., 1991; Liu and Mulvaney, 1992). Diffusion techniques are attractive because they eliminate crosscontamination and require less operator skill and time than sample preparation by distillation (Hauck, 1982). Conway (1957) outlined fundamental principles of diffusion techniques. Diffusion rates increase with increased pH, increased temperature, reduced depth of the sample solution, increased surface area of the sample solution, and increased ionic strength of specific ion's (e.g., K, Na, COs, and OH) in the sample solution. Conway (1957) noted that the rate-limiting processes in a diffusion system are movement of NHa through the liquid sample and movement across the gaseous layer above the sample. Diffusion analysis of N samples is further complicated by the sensitivity of the isotope technique to contamination (Huser, 1966) or any process that causes isotopic fractionation (Hauck, 1982). Various physical arrangements have been proposed for diffusion systems. The diffusion system we evaluated J.A. Lory, USDA-ARS, 119 Keim Hall-East Campus, Univ. of Nebraska, Lincoln, NE 68583; and M.P. Russelle, USDA-ARS and Dep. of Soil Science, 439 Borlaug Hall, Univ. of Minnesota, St. Paul, MN 55108. Joint contribution of the USDA-ARS Plant Science Research Unit, U.S. Dairy Forage Research Center (Minnesota Cluster), and Minnesota Agric. Exp. Stn. Paper no. 20552 of the scientific journal series. Received 7 June 1993. "Corresponding author (jlory@unl.edu). Published in Soil Sci. Soc. Am. J. 58:1400-1404 (1994). was described by Brooks et al. (1989). They reported success concentrating N from 2 M KC1 soil extracts using Devarda's alloy and MgO to reduce NOa and liberate NHs. They tested sample masses between 70 and 508 ng N and recommended using the system for sample volumes up to 60 mL. They suggested that the system be tested if a different matrix is to be used. Subsequent work has demonstrated that N values determined with this system deviate from expected values (Kelley et al., 1991; Liu and Mulvaney, 1992). These latter authors reported success adjusting diffused N values based on contamination detected on diffused reagent blanks. Our objective was to test the method of Brooks et al. (1989) for low-N (<1000 ug N), high-volume (55 mL) simulated Kjeldahl digest samples. The nature of these samples makes them susceptible to the problems associated with the diffusion process and isotope analysis. Low-N, high-volume samples are encountered in soil percolates or solutions, Kjeldahl digests of low-N-content soils, and Kjeldahl digests of small unique samples, such as rhizosphere soil, plant tissues, or plant extracts. METHODS AND MATERIALS