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NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

Suitability of the Diffusion Method for Natural Abundance Nitrogen-15 Analysis

^a Dep. of Forest Resources, Univ. of Idaho, P.O. Box 441133, Moscow, ID 83844-1133
^b current address: Dep. of Life and Physical Sciences, Lincoln Univ., 816 Chestnut St., Jefferson City, MO 65101

^* Corresponding author (stephank{at}lincolnu.edu).

Knowing the isotopic signature of inorganic N in soil is an important component in understanding N cycling processes. We tested the suitability of the diffusion method, a relatively simple and inexpensive method commonly applied to samples containing ¹⁵N at tracer level, for KCl extracts containing low amounts of NH₄⁺ and NO₃^– at natural ¹⁵N abundance. We developed methods to quantify and correct errors affecting target ¹⁵N values resulting from N contamination in reagents and incomplete N recovery from KCl solution. After correcting for these sources of error, we assessed precision and accuracy. We found contamination in reagents to be negligible (0.8–2% of total sample N; sample N = target N + contaminant N) for NH₄⁺ diffusions but considerable (7–13.4% of total sample N) in NO₃^– diffusions containing 50 µg target N. Failure to correct for the isotopic values of contaminants, which we found to be depleted by 10 relative to target ¹⁵N, in NO₃^– diffusions would have resulted in underestimating target ¹⁵N by 0.8 to 1.6, depending on the amounts of reagents used. We found that sample ¹⁵N will be underestimated by approximately 0.2 (NH₄⁺ diffusions) and 0.26 (NO₃^– diffusions) for every 1% short of complete recovery of sample N. After correcting for reagent N contamination and incomplete recovery, we found the precision and accuracy (1 SD) of the diffusion procedure to range from 0.2 and 1.3%. We conclude that the diffusion method can be applied to KCl extracts containing ¹⁵N at natural abundance if a precision or accuracy of <1.3 is not required.

Abbreviations: CI, confidence interval • DA, Devarda's alloy • DI, deionized • IRMS, isotope ratio mass spectrometer • ISIL, Idaho Stable Isotope Laboratory