Suitability of the Diffusion Method for Natural Abundance Nitrogen-15 Analysis
- Kirsten Stephan *ab and
- Kathleen L. Kavanagha
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 δ15N 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 δ15N, in NO3 − diffusions would have resulted in underestimating target δ15N by 0.8 to 1.6‰, depending on the amounts of reagents used. We found that sample δ15N 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.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
Copyright © 2009. Soil Science Society of America