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

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 (15)N at tracer level, for KCl extracts containing low amounts of NH(4)(+) and NO(3)(-) at natural (15)N abundance. We developed methods to quantify and correct errors affecting target delta(15)N 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 NH(4)(+) diffusions but considerable (7-13.4% of total sample N) in NO(3)(-) diffusions containing 50 mu g target N. Failure to correct for the isotopic values of contaminants, which we found to be depleted by 10 parts per thousand relative to target delta(15)N, in NO(3)(-) diffusions would have resulted in underestimating target delta(15)N by 0.8 to 1.6 parts per thousand, depending on the amounts of reagents used. We found that sample delta(15)N will be underestimated by approximately 0.2 parts per thousand (NH(4)(+) diffusions) and 0.26 parts per thousand (NO(3)(-) 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 (15)N at natural abundance if a precision or accuracy of <1.3 parts per thousand is not required.