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ABSTRACT
Sequential diffusion techniques used to speciate inorganic nitrogen-15 (15N) during soil or water analysis are complicated by incomplete recovery of ammonium (NH4+)-N, introducing error in the subsequent determination of nitrate (NO3–)-N. Based on studies to evaluate different strategies for minimizing cross-contamination error in Mason-jar diffusions, a simple cleaning technique was developed that involves an additional 6-h diffusion using 0.6 M boric acid (H3BO3) at room temperature following the recovery of NH4+-N. This technique was 60–87% effective for reducing cross-contamination of unlabeled NO3–-N by labeled NH4+-N and became more effective for controlling analytical error with decreasing sample volumes, lower NH4+-N enrichment, and larger quantities of NO3–-N. When used with the cleaning technique described, sequential diffusions were far superior for 15N analysis of NO3–-N, as compared to the nonsequential approach that involves an isotope dilution calculation after separate diffusions to determine NH4+-N and total mineral N.