Optical emission spectroscopy provides a rapid and precise method for determining nitrogen (N) as the15N/14N ratios of15N‐enriched plant and/or soil samples. The objective of this study was to test whether15N/14N ratios of15N‐enriched soil samples could be determined using direct combustion of KCl extracts without distillation and titration procedures. Twenty soil samples with ranges of mineral N concentration (10 to 43 μg g‐1of dry soil) and15N atom % enrichment (a.e.) (0.37 to 1.10%) were tested. Our data indicate that soluble organic N in the KCl extract contained lower and more variable15N% a.e. levels than those of the mineral N (NO3‐and NH4+) fractions. Thus, direct combustion of KCl extract is not a reliable method for determining15N% a.e. in the soil mineral N pool.