Cavity ring-down spectroscopy (CRDS) provides a low-cost alternative to isotope ratio mass spectrometry (IRMS) for the analysis of ¹⁵N-NO₃⁻ in nitrogen cycle assays, but its performance has not been tested systematically for highly enriched samples. Here we report results from 271 parallel analyses of ¹⁵N-NO₃⁻ in soil extracts after chemical conversion to N₂O, including low (0.36–1.45 at%) and high labelled samples (0.46–8.49 at%). We demonstrate that CRDS measurements deliver comparable accuracy and precision to continuous-flow Gas Bench-IRMS analysis (slope: 0.982 and 0.972, RMSE: 0.0018 and 0.254 at% for low- and high-labelled samples, respectively), thereby offering a robust and affordable alternative for classic ¹⁵N-NO₃⁻ analysis.