A complete procedure for analysing soil and plant samples for total N and atom % excess15N is described. The salicylic acid version of the Kjeldahl method for measuring total N was modified for use in a digestion block, giving quantitative reduction of nitrate in both soil and plant material. Procedures for minimising cross-contamination between samples are specified, including a double-distillation procedure that eliminates ‘memory effects’ when distilling NH3 from Kjeldahl digests. A simple and robust apparatus for converting (NH4)2SO4 to N2 gas for mass spectrometric determination of atom % excess15N is described. The coefficient of variation for replicate measurements of total N in soil and plant material over the range 0.1–2.2% N was 1.0%. The coefficient of variation for measurements of15N in plant material over the range 0.4–2.9 atom % excess15N was 0.2%.
analytical errors determination of N in plants determination of N in soils determination of15N in plants determination of15N in soils isotope mass spectrometry Kjeldahl method