Evaluation of methods for nitrogen‐15 analysis of inorganic nitrogen in soil extracts: I. Steam‐distillation methods

Abstract

A study was conducted to evaluate conventional steam‐distillation techniques for N‐isotope analysis of inorganic forms of N in soil extracts. Extracts obtained with 2 M KCl from 10 diverse soils were treated with: (i) (¹⁵NH₄)₂SO₄ and KNO₃, (ii) (NH₄)₂SO₄ and K¹⁵NO₃, or (iii) KNO₃and Na¹⁵NO₂. Steam distillations were performed sequentially to determine NH₄ ⁺‐N and NO₃ ^‐‐N, and were also carried out to determine (NO₃ ^‐ + NO₂ ^‐)‐N or (NH₄ ⁺ + NO₃ ^‐ + NO₂ ^‐)‐N; a pretreatment with sulfamic acid was used to determine NO₃ ^‐‐N in the presence of NO₂ ^‐‐N. Recovery of added N ranged from 95 to 102%. Significant isotopic contamination was observed in sequential distillation of unlabeled NO₃ ^‐‐N following labeled NH₄ ⁺‐N; otherwise, analyses for ¹⁵N were usually within 1% of the values calculated by isotope‐dilution equations.