DETERMINATION OF NITROGEN BY MICRODIFFUSION IN MASON JARS .2. INORGANIC N-15 IN SOIL EXTRACTS

Simple microdiffusion methods were evaluated for N-isotope analysis of NH4+, NO3(-), and NO2- in soil extracts. In these methods, the soil extract is treated with MgO or MgO and Devarda's alloy in a 473-mL (1-pint) wide-mouth Mason jar, the NH3 liberated being collected in a Petri dish containing 3 mt of boric acid-indicator solution, or in an acidified glass-fiber disk, suspended from the Mason-jar lid. To avoid cross-contamination error from retention of N-15, the lid was immersed in 0.05 M KOH for at least 24 h before reuse. Serious error occurred when NO3--N was diffused following NH4+-N; therefore, separate diffusions should be performed for NH4+-N and (NH4+ + NO3-)-N or (NH4++ NO3- + NO2-)-N. In studies to evaluate the latter approach using labeled soil extracts, analyses for N-15 were within 2% of the corresponding values calculated by isotope-dilution equations. No interference by alkali-labile organic N was observed in diffusion of N-15-labeled extracts from soil receiving various treatments to increase soluble organic N.