Diffusion technique for 15 N and inorganic N analysis of low‐N aqueous solutions and Kjeldahl digests

Diffusion of ammonia is a common sample preparation method for the stable isotope analysis of inorganic nitrogen in aqueous solution. Classical diffusion methods usually require 6–12 days of diffusion and often focus on 15 N/ 14 N analysis only. More recent studies have discussed whether complete N recovery was necessary for the precise analysis of stable N isotope ratios. In this paper we present a newly revised diffusion technique that allows correct and simultaneous determination of total N and 15 N at% from aqueous solutions and Kjeldahl digests, with N concentrations down to sub‐0.5‐mg N L −1 levels, and it is tested under different conditions of 15 N isotope labelling. With the modification described, the diffusion time was reduced to 72 h, while the ratios of measured and expected 15 N at% were greater than 99% and the simultaneous recovery of total N was >95%. Analysis of soil microbial biomass N and its 15 N/ 14 N ratio is one of the most important applications of this diffusion technique. An experiment with soil extracts spiked with 15 N‐labelled yeast showed that predigestion was necessary to prevent serious N loss during Kjeldahl digestion of aqueous samples (i.e. soil extracts). The whole method of soil microbial biomass N preparation for 15 N/ 14 N analysis included chloroform fumigation, predigestion, Kjeldahl digestion and diffusion. An experiment with soil spiked with 15 N‐labelled yeast was carried out to evaluate the method. Results showed a highly significant correlation of recovered and added N, with the same recovery rate (0.21) of both total N and 15 N. A k N value of 0.25 was obtained based on the data. In conclusion, the diffusion method works for soil extracts and microbial biomass N determination and hence could be useful in many types of soil/water studies. Copyright © 2008 John Wiley & Sons, Ltd.