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This article in SSSAJ

  1. Vol. 57 No. 4, p. 1007-1012
     
    Received: Aug 6, 1992
    Published: July, 1993


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doi:10.2136/sssaj1993.03615995005700040021x

Alkaline Persulfate Oxidation for Determining Total Nitrogen in Microbial Biomass Extracts

  1. M. L. Cabrera  and
  2. M. H. Beare
  1. Dep. of Agronomy and Institute of Ecology
    Institute of Ecology, Univ. of Georgia, Athens, GA 30602

Abstract

Abstract

The CHCl3 fumigation-extraction method for determining soil microbial N involves the extraction of CHCl3-fumigated and CHCl3-unfumigated samples with 0.5 M K2SO4, followed by determination of total N in the extracts. Total N is typically determined by Kjeldahl digestion (TKN) with the inclusion of NO2 and NO3 present. The determination of total N by TKN has several procedural constraints such as an initial concentration step, long digestion times, and considerable bumping caused by catalyst salts. In contrast, a persulfate oxidation method (TPN) commonly used for determination of total N in sea- and freshwater samples does not require a concentration step and is much simpler and faster than TKN. The objective of this study was to evaluate and adapt the TPN method to measure total N in 0.5 M K2SO4 extracts for determination of soil microbial biomass N. Studies were conducted to optimize reagent/sample (R/S) ratio and autoclaving time, and to evaluate the effects of C and N concentrations. These studies showed that complete N recovery can be obtained with a R/S ratio of 1 and an autoclaving time of 30 min in extracts containing as much as 250 mg glucose-C L−1 at a C/N ratio of 10:1. A comparison of the optimized TPN method to TKN for extracts of 12 CHCL3-fumigated soils indicated that the methods gave the same N recoveries. The use of the TPN method in 0.5 M K2SO4 extracts should facilitate more rapid and efficient measurements of soil microbial biomass N.

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