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Dichromate Digestion and Simultaneous Colorimetry of Microbial Carbon and Nitrogen

Dep. of Ecology, Evolution and Marine Biology, Univ. of California, Santa Barbara, CA 93106

*Corresponding author (fnapd{at}uaf.edu).

ABSTRACT

Dichromate digestion is used frequently for analysis of organic C and is followed by manual titration. We sought to automate C detection and to include N in the analysis. We optimized digestion parameters (duration, temperature, acids, and catalysts), compared detection methods [manual and automated titration, colorimetric absorbance of Cr(III) and Cr(VI) and automated colorimetry of Cr(VI)], adapted salicylate-indophenol colorimetry for N detection, and compared N digestion efficiency with Kjeldahl digestion. Optimal digestion conditions were 144°C internal temperature for 3 h with 2:1 H₂SO₄/H₃PO₄ and Ag₂SO₄. Automated and manual titrations were reliable but the titrant (ferrous ammonium sulfate) precluded N detection. Colorimetric detection of Cr(VI) with s-diphenylcarbazide was fast and precise, but high blanks and steady decomposition of Cr(VI) necessitated several internal standards. Colorimetric analysis of N was possible after precipitating Ag and it was stable, precise, and accurate. Digestion recovery of yeast extract and soil extract N from birch (Betula papyrifera Marsh.), alder (Alnus tenuifolia Nutt.), and poplar (Populus balsamifera L.) stands was low compared with Kjeldahl N (82, 79, 88, and 78%), but precision of the two digestions was the same. The detection limits were 25 µg C and 2 µg N per digestion (125 mg C and 10 mg N kg^-1 dry soil). While this method is not suitable for work demanding high accuracy, automated C detection combined with N detection provides data acceptable for studies comparing field or laboratory soil treatments.

This work was carried out at the Inst. of Arctic Biology, Univ. of Alaska, Fairbanks, AK 99775.

Received for publication June 2, 1997.

This article has been cited by other articles:

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