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Dichromate Digestion and Simultaneous Colorimetry of Microbial Carbon and Nitrogen

Doyle, Allen, Schimel, Joshua P.
Soil Science Society of America journal 1998 v.62 no.4 pp. 937-941
colorimetry, nitrogen, soil microorganisms, carbon, biomass
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.