DICHROMATE DIGESTION AND SIMULTANEOUS COLORIMETRY OF MICROBIAL CARBONAND NITROGEN

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 (dura tion, 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-indop henol colorimetry for N detection, and compared N digestion efficiency with Kjeldahl digestion. Optimal digestion conditions were 144 degree s C internal temperature for 3 h with 2:1 H2SO4/H3PO4 and Ag2SO4, Auto mated and manual titrations were reliable but the titrant (ferrous amm onium sulfate) precluded N detection. Colorimetric detection of Cr(VI) with s-diphenylcarbazide was fast and precise, but high blanks and st eady 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 (Alnu s tenuifolia Nutt,), and poplar (Populas balsamifera L,) stands was lo w compared with Kjeldahl N (82, 79, 88, and 78%), but precision of the two digestions was the same. The detection limits were 25 mu g C and 2 mu g N per digestion (125 mg C and 10 mg N kg(-1) dry soil). While t his method is not suitable for work demanding high accuracy, automated C detection combined with N detection provides data acceptable for st udies comparing Geld or laboratory soil treatments.