Dichromate Digestion-Spectrophotometric Procedure for Determination of Soil Microbial Biomass Carbon in Association with Fumigation-Extraction

A dichromate digestion and spectrophotometric procedure is proposed for estimating soil microbial biomass carbon (C) in association with fumigation-extraction. The recommended procedure uses a volume (1.6 ml) of 0.5 M potassium sulfate (K2SO4) soil extracts and oxidant solution (dichromate-sulfuric acid, 2.4 mL), mixed with a volume (4 mL) for digestion at 140 degrees C for 30 min. The digested solution is then directly read for absorbance at 350 nm using a spectrophotometer, and the C in the digested soil extracts is measured against glucose standards. The K-EC (indicating the extractable part of microbial biomass C after fumigation) value is estimated as 0.33 for the proposed method. There are good correlations between soil microbial biomass C measured by the proposed method, the dichromate digestion titration, and oven oxidation by total organic C (TOC) analytical method. This method is a simple, rapid, and economical procedure associated with fumigation-extraction for biomass C analysis.