Measurement of dehydrogenase activity in acid soils rich in organic matter

Dehydrogenase activity can be considered to be a good measure of microbial oxidative activity in soils. It is usually determined by measuring the amount of an artificial electron acceptor reduced by microbial activity, such as a soluble tetrazolium salt with a red colored reduced form (a formazan) that can be determined colorimetrically following extraction with a suitable solvent. In an earlier study of acid organic-matter rich forest soils of Galicia (N.W. Spain), measured dehydrogenase activities were low, at variance with respiratory activity data indicating high biological activity. To investigate the possibility that these low dehydrogenase activities were underestimated due to adsorption of the formazan, we have studied the interaction of six soils with iodonitotetrazolium formazan (INTF). At the same time, the capacities of two extractants, methanol and 1:1 dimethylformamide-ethanol (DMF-ethanol), to extract INTF were compared. The results show that INTF is adsorbed by the soils studied with an intensity that closely correlates with soil carbon content, and that dehydrogenase activity is thus underestimated to a different degree for each soil. A mixture of 1:1 DMF-ethanol was more effective than methanol in extracting INTF, thereby improving estimates of dehydrogenase activity. Correction for the effects of INTF adsorption could be achieved by using reference standards containing soil to construct a separate calibration curve for each soil. These standards were prepared by incorporating different concentrations of INTF with the soil under the same conditions used for determination of the dehydrogenase activity. The use of DMF-ethanol and reference standards containing soil is thus recommended for determination of dehydrogenase activity, at least in soils with similar properties to those studied here.