Comparison of chloroform fumigation-extraction, phospholipid fatty acid, and DNA methods to determine microbial biomass in forest humus

Techniques developed to measure microbial biomass in mineral soils may not give reliable results in humus. We evaluated the relationships between three techniques to estimate microbial biomass in forest humus: chloroform fumigation-extraction (CFE), total extractable phospholipid fatty acids (PLFA), and extractable DNA. There was a good relationship between PLFA and CFE (R2=0.96), with a slope slightly different from that previously reported for mineral soils (1 nmol PLFA corresponded to a flush of 3.2 μg C released by fumigation in humus cf. 2.4 μg C in mineral soil). There was no relationship between DNA concentration and the other two measurements of microbial biomass. This may be due, in part, to the high fungal biomass in forest humus, as DNA concentration per unit biomass is much more variable for fungi than bacteria.