Microbial biomass (Cmic) was determined by the SIR method (Anderson and Domsch 1978) that is based on the measurement of CO2 evolution from soil in response to addition of glucose, an easily mineralizable substrate. The magnitude of the respiratory response, as measured after incubation under controlled temperature and humidity conditions, is related to the amount of active biomass in the soil sample, and can be converted to mg of microbial biomass carbon using a conversion factor introduced by Sparling (1995):
Cmic(mg C g-1d.w.) = 50.4 x respiration rate (ml CO2 g-1d.w.h-1)
Microbial biomass C was measured by mixing in 30 ml vials 1 g of each soil sample (sieved through a 2-mm mesh) with 2 ml of 75 mM D-glucose (27.3 mg g-1 soil d.w.). The vials were then sealed tightly and incubated for 4 h in the dark at 25°C. The evolution of CO2 was measured by gas chromatography (Fisons GC 8000 series). The CO2 values were corrected for the CO2 measured in a blanc vial containing only the soil sample and 2 ml of water, and were reported as mg of microbial carbon.
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