Microbial biomass C was measured by the fumigation-extraction method (Wu et al., 1990). Briefly,
the fresh soil samples were carefully dried to 60% of water holding capacity in the dark, passed through
a 2-mm sieve, and incubated for two weeks at 25 ◦C. The moist sub-samples (15 g dry weight equivalent)
were fumigated for 24 h with ethanol-free CHCl3 (Jenkinson and Powlson, 1976). The soluble organic
C was then extracted from the fumigated and the unfumigated samples with 60 mL 0.5 mol L−1 K2SO4
for 30 min on an end-over-end shaker. The extracts were filtered through 0.45-μm filters and frozen at
−20 ◦C prior to the analysis for organic C with a total organic carbon analyzer (Phoenix-8000, Tekmar-
Dohrmann, NY). The microbial biomass C was calculated from the difference between dissolved organic
C in the fumigated and the unfumigated samples using a conversion factor of 0.45. All results were
expressed on an oven-dried soil basis and were the mean of four replicates.