Comparative assessment of soil microbial biomass determined by the methods of direct microscopy and substrate-induced respiration

The content of microbial biomass (MB) was determined in samples of gray forest, chestnut, and tundra soils with different physicochemical properties (0.4-22.7% C-org; 8.4-26.8% silt particles; pH 4.3-8.4) by the methods of substrate-induced respiration (MBSIR) and direct microscopy (MBM). The samples of two upper soil layers, 0-5 and 5-10 cm (without plant litter), from different ecosystems (forest, forest shelter belt, meadow, fallow, and arable) and elements of relief of interfluvial tundra (block/upper land plateau, depression between blocks) have been analyzed. The content of microbial biomass in the 0-5-cm soil layer was 216-8134 and 348-7513 mu g C/g soil as measured by the methods of substrate-induced respiration and direct microscopy, respectively. The MBSIR and MBM values closely correlated with each other: r = 0.90 and 0.74 for 0-5 and 5 10 cm, respectively. The average MBSIR/MBM ratio was 90 and 60% for 0-5 and 5-10 cm, respectively. The portion of microbial carbon in total organic soil carbon was, on average, 4 and 3% (SIR) and 5 and 7% (direct microscopy) for 0-5 and 5-10 cm, respectively. Possible reasons for the differences between MBSIR and MBM values in the soils under study are discussed.