Response of the soil microbial biomass to glucose, and selective inhibitors, across a soil moisture gradient

Methods for investigating the glucose-sensitive component of the soil microbial biomass were applied to soils held at five separate soil moisture contents (i.e. 15,25, 35,45 or 55%) following incubation for 6 h and 3, 10 or 30 days. The O2 uptake method proposed by Van de Werf and Verstraete (1987a, b) for measuring “active” (glucose-sensitive) microbial biomass predicted that this biomass increased with increased moisture status, except for samples that had been previously incubated for 30 days. However, measurements of the glucose-sensitive microbial biomass using total O2 uptake over 10 h following glucose addition [as proposed by Van de Werf and Vestraete (1987a, b)] appears to be influenced in pan by decomposition of microbial biomass killed by prior incubation at low moisture contents, especially between hours 4 and 10. It is proposed that the total O2 uptake in the first 2–4 h following glucose addition is a superior indicator of the response of the soil microbial biomass to soil moisture. Substrate-induced respiration (SIR) and selective inhibition are also proposed for studying the glucose-responsive component of the soil microbial biomass. Selective inhibition checks microbial protein synthesis of this glucosesensitive component. The inhibition of SIR by either bacterial or fungal selective inhibitors was largely independent of soil moisture content when samples were remoistened simultaneously with sample amendment, but was strongly related with soil moisture when samples were not remoistened. The bacterial-fungal ratios derived from these data were typically unaffected by soil moisture.