Interactive effects of pH and substrate quality on the fungal-to-bacterial ratio and QCO2 of microbial communities in forest soils

Microbiological and chemical properties of 40 North German spruce (Picea abies [L] K.) and beech (Fagus sylvatica L.) stands at either acidic (similar to 3.0) or neutral (similar to 6.0) soil pH (A(h) horizon) were investigated to test the effects of pH, substrate type (forest type) and the substrate x pH interactions on soil microbial communities. Soils of each pH set were selected to follow a normal distribution. The percent fungal-to-bacterial respiratory ratio, as determined by the selective inhibition method, was significantly affected by type of forest substrate x pH interactions (P < 0.001). Soil pH influenced the percent fungal-to-bacterial ratio to a greater degree than substrate and the effect of pH was more pronounced under beech than under spruce. Mean percent fungal-to-bacterial respiratory ratios (rounded) of 74-to-26 (beech) and 84-to-16 (spruce) were found at high pH (similar to 6) and ratios of 94-to-6 (beech) and 90-to-10 (spruce) were found at low pH (similar to 3.0), respectively. The metabolic quotient qCO(2) was strongly affected by soil pH (P < 0.001). Also, a strong substrate x pH effect was noted (P < 0 002). Particularly at low pH the qCO(2) was higher under spruce as compared to beech. A relationship emerged between the qCO(2) and the fungal-to-bacterial ratio. With increasing fungal presence the qCO(2) declined. The C-mic- to-C-org ratio was only affected by pH (p < 0.001). (C) 1998 Elsevier Science Ltd. All rights reserved.