Comparison of soil fungal/bacterial ratios in a pH gradient using physiological and PLFA-based techniques

We have compared the total microbial biomass and the fungal/bacterial ratio estimated using substrate-induced respiration (SIR) in combination with the selective inhibition technique and using the phospholipid fatty acid (PLFA) technique in a pH gradient (3.0-7.2) consisting of 53 mature broad-leaved forest soils. A fungal/bacterial biomass index using the PLFA technique was calculated using the PLFA 18:2w6,9 as an indicator of fungal biomass and the sum of 13 bacterial specific PLFAs as indicator of the bacterial biomass. Good linear correlation (p 0.8); for example the PLFAs 16:1w5 and 16:1w7c increased with increasing soil pH, while i16:0 and cy19:0 decreased. A principal component analysis of the total PLFA pattern gave a first component that was strongly correlated to soil pH (R-2 = 0.85, p < 0.001) indicating that the microbial community composition in these beech/beech-oak forest soils was to a large extent determined by soil pH