Distribution of microbial biomass and phospholipid fatty acids in Podzol profiles under coniferous forest

Microbial-derived phospholipid fatty acids (PLFAs) can be used to character ize the microbial communities in soil without the need to isolate individua l fungi and bacteria. They have been used to assess microbial communities o f humus layers under coniferous forest, but nothing is known of their distr ibution in the deeper soil. To investigate the vertical distribution we sam pled nine Podzol profiles on a 100-m-long transect in a coniferous forest a nd analysed for their microbial biomass and PLFA pattern to a depth of 0.4 m. The transect covered a fertility gradient from Vaccinium vitis-idaea for est site type to Vaccinium myrtillus forest site type. The cores were divid ed into humus (O) and eluvial (E) layers and below that into 10-cm sections and designated as either illuvial (B) or parent material (C), or as a comb ination (BC). Two measures of microbial biomass analyses were applied: subs trate-induced respiration (SIR) to determine microbial biomass C (C-mic), a nd the sum of the extracted microbial-derived phospholipid fatty acids (tot PLFA). The soil fertility had no effect on the results. The C-mic correlate d well with totPLFA (r = 0.86). The microbial biomass decreased with increa sing depth. In addition the PLFA pattern changed with increased depth as as sessed with principal component analysis, indicating a change in the microb ial community structure. The composition of the PLFAs in the O layer differ ed from that in the E layer and both differed from the upper part of the B layer and from the rest of the BC layers. The deeper parts of the B layer ( BC1, BC2 and BC3) were similar to one other. The O layer had more 18:2 omeg a6, a PLFA indicator of fungi, whereas the E layer contained relatively mor e of the PLFAs 16:1 omega9, 18:1 omega7 and cy19:0 common in gram-negative bacteria. With increased depth the relative amount of 10Me18:0, the PLFA in dicator for actinomycetes, increased. We conclude that the PLFA method is a promising discriminator between the microbial community structures of the horizons in Podzols.