Microbial and plant community structure across a primary succession gradient

The formation of the organic layer within Scandinavian forest soil started about 10000 yr ago, following the retreat of the: continental ice sheet. Si nce then the land has been slowly rising in northern Europe and uplift stil l occurs on the coast of the Bothnian Bay at a rate of about 0.6-0.9 m per 100 yr. Four, 300 m long, successional gradients were studied from the shor eline to a Scots pine (Pinus sylvestris) stand with a fully developed humus layer of a few centimetres' thickness. The plantless shoreline was followe d by small foredunes and dunes. characterized by Agrostis stolonifera and L eymus arenarius, respectively. and the deflation basin characterized by lic hens and sparse Festuca ovina and Deschampsia flexuosa. The study sites sit uated in Scots pine stands of about 25 and 40 yr age were characterized by sparse dwarf shrubs, lichens and bryophytes. The amount of organic matter i n soil increased along the gradient. When the microbial biomass: estimated as indicative phospholipid fatty acids (PLFAs). was calculated on the organ ic matter basis, the total microbial biomass as well the amount of bacteria l PLFAs decreased from the earlier stages of succession towards the pine fo rest. The ratio of fungal to bacterial PLFAs increased along the succession . The bacterial community structure in the short soil was different to that in the dune soil or in forested zones. On the plantless shore the microbia l community was almost completely described by PLFAs indicative of Gram-neg ative bacteria. In addition to these bacterial PLFAs, dunes were characteri zed by PLFAs indicative of actinomycetes. Thus, the fungal part of the micr obial community seemed to respond most to the accumulation of organic matte r and increasing C/N ratio. while the bacterial biomass and bacterial commu nity structure seemed also to reflect the composition of the vegetation and the varying quality of the organic matter.