Soil microbial activity and biomass in the primary succession of a dry heath forest

Changes in vegetation, soil organic matter content, soil nutrient concentration, microbial activity and microbial biomass were studied in Scots pine (Pinus sylvestris) forests on the post-glacial land uplift island of Hailuoto in Finland, along altitudinal transects representing about 1000 years of primary succession. The characteristics of microbial communities in the humus layer were compared both within altitude classes and within TWINSPAN (two-way indicator species analysis) clusters of field layer vegetation. Non-metric multidimensional scaling (NMDS) was employed to reveal gradients in the data. During succession, the vegetation changed from dominance by bryophytes and deciduous dwarf shrubs to evergreen dwarf shrubs and lichens. The thickness of the humus layer and the amount of organic matter in the soil decreased along the succession, which in turn reduced microbial biomass, microbial activity and soil nutrients when calculated on an areal basis. The nutrient concentration of the soil OM (organic matter) showed no successional trend on a concentration basis but the C-to-N ratio of organic matter increased with increasing soil age and lichen coverage. Thus, the nutrient availability decreased during succession but this could not be demonstrated by calculating results against unit weight of organic matter. Soil basal respiration and microbial biomass increased during the succession when calculated per unit weight of organic matter. The successional decrease in site productivity appeared to be due to leaching of nutrients from the sandy mineral soil and thinning of the humus layer. Plants and soil microbes became increasingly N limited during the course of the succession, suggesting the increased importance of mycorrhizal symbiosis for plant performance and increased energy costs among soil microbes in nutrient uptake.