Native woodland expansion: soil chemical and microbiological indicators of change

The regeneration of native pine woodlands, a habitat of high biodiversity value, is being actively encouraged by conservation agencies as a positive future change in land use. A field study was carried out at Abernethy Forest, Scotland by sampling soil along three parallel transects running from open moorland, through an intermediate zone showing tree seedling colonization, into mature native pinewood forest with the aim of establishing a long-term monitoring site and providing baseline data. The specific objectives were to determine, first, which physical, chemical or microbiological properties in moorland soils would be affected by tree colonization and, secondly, what might be the implications of forest expansion on the soil carbon balance. Moving from moorland to forest, moisture and soil acidity decreased significantly and paralleled vegetation changes but there was little change in %C or %N. Microbial biomass, measured as the total carbon in the soil microflora, decreased markedly towards the forest while the metabolic quotient, a relative measure of microbial activity, showed a significant increase. None of the chemical or microbial indicators were able to detect changes due to seedling colonization at their present stage of development. However, microbial indicators appeared to be more sensitive than physico-chemical properties to the aboveground vegetation. Both the microbial biomass C and the metabolic quotient are likely to be good indicators of change as colonization develops. Estimation of the total sequestered soil carbon indicated more in the moorland (524 t C ha(−1)) than in the forest (288 t C ha(−1)), but this did not include the modest aboveground carbon stored in the tree biomass. At this particular site, forest expansion may result in some loss of soil C that would be partly offset by increases in aboveground C.