Spatial and temporal variability of biological indicators of soil quality in two forest catchments in Belgium

Biological indicators, measurements based on the presence and activity of soil organisms, are increasingly being considered in assessments of forest soil quality. In addition to chemical indicators, such as soil organic carbon and pH, biological indicators can provide an early diagnosis of changes in soil quality and processes in response to environmental change and forest management actions. We investigated the spatial and temporal variability of selected bio-indicators in the forest floor of two catchments over three years. We further evaluated the sensitivity of these indicators to changes in the dominant tree species following reforestation and natural regeneration. Indicators of microbial abundance and activity (microbial biomass, potential respiration) and carbon maintenance costs (metabolic quotient) were higher under young spruce and mixed deciduous stands than pure stands of oak and beech. Our results indicate a greater microbial activity in autumn but a wider range of carbon substrate utilisation in spring. Assessment of seasonal differences in bio-indicator values is vital for the evaluation and planning of long-term studies and the development of reference values for forest soils in Belgium. Our results highlight the usefulness of these bio-indicators in identifying changes in soil quality, particularly in response to management activities, at small spatial scales.