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

Appropriate indicators for assessing forest soil quality are needed to monitor changes in the frame of air pollution and climate change, assess impacts of forest management actions and consequently decide on long-term management objectives. In addition to chemical indicators, such as for example soil organic carbon and pH, biological indicators inform on soil functions/processes and provide an early diagnosis of changes in the environment. The objectives of this work were to provide first baseline values for Belgian forest soils, to evaluate the sensitivity of selected indicators to changes in tree species on the same soil type, and to assess the temporal variability of the indicators.
This work is part of a long-term study on element concentrations and fluxes in 2 small (each ca. 80 ha) forested watersheds (Waroneu and Robinette, east Belgium) in relation to forest management. Both catchments show spatial variability of tree species cover and drainage. In particular, Robinette was partially clear-cut in 1996 and replanted with deciduous tree species in 1998. Six sub-plots with a specific tree species/soil hydrology combination were defined in each catchment. Selected biological indicators (potential respiration, nitrogen mineralization, microbial biomass, metabolic diversity of bacteria) and two ecophysiological indices (microbial quotient and metabolic quotient) were measured in spring and autumn during 3 consecutive years.
Our results suggest that (1) radar plots can be a useful tool to visualize the ranges of several indicators simultaneously (2) the biological indicators showed significant differences between the sub-plots, even though they are situated on the same geological substrate and in a limited geographical region, indicating high sensitivity to detect changes (3) Small scale differences detected raise the question of the appropriate spatial scale and classifying criteria to define baseline values (4) there were no marked seasonal differences, but for some indicators (N mineralization, potential respiration, microbial and metabolic quotients) the sampling moment induced a certain variability.