Forest floor exchange chemistry and microbial biomass in red spruce stands along a calcium/aluminum gradient in the northeastern United States

There are few ecological indicators available to assess the potential health risks and extensive damage associated with environmental stresses in forest ecosystems of the Northeast. The changes in the soil microbial biomass and microbial populations may act as early warning signals for environmentally detrimental effects such as acidic deposition. In this study, the chemical, physical, and soil microbial properties of the forest floor were characterized in six field sites. The results indicated differences in the forest floor chemistry, microbial biomass carbon (MBC), and microbial populations in the Oe and Oa horizons of the forest floor along a Ca/Al gradient. Oe horizons consistently had higher concentrations of exchangeable cations, microbial biomass carbon and microbial populations with the exception of aluminum. Differences between months were not consistent and varied, depending on the site. Unexpectedly, nitrogen-fixing bacteria dominated over fungi. Differences in bacterial and fungal populations may be associated with the tree species composition and the influence of understory vegetation. Tree species data suggest that these forests are no longer predominately red spruce stands. Data also indicated that MBC values were far below the values that appear in the literature. No clear relationships between the microbial populations and forest floor chemistry could be identified. The forest floor experienced a wide range of seasonal differences in temperature and soil moisture. Extreme drought conditions occurred in August 1999 and a significant snowfall occurred in October 1999. Gradients of extreme temperature and soil moisture, substrate availability, and seasonal variations are likely to be the primary factors controlling the properties of forest floor chemistry and fungal and bacterial populations within the study sites. While a number of factors may have concomitantly influenced the soil microbial communities, it may be concluded that the microbial response in the present study was strongly determined by the direct and indirect effects of biotic and abiotic variables as well as the specific site conditions, stand structure, and forest management history. This study suggests that the potential and future use of microbial indicators of forest health require the modification of current research techniques and methods of analyses in order to accurately assess the biological properties in acidic forest soils in the Northeast.