Response of Forest Soil CNP Dynamics to N Enrichment at the Bear Brook Watershed in Maine: Clues from Extracellular Enzyme Activities.

Recent research suggests that extracellular enzyme activities are sensitive indicators of microbial response to ecosystem disturbance, litter decomposition rates, and soil C storage.  We are using this approach to study microbial response to N enrichment in litter, O horizon and mineral soil at the Bear Brook Watershed in Maine (BBWM).  In this long-term paired-watershed experiment, the West Bear (WB) watershed has received bimonthly additions of (NH₄)₂ SO₄  since 1989 while the East Bear (EB) watershed serves as a reference.  Forest composition at the BBWM is characterized by two distinct forest types with different litter qualities: northern hardwoods at lower elevation and red spruce at higher elevation.  We investigate the activities of seven extracellular enzymes involved in microbial acquisition of C, N and P (α-glucosidase, β-glucosidase, xylosidase, phenol oxidase, peroxidase, N-acetylglucosaminidase, and acid phosphatase).  Our initial findings suggest that there are differences in enzyme activity by treatment, forest type and soil horizon.  For example, average phenol oxidase activity in O horizon soil in softwoods is 3x higher in the reference watershed compared to the N-enriched watershed.  Suppressed ligninase activity in N enriched soils is consistent with results from previous research in other ecosystems and can have important implications for soil C storage.  These studies are being expanded through extensive field sampling in order to define microbial mechanisms controlling soil CNP dynamics and their response to chronic anthropogenic disturbance.