The Effect of Nutrient Availability On Soil Extracellular Enzyme Activity and CNP Dynamics.

Soil extracellular enzymes are critical components of organic matter decomposition and nutrient cycling dynamics in forest ecosystems.  There is evidence in the literature that certain soil extracellular enzyme activities are sensitive to nutrient availability, such as the suppression of ligninases by enhanced nitrogen (N) supply.  In ecosystems enriched with N, phosphorus (P) supply may play an important role in regulating enzyme activity and organic matter decomposition.  We examined the response of extracellular enzyme activity to enhanced N and/or P availability in O horizons and mineral soils at a long-term paired-watershed experiment in Maine.  In this experiment, the West Bear watershed has received bimonthly additions of (NH₄)₂ SO₄  since 1989 while the East Bear (EB) watershed serves as a reference.  In addition to the whole-watershed N amendments, a buried bag experiment was conducted in 2008 with experimental manipulations of soil N and P to study extracellular enzyme responses to factorial alterations in soil solution N and P stoichiometry.  Enzymatic indices of microbial function included measurements of α-glucosidase, β-glucosidase, xylosidase, phenol oxidase, peroxidase, N-acetylglucosaminidase, and acid phosphatase.  Results indicate that the activity of most enzymes are lower in the treated watershed compared to the reference watershed, possibly due to lower total microbial biomass in the N-enriched watershed.  Extant phosphatase activity in O horizon soils was higher in EB compared to WB by 31% for hardwoods and 44% for softwoods.  Initial results from buried bag nutrient supply experiments suggest that the response of soil enzyme activity to N and P amendments is also different in the treated versus reference watershed.  For example, B-horizon β-glucosidase activity responded positively to increased P availability only in the treated WB watershed.  These data provide empirical evidence that some microbial processes are now limited more by P than N availability in this N-enriched system.