The Effects of Long-Term Forest N Enrichment and Acidification On Soil CNP Dynamics.

Forest N-enrichment and acidification can alter key aspects of nutrient cycling, including the bioavailability of limiting nutrients.  There is recent evidence that accelerated nitrogen (N) cycling may enhance the role that phosphorus (P) plays in regulating ecosystem processes.  In N-enriched acid forest soils, the nature of nutrient limitation on microbial activity and function is poorly understood. We conducted a plot-level field experiment and a laboratory incubation experiment on soils from the Bear Brook Watershed in Maine (BBWM), a long-term paired watershed study.  At the BBWM, the West Bear watershed has received bimonthly additions of (NH₄)₂ SO₄ since 1989, while the adjacent watershed serves as a biogeochemical reference.  Each watershed contains two distinct forest types: northern hardwoods at lower elevation and red spruce at higher elevation.  In both the field and lab experiments, amendments of N, P or N+P were applied to soils from both forest types in each watershed.  Indices of microbial activity included: C, N and P mineralization, and extracellular enzyme activity.  There was little evidence of a response to N in either watershed, but both N+P and P elicited microbial responses.  The most dramatic response to P was in soils from hardwoods in the reference watershed; in the incubation experiment, soils amended with P from the reference hardwoods had as much as 42% higher CO₂ flux than soils from the N-enriched watershed.  The reason our N-enriched soils responded less to P treatments may be due to the indirect effects of soil acidification, whereby mineral P pools have been mobilized, biocycled, and made more available in surface horizons of the treated watershed.  We postulate that this mechanism may produce a transient delay in the onset of microbial P limitation in ecosystems simultaneously subjected to N enrichment and acidification.