Effects of long-term fertilization on peat stoichiometry and associated microbial enzyme activity in an ombrotrophic bog

Ombrotrophic bogs are nutrient-poor systems and important carbon (C) sinks yet there remains a dearth of information on the belowground stoichiometry of C, nitrogen (N), phosphorus (P), and potassium (K), important determinants of substrate quality for microorganisms, in these systems. In this study, we quantified the C, N, P, and K concentrations of both hummock peat and dissolved matter as well as microbial extracellular enzyme activity from 0 to 10 cm depth in the long-term NPK fertilization plots at Mer Bleue bog. Greater N-loading resulted in significantly reduced C:N and greater C:P and C:K (p < 0.05) while the addition of NPK resulted in significantly lower C:P and C:K (p < 0.05) and lower C:N relative to N-only treatments but the difference in C:N was not significant (p > 0.05). Hydrolase enzyme activity followed resource allocation models with significantly suppressed N-acetyl-β-d-glucosaminidase and phosphatase activity (p < 0.05) and enhanced β-d-glucosidase activity with greater N- and P-availability. The concentration of inhibitory phenolics was significantly greater with N-loading (p < 0.05), due in part to shifts in surface vegetation, while phenol oxidase activity was significantly suppressed by N (p < 0.05) with results suggesting greater suppression by NH4+ than NO3−. Taken together, these results suggest that higher levels of nutrients impact both microbial substrate quality as well as the activity of microbial enzymes that are key to the decomposition process that may ultimately decrease carbon sequestration in bogs.