Effects of Nutrient Addition on Belowground Stoichiometry and Microbial Activity in an Ombrotrophic Bog

Abstract:

Ombrotrophic bogs are both nutrient-poor systems and important carbon (C) sinks yet there remains a dearth of information on the stoichiometry of C, nitrogen (N), phosphorus (P), and potassium (K), an important determinant of substrate quality for microorganisms, in these systems. In this study, we quantified the C, N, P, and K concentrations and stoichiometric ratios of both soil organic matter (SOM) and dissolved organic matter (DOM) as well as microbial extracellular enzyme activity from 0 – 10cm depth in a long-term fertilization experiment at Mer Bleue bog, Ontario, Canada. Though trends in C:N, C:P, and C:K between SOM and DOM seem to follow one another, preliminary results indicate that the stoichiometric ratios of DOM were at least an order of magnitude smaller than those of DOM suggesting that nutrient fertilization impacts the quality of DOM as a microbial substrate to a greater degree than SOM. C:N decreased with greater nitrogen addition but C:P and C:K increased; the magnitude of that increase being smaller in NPK treatments relative to N-only treatments suggesting co-limitation by P and/or K. This is further supported by the increase in activity of both the C-cycling enzyme, β-D-glucosidase (bdG), and the P-cycling enzyme, phosphatase (Phos), with greater nitrogen addition; particularly in NPK-treatments for bdG and N-only treatments for Phos. The activity of the N-cycling enzyme, N-acetyl-β-D-glucosaminidase, and the C-cycling enzyme, phenol oxidase, with greater N-addition suggests a decreased need to breakdown organic nitrogen to meet microbial N-requirements in the former and N-inhibition in the latter consistent with findings in the literature. 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 in the decomposition process which may ultimately decrease the ability of peatlands to sequester carbon.