Peat-accumulating wetlands have undulating surfaces comprised of raised areas called hummocks and depressions called hollows. This micro-topography, which can vary in height between 10-30 cm, influences the distribution of plant species, root density, and microbial community composition and abundance. These variations could in turn influence carbon (C) and nitrogen (N) cycling within peatlands, which has implications for characterizing peatland biogeochemistry at regional and global scales. Using paired hummock and hollow cores from a boreal peatland, we asked how does microtopography influence peatland microbial function and, in turn, ecosystem C and N cycling? We hypothesized that the hummocks, with greater root biomass and increased aerobic conditions, would have (1) greater microbial abundance and (2) higher rates of enzyme activity. We further hypothesized that (3) the hummock peat above the water table would be biogeochemically distinct given the higher abundance of roots and visibly higher concentrations of fungal biomass. The peat was analyzed for microbial biomass and potential enzyme activity within 10 cm depth increments, resulting in three increments for hummocks (+10, -10, -20), and two for hollows (-10, -20).
Results/Conclusions
Across hummocks and hollows, there was generally a decrease in microbial biomass carbon and nitrogen, fungal biomass (as ergosterol), and dissolved organic carbon with depth from the peat surface. In contrast, potential enzyme activity of C-N degrading peptidases increased with depth. These results support our first hypothesis that microbial biomass is greater in the surface hummock peat (+10) but contradicts the second hypothesis. Within this functional enzyme group, the C-N degrading peptidases differed across the five topography × depth increments. Potential enzyme activity in the hummock surface peat (+10) was significantly lower than the surface peat in the hollows (-10). When examining peat increments at the same depth from the water table (i.e. -10 hummock and -10 hollow), activity of the C-N degrading peptidases in the hummocks remained significantly lower than the hollows. These results supported our third hypothesis that the surface hummocks are biogeochemically distinct. At our study site, hummocks compose approximately 87% of the land area. Our results suggest that regional or global estimates of C and N cycling within peatlands may be underestimated or overestimated if microtopography is not considered.