Long-term carbon and nitrogen dynamics at SPRUCE revealed through stable isotopes in peat profiles

Peatlands encode information about past vegetation dynamics, climate, and microbial processes. Here, we used delta N-15 and delta C-13 patterns from 16 peat profiles to deduce how the biogeochemistry of the Marcell S1 forested bog in northern Minnesota responded to environmental and vegetation change over the past similar to 10 000 years. In multiple regression analyses, delta N-15 and delta C-13 correlated strongly with depth, plot location, C/N, %N, and each other. Correlations with %N, %C, C/N, and the other isotope accounted for 80% of variance for delta N-15 and 38% of variance for delta C-13, reflecting N and C losses. In contrast, correlations with depth and topography (hummock or hollow) reflected peatland successional history and climate. Higher delta N-15 in plots closer to uplands may reflect upland-derived DON inputs and accompanying shifts in N dynamics in the lagg drainage area surrounding the bog. The Suess effect (declining delta(CO2)-C-13 since the Industrial Revolution) lowered delta C-13 in recent surficial samples. High delta N-15 from -35 to -55 cm probably indicated the depth of ectomycorrhizal activity after tree colonization of the peatland over the last 400 years, as confirmed by the occasional presence of wood down to -35 cm depth. High delta C-13 at similar to 4000 years BP (-65 to -105 cm) could reflect a transition at that time to slower rates of peat accumulation, when C-13 discrimination during peat decomposition may increase in importance. Low delta C-13 and high delta N-15 at -213 and -225 cm (similar to 8500 years BP) corresponded to a warm period during a sedge-dominated rich fen stage. The above processes appear to be the primary drivers of the observed isotopic patterns, whereas there was no clear evidence for methane dynamics influencing delta C-13 patterns.