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

Abstract. Peatlands encode information about past vegetation dynamics, climate, and microbial processes. Here, we used δ¹⁵N and δ¹³C 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  ∼  10 000 years. In multiple regression analyses, δ¹⁵N and δ¹³C 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 δ¹⁵N and 38 % of variance for δ¹³C, reflecting N and C losses. In contrast, correlations with depth and topography (hummock or hollow) reflected peatland successional history and climate. Higher δ¹⁵N 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 δ¹³CO₂ since the Industrial Revolution) lowered δ¹³C in recent surficial samples. High δ¹⁵N 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 δ¹³C at  ∼  4000 years BP (−65 to −105 cm) could reflect a transition at that time to slower rates of peat accumulation, when ¹³C discrimination during peat decomposition may increase in importance. Low δ¹³C and high δ¹⁵N at −213 and −225 cm ( ∼  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 δ¹³C patterns.