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 γ¹⁵N and γ ¹³C 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, γ ¹⁵N and γ ¹³C 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 γ ¹⁵N and 38% of variance for γ ¹³C, reflecting N and C losses. In contrast, correlations with depth and topography (hummock or hollow) reflected peatland successional history and climate. Higher γ ¹⁵N 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 γ ¹³CO₂ since the Industrial Revolution) lowered γ ¹³C in recent surficial samples. High γ ¹⁵N 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 γ ¹³C at -4000 years BP (-65 to -105 cm) could re- flect a transition at that time to slower rates of peat accumulation, when 13C discrimination during peat decomposition may increase in importance. Low γ ¹³C and high γ ¹⁵N 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 γ ¹³C patterns. [ABSTRACT FROM AUTHOR]