Long-term Carbon and Nitrogen Dynamics at SPRUCE Revealed through Stable Isotopes in Peat Profiles

Abstract:

Carbon and nitrogen turnover in peatlands is of considerable interest because peat is a large reservoir of stored carbon that could emit greenhouse gases in response to climate change. Because peat cores preserve a long-term record of system carbon and nitrogen dynamics, it is possible to use stable isotopes as markers of changes in carbon (C) and nitrogen (N) dynamics over time. Here, we used δ¹⁵N and δ¹³C patterns throughout the depth profile of peat cores to understand controls over C-N cycling in the Marcell S1 forested bog in northern Minnesota. In multiple regression analyses, δ¹⁵N and δ¹³C correlated strongly with depth, plot location, %C, %N, and each other. Negative correlation of δ¹⁵N with %N presumably reflected removal of ¹⁵N-depleted N via denitrification, diffusion, or plant N transfer via mycorrhizal fungi. A step increase in the depth coefficient for δ¹⁵N of ~3‰ from -25 cm to -35 cm suggested that the N removal process primarily operates at a discrete depth corresponding to the juncture between aerobic and anaerobic layers defined by the water table. Higher δ¹⁵N and lower δ¹³C in plots closer to uplands may reflect distinct hydrology and accompanying shifts in C and N dynamics in the lagg area fringing the bog. The Suess effect (declining δ¹³CO₂ since the Industrial Revoluation) and aerobic decomposition lowered δ¹³C in recent surficial samples. Small increases in δ¹³C at -112 cm (4300 calibrated years BP) and -85 cm (3800 calibrated years BP) may reflect C dynamics during a suspected transitional fen stage (based on paleoecology at a nearby bog), when reduced methanotrophy retained less ¹³C-depleted carbon derived from methane than in later periods. The C/N decreased until about -85 cm and thereafter remained steady, suggesting that the active zone of aerobic processing during drought may extend to this depth. The inflection point in calculated carbon accumulation rates at this depth supports this conclusion.