Long-Term Carbon Accumulation in Temperate Swamp Soils: a Case Study from Greenock Swamp, Ontario, Canada

Wetlands that develop peat are a globally significant pool of soil carbon. While some wetland types such as bogs and fens are well characterized by the consistent development of carbon-rich peat, swamps soils are more variable both in terms of their carbon densities and accretion rates. Subcategorizing swamps by forest type may be a useful way of understanding this variability. Here we provide a case study of carbon accumulation in two distinct forest stands of Greenock Swamp located in the Great Lakes – St Lawrence mixed forest region in Bruce County, Ontario, Canada: Acer-Fraxinus (maple-ash) swamp (i.e., broad-leaf swamp) prevalent across the site, and a Thuja occidentalis (cedar) swamp stand (i.e., needle-leaf swamp). Organic matter and organic carbon contents were analyzed among seven broad-leaf swamp soil cores and one needle-leaf swamp core collected from Greenock Swamp. The broad-leaf swamp cores had peat depths ranging from 18–60 cm with a mean organic matter content of 54% and an organic carbon content of 34% of dry mass. The needle-leaf swamp core had at least 4 m of almost homogeneous peat with a mean organic matter content of 89%. Radiocarbon dating indicates that the broad-leaf swamp accumulates peat episodically, but can contain organic matter thousands of years old; the needle-leaf swamp shows continuous peat accumulation since the Middle Holocene. While broad-leaf swamp soils contain lower carbon stocks than needle-leaf swamp soils, they extend over large areal extents at Greenock Swamp and elsewhere in the temperate zone and contain important pools of recalcitrant organic matter, in some cases thousands of years old. Thus, both swamp types need to be considered to fully represent the carbon pools and potential sink of temperate wetlands.