Beaver pond effects on carbon storage in soils

Abstract Beaver ponds have been shown to be hot spots of carbon trace gas flux, which begs the question, “What is the source of the carbon?” The density and stratigraphy of carbon storage was studied in soils of boreal beaver meadows, graminoid wetlands that develop after a beaver pond is abandoned and drained, at Voyageurs National Park on the U.S.–Canada border, where beaver re-colonized the landscape during the latter half of the 20th century. Carbon density was measured to a depth of 60 cm by collecting volumetric samples in mineral soils from the side walls of hand-dug trenches and in a Histosol using a box corer. Interpretation of pre-beaver (1940) aerial imagery showed the presence of cedar swamps in locations that currently have Haplosaprists and Argiaquolls, with deep carbon storage that cannot be attributed to recent beaver activity. Excluding these sites, carbon density to a depth of 60 cm was still significantly greater in beaver meadow pedons (15.1 ± 6.8 kg m− 2) than in adjacent forest soils that had not been impounded (8.2 ± 2.9 kg m− 2). The difference was attributed to the accumulation of graminoid plant debris in thick surface O horizons. Volumetric carbon concentrations were greatest in O horizons, and a linear regression between O horizon thickness and carbon density was significant. Parent material origin (i.e., glacio-fluvial vs. glacio-lacustrine) did not significantly affect carbon density. This research confirms the presence of soil carbon adequate to supply trace gas fluxes, but suggests that considerable carbon is sequestered by beaver impoundments in the meadow stage, which may offset carbon release that occurs during the pond stage.