Controls on Organic Carbon Burial and Masses in Restored Marsh Soils in Highly Altered Watersheds of the Long Point Region of southern Ontario, Canada

Wetland restoration following drainage and large-scale anthropogenic hydrological alteration has become a common management practice to re-establish former wetland functions and has been brought forward as a natural climate solution for sequestering soil organic carbon. However, factors that drive organic carbon burial in restored freshwater marsh soils in highly altered landscapes in the temperate region and that promote preservation within policy-relevant timescales to achieve emission reduction targets remain highly uncertain.

In this study, we examine paleoenvironmental reconstructions from an undrained reference marsh and a passively-restored marsh (reflooded after 1954 AD), both of which are freshwater, riverine-influenced and situated in the Long Point Biosphere Reserve of southern Ontario, Canada on the northeast shore of Lake Erie. Significant human alteration in this region began in the late 1700s with the arrival of Euro-American settlers and wetlands were extensively drained to create suitable land for agriculture. Although impacts on the local hydrology persist to this day, active restoration efforts have been increasing to restore wetland habitat for improving water quality and conserving biodiversity in this region. We couple the paleorecords with a chronosequence of actively-restored (i.e., excavated), depressional freshwater marshes (<10 years in age) located in the same Lake Erie watersheds to identify temporal and spatial controls on organic carbon burial. We dated sediment cores from the reference and passively-restored marshes using radiometric analyses and trace pollutants to analyze and compare rates of organic carbon accumulation over time. We also measured soil organic carbon masses in the surface soils, where recent accumulation has occurred, of actively-restored sites to determine whether carbon masses increase with time since restoration, and to evaluate a series of landscape attributes including slope, permeability of underlying parent material and vegetation cover as drivers for carbon masses in wetland soils. Lastly, we compare rates of organic carbon accumulation and masses measured in the study sites with undrained and restored freshwater marshes in other agricultural-intensive areas in temperate North America.

Results demonstrate that the reference site has been influenced by hydro-fluvial and inundation events over thousands of years, and has high apparent rates of organic carbon accumulation (average short-term rate of 274 g C m-2 yr-1) and the highest mass of soil organic carbon (122 tC ha-1) at 0-30 cm depth. Despite the study sites being in proximity, we found that organic carbon masses at 0-30 cm among all restored wetlands (16-115 tC ha-1) are highly variable, and in many cases are markedly lower than that of the reference site and those of reference sites in other agricultural-intensive areas of North America (e.g., the Prairie Pothole Region). Furthermore, we found that time does not have a strong effect on surface organic carbon stocks within ten years of active restoration; instead, underlying parent material is the strongest factor driving organic carbon masses in the actively-restored marsh soils.

Our findings show that conservation of undrained freshwater marshes is preferred to restoration in terms of potential as a natural climate solution in highly fragmented landscapes. Nonetheless, our findings also show that passive wetland restoration provides a means of restoring organic carbon accumulation (short-term rates ranging between 44-290 g m-2 yr-1) on a multidecadal scale where sites are low-lying, underlain by alluvial deposits, and connected to larger ground and surface water networks. Active restoration measures that involve removal of drainage tiles, excavation and/or installation of berms may promote organic carbon preservation, especially where soils have low infiltration capacity, but may also disturb pre-existing organic carbon stocks and alter soil properties that promote organic carbon burial. In this presentation, we will highlight the importance of landscape attributes in selecting sites that sustain hydric soils and maximize carbon sequestration for marsh restoration, and argue that the pre- versus post-settlement context and reference paleorecord provide necessary baselines and benchmarks for directing successful wetland restoration under environmental change.