Survival of the Deepest? Peat Depth as a Control on Peatland Hydraulic Structure and Pore-water Residence Time

Abstract:

Northern peatlands persist on the landscape through hydrological feedbacks (Waddington et al., 2015) that confer resilience to disturbances such as seasonal moisture deficits. One such feedback is between pore-water residence time and decomposition, whereby peat hydrophysical properties facilitate the accumulation of decay end-products deep in the peatland profile that may thermodynamically suppress further decomposition. While peatland hydrological models suggest that water in deeper (and larger) peatlands experience less turnover and confer a greater resilience than shallower peatlands, this has not been systematically examined in natural systems. We examined the role of peat depth on this feedback at several shallow peat deposits and deeper peatlands in bedrock depressions of the Canadian Shield. Despite their much smaller size and younger peat, the hydraulic structure of the shallow sites exhibited similar controls as typical deeper northern peatlands. However, their hydrologic behaviour varied considerably, with the shallow sites prone to flooding and desiccation. Profiles of pore water isotopic composition (δD and δ¹⁸O) were sampled weekly in the spring/summer alongside rainfall to estimate relative residence times using the inverse transit time proxy (ITTP). These relative residence times varied by similar site parameters as peat hydrophysical properties, and the ITTP estimated using δD exhibited strong correlation with a depth-averaged hydraulic conductivity. Relative residence times were fairly consistently the longest at the bottom of peat profiles and the deeper sites, even though wet conditions mitigated much of the typical seasonal drought-induced pore-water turnover. This was supported by greater turnover in all but the bottom of deep sites in a dry summer. This work suggests that peat depth controls the strength of crucial resilience-conferring feedbacks. This 'survival of the deepest' concept can inform both peatland restoration efforts and potential climate change impacts on carbon cycling.