The effect of compression on Sphagnum hydrophysical properties: Implications for increasing hydrological connectivity in restored cutover peatlands

Abstract

Sphagnum moss, a dominant peat-forming species in northern peatlands, relies on capillary rise to sustain metabolic activities. However, in restored peatlands, a capillary barrier resulting from distinctly different pore-size distributions in the degraded remnant cutover peat and regenerated Sphagnum moss limits capillary rise. Space-for-time analogues suggest it could take >40 years for decomposition and degradation of the moss profile to overcome this capillary barrier effect. This paper explores the feasibility of mechanical compression to ameliorate the capillary barrier effect and accelerate the return of ecohydrological function in restored Sphagnum moss. Seven reference cores (10 × 20 cm) and 29 samples (10 × 5 cm) representing various depths from surface (0-5 cm, n = 9; 5-10 cm, n = 9; 10-15 cm, n = 7; 15-20 cm, n = 4) were tested in a laboratory setting to analyse pre-compression and post-compression water retention-unsaturated hydraulic conductivity relationships, proportion of macropores, and other hydrophysical parameters. Post-compression, reference core moss height (originally 20 cm) decreased by 5.5 ± 1.2 cm, and sample bulk density increased, while porosity decreased (Wilcoxon signed rank test, p < 0.01). Increased water retention and subsequent increase in unsaturated hydraulic conductivity post-compression was a result of a decrease in macropores (diameter > 75 μm). Simulation results with Hydrus-1D indicate that the post-compression moss profile was better suited to maintain pressure heads above critical values for photosynthesis. These findings suggest that mechanical compression may be used to ameliorate the capillary barrier effect in restored cutover peatlands; however, field scale studies are required.

Heterogeneity of the peat profile and its role in unsaturated sodium chloride rise at field and laboratory scales

“…This transmitted water well during saturated conditions, as shown by the lack of water table rise above 5 cm bgs during precipitation events (Figure 3), as water readily drains away. In unsaturated conditions, the low water retention at F1‐5 restricts upward movement of water (Gauthier et al., 2018; McCarter & Price, 2014), only facilitating capillary rise from the water table during its peak on Day 19 (Figure 3, θ = 0.3); this corresponds to the only notable increase in SC pw at this location. The relatively low water content suggests that only the smaller pores remained saturated after this, and the steady SC pw suggests these remained isolated for the duration of the experiment (Figure 4).…”

“…Deeper in the acrotelm, the more decomposed peat has a higher water retention capacity, and thus higher unsaturated hydraulic conductivity at a given negative pressure, compared with acrotelm peat (Price et al., 2008). A gradual transition in soil hydraulic properties from the lower to upper acrotelm provides good vertical connectivity that can sustain water rise, though this may also enhance loss to evapotranspiration (Gauthier, McCarter, & Price, 2018; McCarter & Price, 2014; Nungesser, 2003).…”

Near‐surface controls on peatland hydrology: Implications for rapid adaptation and enhanced resilience to disturbances

“…Further as the peat layer extends to depths greater than 5 cm in depth, transitions in peat hydrophysical properties that are widely reported (Gauthier et al, 2018;Golubev et al, 2021;McCarter & Price, 2015) will occur. Note in this latter case, the experimental modelling undertaken here with this binary structure would strongly indicate that the surface layer would drive the hydrological dynamics of such complex vertical variations in peat profiles.…”

“…For example, after periods of disturbance Sphagnum can allocate more resources to vertical rather than structural growth (Turetsky et al, 2008;Waddington et al, 2011). This growth pattern results in a pore network dominated by large pore throat diameters, with a lower capillary rise potential in this new moss (Gauthier et al, 2018;McCarter & Price, 2015;Taylor & Price, 2015). With such examples, the connectivity between the water table and the moss surface will be lower than the simulated layers of undisturbed Sphagnum peat.…”

“…Despite the importance of these near-surface controls on peatland hydrology and carbon sequestration, historically the quantification of peatland hydrophysical properties and the hydrological simulation of the ecosystems has focused on peat properties at depth (Wang et al, 2021), linking surface ecological dynamics with the water table depth (WTD) (Ahmad et al, 2021). However, more recently, notably Canadian, peatland studies evaluated moss moistures stresses via known ecohydrological thresholds by quantifying bulk densities, hydrophysical properties, water table depths and peat moistures of very near-surface layers regrowing or recolonizing onto the cutover surface (Cagampan & Waddington, 2008;Gauthier et al, 2018;Golubev et al, 2021;McCarter & Price, 2015;Turetsky et al, 2008;Waddington et al, 2011) This focus on near-surface properties and hydrological dynamics is important. Whilst the restriction of drainage through the blockage of drainage channels provides core restoration technique, other restoration approaches target the peatland surface, promoting regime shifts in vegetation composition (Tuittila et al, 2000), the reintroduction of Sphagnum moss species (Limpens & Tomassen, 2020), and the transition from a decomposing peat profile to a peatland that is accumulating organic matter in the near-surface.…”