Unsaturated hydraulic properties of Sphagnum moss and peat reveal trimodal pore‐size distributions

Abstract

In ombrotrophic peatlands, the moisture content of the vadose zone (acrotelm) controls oxygen diffusion rates, redox state, and the turnover of organic matter. Whether peatlands act as sinks or sources of atmospheric carbon thus relies on variably saturated flow processes. The Richards equation is the standard model for water flow in soils, but it is not clear whether it can be applied to simulate water flow in live Sphagnum moss. Transient laboratory evaporation experiments were conducted to observe evaporative water fluxes in the acrotelm, containing living Sphagnum moss, and a deeper layer containing decomposed moss peat. The experimental data were evaluated by inverse modeling using the Richards equation as process model for variably‐saturated flow. It was tested whether water fluxes and time series of measured pressure heads during evaporation could be simulated. The results showed that the measurements could be matched very well providing the hydraulic properties are represented by a suitable model. For this, a trimodal parametrization of the underlying pore‐size distribution was necessary which reflects three distinct pore systems of the Sphagnum constituted by inter‐, intra‐, and inner‐plant water. While the traditional van Genuchten‐Mualem model led to great discrepancies, the physically more comprehensive Peters‐Durner‐Iden model which accounts for capillary and noncapillary flow, led to a more consistent description of the observations. We conclude that the Richards equation is a valid process description for variably saturated moisture fluxes over a wide pressure range in peatlands supporting the conceptualization of the live moss as part of the vadose zone.

A pore-size classification for peat bogs derived from unsaturated hydraulic properties

“…Under field conditions, soil hydraulic properties (SHP) control the water regime in the topmost, growing part of the peat profile and are thus a critical factor for moss growth and survival (Hájek and Beckett, 2008). The importance of capillary, film, and vapour flow processes for upward water fluxes in moss and peat has been emphasized by Hayward and Clymo (1982) and Price et al (2009), while SHP accounting for these processes have only recently been identified by Weber et al (2017a) for a limited number of samples.…”

“…Quinton et al, 2008;Price et al, 2008;Price, 2012, 2014;Morris et al, 2015;Weber et al, 2017a). As Sphagnum decomposes, the fibrous material is biochemically broken up into smaller solid fragments (Rezanezhad et al, 2016).…”

“…In summary, the afore-mentioned processes constitute the entire continuous pedogenesis shaping the soil profile as an ongoing process (Blume et al, 2016). It should be noted that this includes the actively growing Sphagnum mosses, as Weber et al (2017a) define the actively growing and living part of the Sphagnum mosses as part of the vadose zone, i.e. as part of the soil profile.…”

“…Recently, Weber et al (2017a) demonstrated that the Richards equation is an adequate process model for unsaturated water flow under evaporative conditions even for the uppermost layer of a bog, where the living Sphagnum moss can be treated as part of the soil. The solution of the Richards equation requires the parameterization of SHP.…”

“…However, the investigation of the WRC of organic soils has revealed bimodal pore-size distributions which have been found for different but limited pressure head ranges (Rezanezhad et al, , 2010(Rezanezhad et al, , 2016Quinton et al, 2008Quinton et al, , 2009Kettridge and Binley, 2011), and have so far rarely been expressed in terms of effective SHP over the full pressure head range. Weber et al (2017a) applied the highly sensitive method of inverse modelling evaporation experiments performed on Sphagnum moss and peat over a wide moisture range to parameterize SHP. The WRCs over the pressure head range from −1500 to 0 cm revealed a trimodal pore-size distribution (PSD) and clearly showed the necessity to account for adsorbed water in drying soil.…”