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

Abstract

Abstract. In ombrotrophic peatlands, the moisture content of the acrotelm (vadoze zone) controls oxygen diffusion rates, redox state, and the turnover of organic matter. Thus, variably saturated flow processes determine whether peatlands act as sinks or sources of atmospheric carbon, and modelling these processes is crucial to assess effects of changed environmental conditions on the future development of these ecosystems. We show that the Richards equation can be used to accurately describe the moisture dynamics under evaporative conditions in variably saturated peat soil, encompassing the transition from the topmost living moss layer to the decomposed peat as part of the vadose zone. Soil hydraulic properties (SHP) were identified by inverse simulation of evaporation experiments on samples from the entire acrotelm. To obtain consistent descriptions of the observations, the traditional van GenuchtenMualem model was extended to account for non-capillary water storage and flow. We found that the SHP of the uppermost moss layer reflect a pore-size distribution (PSD) that combines three distinct pore systems of the Sphagnum moss. For deeper samples, acrotelm pedogenesis changes the shape of the SHP due to the collapse of inter-plant pores and an infill with smaller particles. This leads to gradually more homogeneous and bi-modal PSDs with increasing depth, which in turn can serve as a proxy for increasing state of pedogenesis in peatlands. From this, we derive a nomenclature and size classification for the pore spaces of Sphagnum mosses and define inter-, intra-, and inner-plant pore spaces, with effective pore diameters of > 300, 300-30, and 30-10 µm, respectively.

Saturated and unsaturated salt transport in peat from a constructed fen

“…The retention curve (Fig. 3) does not have the classical shape that would point at discrimination between an active and inactive porosity (Rezanezhad et al, 2016;Weber et al, 2017b). Measured water retention and hydraulic conductivity data closely fit the van Genuchten-Mualem unimodal model with an RMSE of 0.03 and AICc value of −342 ( Fig.…”

“…This physical structure, along with surface adsorption of reactive solutes (Rezanezhad et al, , 2016 will lead to delayed arrival of solutes which are likely to affect vegetation community development in oil sands reclamation landscapes. The physical and hydraulic properties of undisturbed peat changes along a continuous vertical profile (Weber et al, 2017b;Limpens et al, 2008), whereby deep peat layers are generally more decomposed (Clymo, 1983). In addition to pore-scale effects, the systematic layered heterogeneity common in natural peatlands influences mixing and transport (Hoag and Price, 1997).…”

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

“…Unsaturated hydraulic conductivity is, in turn, a function of the volumetric moisture content (VMC) of the moss (θ) that is governed by the pore network within the moss matrix. In natural peatlands, a gradual gradient of living mosses to dead and decomposed mosses is observed, creating a gradation of larger pores at the surface, to smaller pores below (Boelter, 1968;Goetz & Price, 2015;McCarter & Price, 2014;Rezanezhad et al, 2010;Weber, Iden, & Durner, 2017a). In natural peatlands, a gradual gradient of living mosses to dead and decomposed mosses is observed, creating a gradation of larger pores at the surface, to smaller pores below (Boelter, 1968;Goetz & Price, 2015;McCarter & Price, 2014;Rezanezhad et al, 2010;Weber, Iden, & Durner, 2017a).…”

“…This pore network is generated by a series of overlapping Sphagnum leaves and branches, which undergo decomposition and collapse with increasing depth from the surface . Effective pore diameters decrease with depth corresponding to an increase in bulk density caused by decomposition and compression (Carey, Quinton, & Goeller, 2007;Quinton, Hayashi, & Carey, 2008;Rezanezhad et al, 2010;Weber et al, 2017a). Effective pore diameters decrease with depth corresponding to an increase in bulk density caused by decomposition and compression (Carey, Quinton, & Goeller, 2007;Quinton, Hayashi, & Carey, 2008;Rezanezhad et al, 2010;Weber et al, 2017a).…”

Using Sap Flow Data to Parameterize the Feddes Water Stress Model for Norway Spruce

“…Moreover, the concept of REW seems to apply fairly well to the conditions at the Slope plot and to other soils in central European high and low mountain ranges [4,31,61,62]. However, Norway spruce is also widely distributed in boreal forest landscapes, where typical critical REW values seem to be lower than 0.3 [23,24] and soils are often peaty [71], hence showing similar characteristics (low residual water content, high saturated water content; [72][73][74]) as our plot in the riparian zone. Our data indicates that for such soils the concept of REW is likely to fail.…”