Hydraulic properties of peat soils along a bulk density gradient—A meta study

Abstract

Our understanding of hydraulic properties of peat soils is limited compared with that of mineral substrates. In this study, we aimed to deduce possible alterations of hydraulic properties of peat soils following degradation resulting from peat drainage and aeration. A data set of peat hydraulic properties (188 soil water retention curves [SWRCs], 71 unsaturated hydraulic conductivity curves [UHCs], and 256 saturated hydraulic conductivity [Ks] values) was assembled from the literature; the obtained data originated from peat samples with an organic matter (OM) content ranging from 23 to 97 wt% (weight percent; and according variation in bulk density) representing various degrees of peat degradation. The Mualem‐van Genuchten model was employed to describe the SWRCs and UHCs. The results show that the hydraulic parameters of peat soils vary over a wide range confirming the pronounced diversity of peat. Peat decomposition significantly modifies all hydraulic parameters. A bulk density of approximately 0.2 g cm−3 was identified as a critical threshold point; above and below this value, macroporosity and hydraulic parameters follow different functions with bulk density. Pedotransfer functions based on physical peat properties (e.g., bulk density and soil depth) separately computed for bog and fen peat have significantly lower mean square errors than functions obtained from the complete data set, which indicates that not only the status of peat decomposition but also the peat‐forming plants have a large effect on hydraulic properties. The SWRCs of samples with a bulk density of less than 0.2 g cm−3 could be grouped into two to five classes for each peat type (botanical composition). The remaining SWRCs originating from samples with a bulk density of larger than 0.2 g cm−3 could be classified into one group. The Mualem‐van Genuchten parameter values of α can be used to estimate Ks if no Ks data are available. In conclusion, the derived pedotransfer functions provide a solid instrument to derive hydraulic parameter values from easily measurable quantities; however, additional research is required to reduce uncertainty.

Effect of anisotropy on solute transport in degraded fen peat soils

“…The obtained K s values of the investigated peat soils ranged over two orders of magnitude from 0.06 to 15.01 cm h −1 , which matches roughly the previous reported range of values from 0.6 to 71.8 cm h −1 (Kruse et al, ; Liu & Lennartz, ) of fen peat. The K s values differed significantly between the two sites ( p < .001) although the peat of both sites is in a comparable degradation stage (e.g.…”

“…Total porosity was estimated based on bulk density and particle density. Macroporosity was estimated by the differences between total porosity and volumetric water content at −60 cm H 2 O pressure head assuming a contact angle of 0° for degraded fens (equivalent pore diameter of 50 μm; Liu & Lennartz, ; Schindler, Behrendt, & Müller, ). Recently, a contact angle of 51.7° was reported for bogs by Gharedaghloo and Price ().…”

“…More than 40% of European peatlands have been anthropogenically altered and are degraded because of drainage and climate change, losing their ecosystem functions as water storage and water filter (Joosten, ; Lennartz & Liu, ). Several studies have been conducted on hydro‐physical (Holden, Chapman, & Labadz, ; Liu & Lennartz, ) and solute transport properties (Liu, Forsmann, Kjærgaard, Saki, & Lennartz, ; McCarter, Rezanezhad, Gharedaghloo, Price, & Van Cappellen, ), as well as pore water chemistry (Tiemeyer, Pfaffner, Frank, Kaiser, & Fiedler, ; Zak & Gelbrecht, ) of degraded peat soils and their significant impact on carbon and nitrogen cycles could be demonstrated (Baird, Belyea, Comas, Lee, & Slater, ; Limpens et al, ; Liu, Zak, Rezanezhad, & Lennartz, ). Water flow and solute transport in peat soils are controlled by soil physical properties (e.g.…”

“…Peat soils exhibit highly heterogeneous and anisotropic properties. For instance, the saturated hydraulic conductivity ( K s ) may range over about two orders of magnitude for a specific peat soil (Cunliffe, Baird, & Holden, ; Liu & Lennartz, ). The anisotropic behaviour of K s has been studied over the last two decades (Beckwith, Baird, & Heathwaite, ; Cunliffe et al, ; Gharedaghloo, Price, Rezanezhad, & Quinton, ; Kruse, Lennartz, & Leinweber, ; Lewis, Albertson, Xu, & Kiely, ; Liu, Janssen, & Lennartz, ; Morris, Baird, Eades, & Surridge, ; Rosa & Larocque, ).…”

“…In more pristine peat, the dominant flow and transport direction depends on the peat forming process and how dying plants and decaying plant materials were deposited. With advancing peat degradation, the volume fraction of macropores and pore connectivity decrease significantly (Liu & Lennartz, , ; Liu et al, ) resulting in a relative isotropic structure of highly degraded peat soils (Kechavarzi, Dawson, & Leeds‐Harrison, ; Liu et al, ). In addition, cracks occur in peat soils in dry summers, which may increase macroporosity of peat soils (Holden, Burt, & Cox, ).…”