Hydrophysical properties of ombrotrophic peat show anisotropic patterns along a degradation transect

In previous studies we found that the vertical movement of solutes within water-saturated peat of Puergschachen bog is limited. Furthermore, it is evident from literature that the hydrophysical and chemical properties of peat are influenced by the parent material of peat (i.e. plant material and layering), land use and the decomposition of peat. The study site, Puergschachen bog (an Long Term Ecosystem Research (LTER) site located in the Enns Valley, Styria), exhibits different stages of degradation, ranging from slightly degraded peat (Center), intermediately degraded (Inter), and two more strongly influenced sites covered with Betula pubescens (Birch) and Pinus mugo (Edge), which allows investigations along a degradation transect.

The objective of this study is to address the following research questions: how do hydrophysical and chemical properties of peat vary along a degradation transect and to what extent does depth influence these properties? We hypothesized that the degradation of peat influences the hydrophysical (saturated hydraulic conductivity (kF), water retention (pF2.5), bulk density (BD)) and chemical properties of peat (dissolved organic carbon (DOC), aromaticity of DOC (SUVA₂₅₄) and total dissolved nitrogen (TDN)), and that these parameters vary with depth.

Hydrophysical parameters were measured under laboratory conditions using undisturbed peat samples from sites along a degradation transect in two depths (10–20 cm and 20–30 cm). For each site and depth, 5 replicates in vertical and horizontal direction were taken. Chemical parameters were measured for bog water sampled seasonally in 4 depths (10–20, 35–45, 60–70 and 85–95 cm). A non-parametric Man-Whitney-Test was used to test for significant differences between groups.

Our results revealed that BD differed significantly between Center (0.053 ± 0.011 (mean ± SD)) and Birch (0.071 ± 0.023) and Edge (0.076 ± 0.014 g cm^-3) and were generally slightly higher in upper horizons (10–20 cm). kF measurements showed that horizontal and vertical flow directions differ between sites as an anisotropic behavior of peat with higher horizontal conductivities in the upper (10–20 cm) and lower (20–30 cm) horizons for Center and Birch and higher vertical conductivities (both depths) for Edge, was observed. Water retentions at pF2.5 differed between sites and depths and were generally higher for deeper horizons, indicating reduced pore sizes, binding water stronger in pores. Also, differences between horizons were highest for Edge peat. Birch showed the highest DOC concentrations together with the highest aromaticity. DOC concentrations decreased with depth at all sites, while TDN and SUVA₂₅₄ showed no constant depth-related pattern.

Our results indicate that water and solute transport through peat is linked with peat degradation, which inhibits or allows movement within the soil. As shown, hydraulic conductivities can develop highly heterogeneous and anisotropic patterns of directional movement. Further studies are needed to assess the extent to which these heterogeneous hydrophysical properties affect solute transport and how this might influence peat decomposition processes.