Soil Hydraulic Properties of Sphagnum Moss and Peat.

In peatlands, the moisture state of the vadose zone (acrotelm) decisively controls the organic matter turnover and consequently determines whether they act as sinks or sources of atmospheric carbon. Water and air content inherently control oxygen diffusion rates, heat, solute, and particle transport and thus influence the redox state. For prediction purposes, the hydrological processes must be epitomised in computer models to link the terrestrial water cycle and the carbon cycle. This requires an accurate determination of effective soil hydraulic properties which are an intrinsic input to the Richards equation, the standard model for variably-saturated flow processes in porous media. Currently we are testing and improving parametric soil hydraulic models to select the most suitable for describing sphagnum moss and peat hydraulic properties. Therefore, we conducted a series of laboratory evaporation experiments on samples from an ombrotrophic peatland. The samples were taken in large blocs, saturated with water, and frozen. Using a drill rig, vertical cores of 8 cm diameter were obtained and cut into 5 cm segments, yielding subsamples of approximately 250 cm³ corresponding to 8 sampling depths over the initial 40 cm below the surface. The soil hydraulic function parameters were identified by inverse modelling of the experiments with the Richards equation. Noteworthy, is that we conceptualise the peatlands vadose zone as rigid mineral soils, simplifying that they exhibit entirely different inherent attributes. Notwithstanding, our results indicate the validity of this assumption. Further, a shift of the hydraulic properties with depth, reflecting the pedogenesis of the acrotelm, is apparent. In peatlands, the unimodal van Genuchten Mualem model finds frequent application, whose suitability we critically assess. Finally, we discuss alternatives which account for multimodal pore size distributions over a wide moisture range, accounting for capillary and non-capillary storage and flow, and isothermal water vapour transport in the process model.