Method to Determine Unsaturated Hydraulic Conductivity in Living and Undecomposed Sphagnum Moss

Sphagnum mosses (Sphagnum L.) are the primary peat-forming plant in northern peatlands and rely on capillary transport of water to facilitate physiological processes. The unsaturated hydraulic conductivity of the living, undecomposed, and poorly decomposed mosses is needed to estimate and model water flux to their growing upper layer. This study describes a new apparatus to measure this in the highly porous (approximately 90%) hummock profile where the pore sizes are large and the mosses delicate, in which established methods do not work. Independent tension disks controlled the pressure head (psi, between 0 and -35 cm of water) and the pressure gradient and thus flow. The uppermost 5-cm layer of moss had a saturated hydraulic conductivity of 1800 micrometer s-1, and decreased when unsaturated (psi = -25 cm of water) to 0.03 micrometer s-1. Moss 25 cm below the surface had equivalent values of 230 and 11.0 micrometer s-1 at moisture contents of 0.18 to 0.22 m3 m-3. The The soil water retention model RETC provided a good fit for both hydraulic conductivity and water retention when fitted simultaneously, but did not perform well to predict hydraulic conductivity from water retention data alone.