Soil water release curve construction

A soil moisture release curve was constructed by dew point psychrometry to estimate Soil water potential (ψ_soil, MPa) from soil volumetric water content (θ_g, g g⁻¹). Six dry peat plugs were hydrated, and its volume and weight measured. The plugs were broke-down into a layer and placed in individual trays to facilitate homogeneous drying in a lab bench. At intervals determined by weight loss, soil in each cup was homogenised and a small volume (0.21 cm³) was placed in a sample holder cup and packed to match the average apparent density in the hydrated plugs. Soil and holder were weighed and inserted in a psychrometric chamber (C-52, Wescor, USA). Water potential was determined after at least 6 h of equilibrium. This was repeated until the samples were dried down to approximately soil volumetric water content θ_v = 0.10 g g⁻¹ (Soil water potential ψ_soil = − 3 MPa). This θ_v corresponds to a plug weight of approximately 10 g. After each measurement, the sample and the holder were oven dried to constant weight to calculate the water content of each sample, which was used to determine θ_v at each point. The whole dataset was fitted to an exponential decay curve with Sigmaplot 12 (Systat Software Inc., San Jose, USA).