In situ measurements of the thermal properties of a northern peatland: Implications for peatland temperature models

Carbon uptake/release in peat soils (the latter especially in the form of methane) is strongly dependent on soil temperature. However, previous peatland temperature models have not represented soil temperatures accurately due to either errors in their parameterization or errors in the representation of heat transfer processes. Dual probe heat pulse sensors (DPHPS) and triple-probe heat pulse sensors (TPHPS) were used to provide detailed in situ measurements of the soil thermal properties of Sphagnum peat and the advective liquid heat flux resulting from percolating rainfall. The difficulties of applying the DPHPS approach within poorly decomposed Sphagnum peat are first considered. Our results suggest that measurement-induced advective vapor and liquid heat fluxes occurred when using the DPHPS. The analytical equations normally used to calculate the thermal diffusivity and volumetric heat capacity (C) from DPHPS measurements, therefore, cannot be applied within unsaturated Sphagnum peat. However, a numerical approach enabled the soil thermal properties to be estimated when an instrument-induced advective vapor flux was significant. When an instrument-induced advective liquid heat flux was significant, the relevant DPHPS data were identified and rejected. The measurements of the soil thermal properties were used to (1) develop an empirical model of the vertical variations in C through the unsaturated zone of poorly decomposed Sphagnum peat and (2) to develop an empirical relationship between the thermal conductivity (k) and C. The TPHPS measurements showed that the advective heat flux due to percolating rainfall occurred for a maximum of 24 hours after rainfall events. Its influence on soil temperatures may therefore be negligible.