Ninth International Conference on Permafrost ... - IARC Research

The Effect of Soil Moisture and Ice Content on the Thermal Conductivity ofOrganic Soil Horizons Underlain By Discontinuous PermafrostJonathan A. O’DonnellDepartment of Biology and Wildlife, University of Alaska FairbanksVladimir E. RomanovskyGeophysical Institute, University of Alaska FairbanksJennifer W. HardenUnited States Geological Survey, Menlo Park, CAKenji YoshikawaWater and Environmental Research Center, University of Alaska FairbanksIntroductionPermafrost temperatures in Alaska have warmed in recentdecades in response to changing climatic conditions athigh latitudes (Lachenbruch & Marshall 1986, Osterkamp& Romanovsky 1999). In Interior Alaska, where forestsare underlain by discontinuous permafrost, mean annualground surface temperature (MAGST) often exceeds 0°C,yet permafrost can exist in a stable state (< 0°C) due tothermal offset (the difference between MAGST and meanannual permafrost surface temperature, or MAPST; Burn& Smith 1988). Thermal offset persists because the thermalconductivity of ice is greater than that of water, so soilsconduct heat more effectively in winter than in summer.Soil thermal models typically use a fixed thermalconductivity value for frozen and thawed soils. This bimodalapproach, however, may be too simplistic to accuratelypredict soil temperatures and active layer depth (Overduinet al. 2006). Thermal conductivity of organic soil horizonsvaries considerably with moisture and ice content and is acritical control on active layer depth (Yoshikawa et al. 2003).Organic horizon type, which varies in bulk density, moisturefield capacity, and extent of decomposition, may alsoinfluence thermal conductivity values. The primary objectiveof this study is to evaluate the effects of soil moisture and icecontent on the thermal conductivity of three organic horizontypes from black spruce forests of Interior Alaska.A. David McGuireInstitute of Arctic Biology, University of Alaska FairbanksIn the lab, all samples (n = 5 per site per horizon, total= 90 samples) were saturated and dried at air temperature,during which time we weighed each soil block andmeasured thermal conductivity using a KD2 Pro ThermalProperties Analyzer (Decagon Devices, Inc., Pullman,WA, USA) to generate a thermal conductivity—moisturecontent relationship for each sample from each horizontype. Measurements of thermal conductivity as a functionof ice content are in progress. In 2005, we measured VWCin the field near Hess Creek, Alaska, using ECH2O probes(Decagon Devices, Inc., Pullman, WA) in replicate plots.Following field measurements, all probes and soil blockswere calibrated (O’Donnell et al. in review).ResultsVolumetric water content (VWC) of feather moss species,such as Hylocomium splendens, is generally around 10% (byvolume) during summer months (Fig. 1). VWC of Sphagnumspp. is generally higher, ranging from 15–40% (by volume;Fig. 1).We observed strong positive and linear correlationsbetween thermal conductivity and organic VWC across all60Methods50Feather mossSphagnum spp.We conducted a laboratory experiment to examinethe effects of moisture and ice content on the thermalconductivity of organic soils collected from black spruceforests underlain by permafrost. Soil samples were collectedfrom three moderately drained and three poorly drained blackspruce forests in Interior Alaska. Organic soils were dividedinto three distinct horizons (live/dead moss, fibric, mesic/humic), that differ with respect to bulk density and extentof decomposition. Feather moss (Hylocomium splendensand Pleurozium schreberi) was the dominant moss speciesfrom the moderately drained stands, whereas Sphagnum spp.(most commonly S. fuscum) dominated forest floor cover inthe poorly drained stands.Organic VWC (%)403020100May Jun Jul Aug SepFigure 1. Seasonal variation in volumetric water content (VWC)of fibric organic matter near Hess Creek, Alaska (O’Donnell el al.in review).227