Slow Molecular Diffusion in Frozen Soil as Possible Restriction Factor

Pure ice does not allow gas diffusion; that is why air entrapped in the Greenland and Antarctic ice has been used for chronological reconstruction of the Earth atmosphere (Brook et al. 1996). There are ice lenses in polar soils and subsoils which serve as barriers to gas diffusion. However, the bulk of permafrost and seasonally frozen soils represented by mosaic of frozen water, solid organo-mineral particles and fine network of gas-filled pores and channels should be conductive for gases and probably to soluble compounds. To find out the rate of gas diffusion, we used the following experimental approach. First, we obtained intact permafrost aggregates by gentle crashing the core avoiding its melting. One single intact aggregate (ca. 15 mm in diameter) was placed into a vial precooled to -20°C, 14CO2 was

Microbial Population, ng C (g soil)-1

Microbial Population, ng C (g soil)-1

Frozen Soil Distribution
Fig. 9.5 Vertical distribution of microbial groups as determined by membrane fatty acid analysis. Site: Fairbanks, forest soil with buried organic layer

injected into headspace, and after an exposure over 0.5-4 days, the label penetration was quantified by serial washing of the aggregate with cooled (~ 0°C) 0.5 N NaON. The alkaline solution was used to remove layer-by-layer the surface material containing label, leaving the aggregate core frozen. The accompanying reduction in the aggregate size was recorded with a TV camera and converted to volumes by image analysis, and the leached label was counted by scintillation.

Results are presented in Fig. 9.6 for the Fairbanks soil sample taken from the second buried organic layer. Contrary to pure ice, this permafrost is highly conductive to gases. The apparent diffusion coefficient for CO2 as estimated from its spatial gradient was found to be 6.9x10-9 cm2 s-1. For comparison, the diffusion coefficient for CO2 in air at the same temperature of -20°C is 0.119 cm s-1, or 107 times higher. Another Fairbanks sample taken from the lower mineral layer (70-80 cm) displayed 15 times slower 14CO2 diffusion. The most probable mechanism of gas

012345678 Distance from surface of aggregate (mm)

Fig. 9.6 Diffusion of 14CO2 to inner space of the frozen aggregate. The insert shows 14CO2 concentration distribution inside of 14.5 mm permafrost aggregates from Fairbanks, layer 50-60 cm, after 48 h of exposure to labeled gas at -20°C

penetration into permafrost is molecular diffusion via tiny aeration pores. Judging from gas penetration dynamics, the partial contribution of aeration pores to bulk volume of this permanently frozen organic soil layer (about 50% of organic matter) was as low as 5.8x10-8 (compared with the typical value of 0.2-0.4 for top soils at a moisture content of 50% of the maximum water-holding capacity). Obviously these frozen mineral soils are even less conductive. In any case, the tested soils have enough air-filled micropores to support slow aerobic growth. Apart from CO2 and O2, frozen soils should allow also the delivery of volatile organic substrates (alcohols, hydrocarbons, fatty acids) as a carbon and energy source for heterotrophic microorganisms.

We have not measured the mobility of non-gaseous compounds, and respective rates are expected to be slower by a factor of 105, the difference in diffusivity of gases in gas and liquid phases. Even such low mobility could be sufficient to deliver compounds soluble in unfrozen water films around cells. Indirect confirmation of this possibility comes from our data on oxidation of 14C-glucose added to permafrost from Barrow in the temperature range from 0 to -35°C (Panikov et al. 2006).

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