Vertical Profiles of DF

There are data on four Alaskan sites: Barrow (N 71°18', W 156 °47'), Franklin Bluffs (N 69°40', W 148 °41'), Sagwon (N 69°25', W 148 °41'), Fairbanks (N 64° 52', W 147 °52'). All four sites were represented by wet tundra with some peat accumulation; three sites are acidic (pH 4.5-5), and Franklin Bluffs is neutral carbonaceous soil. Figure 9.4 shows vertical variation of DF along soil profiles measured at -11°C. It was highest in the top soil layers, and declined with soil depth. The closest correlation was found between DF and the content of organic matter, as well as between DF and the total amount of microbial biomass estimated as phos-pholipids fatty acids (PLFA) (Fig. 9.5). This correlation was particularly clearly expressed in the Fairbanks profile, where two peaks of soil C were observed: the top modern humus layer at 0-25 cm and the old buried humus layer at 50-60 cm.

The coldest sites (Franklin Bluffs and Sagwon) had higher DF activity than the warmer Fairbanks site, although their activity above the freezing point of water was approximately the same (data not shown). Earlier, we have shown that also tundra and permafrost have higher below-zero activity than boreal soils, and in the Barrow

(•)14CO2 Uptake Rate, nmol d 1 (g soil) 1 or (O) Loss on Ignition, g (g soil) 1

(•)14CO2 Uptake Rate, nmol d 1 (g soil) 1 or (O) Loss on Ignition, g (g soil) 1

Fig. 9.4 Vertical distribution of below-zero microbial activity in three Alaskan sites: dark 14CO2 uptake (•) and loss on ignition (o) as a measure of the organic matter content. Note the difference in scales

site soil respiration at -20°C was consistently higher in deeper permafrost layers than in the top seasonally frozen soil, while the above-zero respiration displayed the reverse trend (Panikov et al. 2006). This observation supports the view on the adaptive nature of below-zero activity, acquired by specialized microorganisms under pressure of natural selection in polar regions. It is in agreement with the emergent ecological theory on acclimation and adaptation of Arctic organisms to cold conditions. This theory has been firmly established for plants and animals (Mooney and Billings 1961; Tjoelker et al. 1999) and now can be confirmed for microorganisms.

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