The Soils and Their Nutrients 



223 



of the soil formed before elevation of the rims and centers. Soils of the 

 basins of low-centered polygons have low bulk densities. The general in- 

 crease in bulk density between soils of basins and rims of these microtop- 

 ographic units is partly due to the more highly decomposed organic mat- 

 ter in the rims and partly because the rims generally contain a higher con- 

 tent of fine-textured mineral matter. The relatively low content of organ- 

 ic matter in the Cryaquept soils found in most polygon troughs results in 

 high bulk densities, although some soils in the troughs are highly 

 organic, with correspondingly low bulk densities. The highest bulk den- 

 sities of the coastal tundra at Barrow are generally encountered in the 

 sandy soils along stream margins. 



The bulk density of a tundra soil, as it reflects mineral content and 

 water-holding capacity, is an important determinant of the depth of thaw 

 (Gersper and Challinor 1975). Under similar moisture regimes, mineral 

 soils generally thaw more deeply than organic soils because of their 

 higher bulk density and consequent increased heat conduction. However, 

 soils with low bulk density also tend to have high moisture content, 

 which is also associated with deeper thawing. Although the variables are 

 strongly covariant, multiple regression analysis suggests that bulk density 

 alone accounts for 35% of the variation in thaw depth, while moisture 

 content accounts for an additional 31%. 



The soils are highly porous, with a range of porosity from 50 to 65% 

 for mineral layers, increasing to more than 90% for organic layers (Fig- 

 ure 7-3). Thus, these soils are more porous than mineral soils of temper- 

 ate regions, which range from 30 to 60% porosity (Hausenbuiller 1974). 



Pore Volume, % 

 25 50 75 



GO 



0.25 0.50 



Bulk Density, g cm 



0.75 



3 



FIGURE 7-3. Bulk density, per- 

 centage total pore volume, and 

 1 00 percentage air-filled pore volume 

 in soil of a moist meadow. 



