222 P. L. Gersper et al. 



identifiable plant components can be recognized which disintegrate only 

 after considerable mechanical manipulation. Such materials commonly 

 dominate the entire soil profile in basins of low-centered polygons and in 

 some of the large, very low relief orthogonal polygons of mesic and wet 

 meadows. 



Differences in the amount of organic matter over lateral distances of 

 only 1 m can be more marked than differences with depth. In an appar- 

 ently uniform area of wet non-polygonal terrain, the organic carbon in 

 two soil profiles sampled only a meter apart differed by a factor of 2. 

 However, differences between microtopographic units are generally 1.5 

 to 2 times greater than variations within these units. 



Bulk Density, Porosity, and Texture 



The bulk density of a soil has a strong effect on heat conduction and 

 temperature, depth of thaw, soil water content and movement, soil por- 

 osity and air content, and the penetration of roots. Bulk densities in the 

 soils of the coastal tundra at Barrow range from about 0.05 to about 1 .50 

 g cm'\ Differences in bulk density (D,,) along a microtopographic gradi- 

 ent are strongly associated with the percentage of organic carbon (C.„«) 

 present: 



C.,(% by weight) = 1.1-57.15 log(A,) r = 0.93, n > 200 



The lowest bulk densities are found in surface horizons, which have 

 high contents of fibric organic matter, live and dead roots and moss (Fig- 

 ure 7-2). Within the thawed layer, soils of wet and mesic meadows have 

 bulk densities that range from moderately high to low, with a tendency 

 toward increasing bulk density with increasing wetness. Rims of low- 

 centered polygons and centers of high-centered polygons have soils that 

 range in bulk density from low to intermediate, depending on the nature 



Meadow 



Basin 



Trough 



Rim 



BULK DENSITY, g cm 



FIGURE 7-2. Bulk density of soils from four dif- 

 ferent microtopographic units. 



