The Soils and Their Nutrients 227 



moisture regimes are sufficiently influenced by microtopography to re- 

 sult in differences in species composition and physiognomy of the above- 

 ground plant community, and in the soil microflora and fauna. 



Oxygen concentrations at 10 cm ranged from 40 Vo saturation to 0%, 

 with the highest values found in the tops of high-centered polygons and 

 the lowest in the wet sloughs (Figure 7-6). As the soil thaws, the depth to 

 fully anaerobic conditions follows the thaw front downward, and by 

 mid-season oxygen saturation is zero at about 25 cm depth. Although the 

 soil continues to thaw, the aerated layer in the wet meadow soil seldom 

 exceeds 25 cm because oxygen flux is impeded by high bulk density and 

 water saturation in the mineral horizons (Benoit, unpubl.). 



Alternating organic and mineral layers in the soil can produce a very 

 complicated pattern of air and water movement through the active layer 

 (Figure 7-3). Histosols generally lack a continuous mineral layer and con- 

 tinue to drain freely as thaw progresses. These soils are unsaturated in 

 the upper part, permitting air movement within the soil, except in wet 

 summers. In contrast, the mineral layers of Inceptisols restrict drainage, 

 and these soils often remain at or near saturation throughout the sum- 

 mer. Air movement is restricted, and reducing conditions prevail in and 

 below the mineral layers. 



Cation Exchange Capacity and Acidity 



The cation exchange capacity (CEC) of the soils is dominated by the 

 organic fraction. Thus there is a strong correlation between CEC and or- 

 ganic carbon content. For example, within the upper 30 cm of meadow 

 soils this relationship was: 



CEC[meq(100 g)"'] = 2A5C^^^{%) + 15.54 r = 0.97, n = 86. 



This equation indicates that the mineral clay fraction, which makes up an 

 average of 21% by weight within the upper 30 cm of these soils, contrib- 

 utes 15 milliequivalents (meq) per 100 grams of soil to the CEC, while the 

 organic fraction, which averages approximately 2097o carbon, contributes 

 40 meq (100 g)"'. These combine to give the average soil in the meadows a 

 total CEC of 55 meq (1(X) g)'', which is well above that of most mineral 

 soils. 



In general, poorly decomposed fibric organic matter contains rela- 

 tively few phenolic hydroxyl and carboxyl groups, and thus contributes 

 comparatively little CEC to soil horizons in which it occurs. On the other 

 hand, well-humified sapric organic matter generally contains many such 

 groups, and in many of the soils may be the main source of the CEC. 



Cation exchange capacities ranged widely among soils of the differ- 



