220 P. L. Gersper et al. 



SOIL PHYSICAL PROPERTIES AND NUTRIENTS 



Biological processes in the soils of the coastal tundra at Barrow oc- 

 cur in an organic-rich layer less than 50 cm thick that is thawed for less 

 than four months of the year. This layer contains over 70% of the living 

 biomass of the tundra ecosystem. In it, roots grow and take up nutrients 

 and water, organic matter decomposes, invertebrates graze and prey 

 upon one another, and lemmings burrow for summer protection from 

 predators. 



This shallow layer of thawed soil is the reservoir from which inor- 

 ganic nutrients are initially supplied to the living organisms. Calcium, 

 magnesium, potassium and sodium are all retained by the cation ex- 

 change complex, which is made up largely of humified soil organic mat- 

 ter. The organic matter itself contains the major pools of nitrogen and 

 phosphorus. However, most of the available nitrogen is in the form of 

 ammonium and is retained on the soil exchange complex, while most of 

 the available phosphorus is bound to iron or aluminum ions. The pools 

 of available nutrients are highly variable, both spatially, because of the 

 different kinds of soils associated with the different microtopographic 

 landforms, and temporally, in response to fluctuations in environmental 

 conditions. The underlying permafrost affects the nutrient supply 

 through its effects on temperature gradients in the thawed soil and by 

 isolating large quantities of nutrients contained in the frozen soil. 



Organic Matter 



The predominant characteristic of the soils is their high proportion 

 of organic matter. More than 95% of the total organic matter in the ter- 

 restrial tundra ecosystem is below the ground surface, and one-third is in 

 the upper 10 cm of soil, where biological activity is concentrated. The 

 large amounts of organic matter impose a particular structure on the soil, 

 influence the flux of moisture, oxygen and heat, and modify the chemical 

 properties, particularly in the cation exchange complex. The large pro- 

 portion of organic matter in these soils has a strong effect on the nutrient 

 supply, as is typical for arctic tundra soils (Babb and Whitfield 1977, 

 Chapin and Van Cleve 1978). 



Carbon and nitrogen, in an average ratio of 20:1, make up from 10 

 to 40% of the total soil weight (Figure 7-1). Total carbon contents in the 

 upper 15 cm of soil typically range from about 12,000 to 16,000 g m"^ but 

 may be less than 10,000 g m"^ in comparatively warm, nutrient-rich, wet 

 Pergelic Cryaquepts of polygon troughs, where decomposition rates are 

 high. 



