The Soils and Their Nutrients 253 



SUMMARY 



Organic matter, generally in a partially decomposed (hemic) state, 

 dominates the soil profiles of the coastal tundra at Barrow, and consti- 

 tutes the major pool of fixed carbon in the ecosystem. The bulk density 

 of the highly organic soil is low, but increases with advancing decomposi- 

 tion. The soils remain very moist throughout most summers, have high 

 cation exchange capacities, and are moderately acid, with the lower pH 

 levels correlated with lower primary productivity. 



Almost all the nitrogen in the soil is present in organic form, and a 

 large fraction of this is associated with poorly decomposed material. A 

 small and variable amount of labile organic N is also present. In wet 

 meadows inorganic nitrogen is mainly in the form of ammonium, and 

 more than half is found below the primary rooting zone. In the wet 

 areas, nitrate concentrations in the soil solution are very low, but in the 

 driest units, nitrate concentrations exceed those of ammonium. Most of 

 the soil phosphorus is also in organic forms, and the concentrations of 

 inorganic phosphorus in the soil solution are extremely low. 



Nitrogen fixation by blue-green algae is the major input mechanism 

 for nitrogen. These algae may be free-living forms, but in many cases are 

 associated with mosses or occur symbiotically in lichens. The predomi- 

 nant algal and lichen forms involved in nitrogen fixation are Nostoc 

 commune and Peltigera aphthosa, respectively, although several other li- 

 chen species are also active. The biomass of nitrogen-fixing organisms is 

 highest in wet, mossy areas, and is extremely low in dry areas. In mesic 

 sites, moisture is usually the major factor controlling the input of nitro- 

 gen, but oxygen concentration and temperature are also important. The 

 low oxygen concentrations that occur in wet, mossy areas may enhance 

 the rates of nitrogen fixation. A simulation model indicates that a simple 

 multiplicative interaction between these factors may be involved, and 

 that the control mechanisms for nitrogen fixation may be similar at other 

 tundra sites. 



Inorganic nitrogen and phosphorus enter the system through precip- 

 itation. The amounts added are small in comparison to the total pools of 

 these elements, but substantial with respect to available inorganic pools. 

 The major losses of nitrogen and phosphorus occur in runoff during 

 snowmelt and are mainly of organic forms. The combination of precipi- 

 tation and runoff yields a net loss of nitrogen and phosphorus. Some ni- 

 trogen is also lost by denitrification, but the rate is low compared to the 

 potential for nitrate reduction that exists in the wetter microtopographic 

 units. Nitrogen fixation is sufficient to lead to a net accumulation of soil 

 nitrogen. 



The ratios of carbon to nitrogen and organic phosphorus are suffi- 



