234 P. L. Gersper et al. 



On the rims of low-centered polygons and the tops of high-centered 

 polygons with mineral soil, total phosphorus in the surface horizon is 

 somewhat more abundant than in the wet meadow soils, but a greater 

 fraction of the total phosphorus is in an organic form, and inorganic 

 phosphorus is less abundant. The inorganic phosphorus of the drier site 

 is mainly NH4-F soluble, and considered to be readily available to plants 

 (Barel and Barsdate 1978). Increased availability of phosphorus is also 

 indicated by a higher average value of resin-extractable phosphorus (22.6 

 mg m"^) in the drier soils. However, the amount of inorganic phosphorus 

 in solution is lower than in the wet meadow. On the tops of high-centered 

 polygons the phosphate in the soil solution drops from 8 to 4 ppb at the 

 boundary between organic and mineral soil, 4 cm below the soil surface 

 (Barel and Barsdate 1978). 



INPUTS AND OUTFLOWS 



OF NITROGEN AND PHOSPHORUS 



The major input of nitrogen to the soils of the coastal tundra at Bar- 

 row is through the fixation of atmospheric nitrogen by blue-green algae, 

 either alone or in symbiotic relationships. The inorganic ions in precipi- 

 tation are the major sources of phosphorus and also add to the pool of 

 inorganic nitrogen. Inorganic forms of nitrogen move in the soil by dif- 

 fusion, but phosphorus ions are relatively immobile. Losses of nitrogen 

 and phosphorus, in both inorganic and dissolved or suspended organic 

 forms, occur through surface and subsurface flow. Nitrogen can also be 

 lost through reduction to gaseous forms, nitrogen oxides and nitrogen 

 gas. Even though a large portion of the tundra surface is covered with 

 lakes and small ponds, movement of nutrients between the terrestrial and 

 aquatic subsystems seems to be restricted to the period of snowmelt, 

 when there is a small net loss of nitrogen and phosphorus to the ponds 

 (Prentki et al. 1980). 



Nitrogen Fixation 



Fixation supplies the bulk of the nitrogen input to the terrestrial sys- 

 tem, although amounts entering by this pathway vary markedly between 

 microtopographic units. Measured rates increase from 8 to 180 mg N m"^ 

 yr"' along a moisture gradient from dry polygon rims to wet meadows. 



Blue-green algae are the most important agents of nitrogen fixation. 

 These algae, primarily Nostoc commune, occur as free-living or moss- 

 associated filaments, or symbiotically in lichens of several genera. Al- 

 though Peltigera aphthosa is the most abundant nitrogen-fixing lichen. 



