254 P, L. Gersper et al. 



ciently high to suggest that weak nitrogen immobiHzation and strong 

 phosphorus immobiHzation should be associated with decomposition. 

 However, nitrogen mineraUzation has been shown to occur under cold, 

 anaerobic conditions, perhaps because of low tissue nitrogen concentra- 

 tions and low efficiency in the decomposer population. Phosphorus min- 

 eralization may respond to these same factors and be further facilitated 

 by selective degradation of phosphorus-rich substrates. 



Nitrifying bacteria are not common in the soil, and their activities 

 are inhibited by low temperatures. Changes in the amount of nitrate 

 present indicate low rates of nitrification in the wet meadows, and higher 

 rates in drier microtopographic units. 



Several internal pathways may aid in replenishing inorganic nutri- 

 ents in the rooting zone. Studies with "N indicate a substantial flux of N 

 from the subsoil to the surface. Freezing and rethawing of the soil liber- 

 ate some available nitrogen and phosphorus. Weathering of minerals 

 containing non-exchangeable phosphorus may also occur. In a high lem- 

 ming year, lemming excreta contribute substantial amounts of available 

 nitrogen and phosphorus. 



