50 J. E. Hobbie 



proposed: the entire system is in balance but there is accumulation on land 

 and net decomposition in ponds; the system is accumulating organic 

 matter and this will continue until conditions change. 



Nutrients are tied up in the soil organic matter; less than 1% is in the 

 biota. To replenish the nutrients absorbed by plants, the pool of soluble 

 plus exchangeable nutrients must turn over 1 1 and 200 times a year for 

 nitrogen and phosphorus. It is likely that this supply ratio, and also the 

 primary productivity, depend upon the rate of decomposition. Despite 

 differences in nutrient concentrations in the soil, almost all the nutrients 

 are present in similar amounts from site to site. The exception is 

 phosphorus where plants from the most productive sites have the highest 

 concentrations. This evidence, plus evidence from fertilization studies, 

 suggests that phosphorus more strongly limits primary production than do 

 other nutrients such as nitrogen, calcium, or potassium. Nutrients may 

 become concentrated into troughs between polygons during the winter. 

 Lemmings build nests in the troughs and deposit most of their feces there 

 but forage over a larger area. 



A study of the nitrogen budget revealed that the most important input 

 was nitrogen fixation which was 3 times the precipitation input. Outputs 

 due to denitrification and runoff were small; 65% of the input was stored in 

 organic matter. 



It was concluded that the terrestrial ecosystem of the tundra is rich in 

 total nutrients and energy but poor in amounts of nutrients and energy 

 actually available and circulating. Decomposition rates regulate the 

 system. 



