Carbon and Nutrient Budgets 473 



Nitrogen is lost from the system primarily in spring runoff, but these 

 losses are less than the annual gain through precipitation and N fixation. 

 The large accumulation of nitrogen in tundra systems (Table 12-1) sug- 

 gests that nitrification and/or denitrification are restricted more severely 

 than nitrogen fixation under tundra conditions. Denitrification rates are 

 very low, even though facultative denitrifying organisms are abundant in 

 the tundra at Barrow (Chapter 7). Low phosphorus availability is one 

 factor limiting denitrification rates (Barsdate and Alexander 1975), and 

 phosphorus thereby plays a role in the accumulation of organic nitrogen. 

 Assuming that organic carbon accumulates in parallel with organic nitro- 

 gen, the continued accumulation of organic matter will further reduce 

 phosphorus availability (Chapter 7) and hence denitrification rate. This 

 positive feedback loop may continue to operate in the absence of large 

 external inputs, as discussed above. 



Transfer Within the Ecosystem 



Because of the low nutrient input to the coastal tundra the function- 

 ing of this ecosystem depends greatly upon internal recycling of the exist- 

 ing nutrient capital, more so than do temperate systems. Yet the low tem- 

 perature regime of tundra restricts these rates of nutrient cycling, both 

 directly through temperature effects upon biological processes and wea- 

 thering and indirectly through the occurrence of permafrost, which re- 

 stricts drainage and results in poorly oxygenated soil. 



Fungi are important in the breakdown of nutrient-containing com- 

 pounds in litter and in better drained soils (Chapter 9). However, in 

 waterlogged, low-oxygen soils, fungal metabolism is depressed and bac- 

 teria dominate (Chapter 8). In contrast to fungi, bacteria at Barrow lack 

 the capacity to break down complex substrates at low temperature 

 (Chapter 9). Thus, any nitrogen and phosphorus contained in complex 

 organic molecules might tend to accumulate in the anaerobic zone be- 

 cause of the absence of fungi and the inability of bacteria to attack such 

 substrates at low temperature and low oxygen. Such nutrients would be 

 largely removed from active cycling within the ecosystem. Low tempera- 

 ture also has direct and indirect effects upon microbial growth (Chapter 

 9), such that microbial biomass is an order of magnitude less than that 

 characteristic of temperate grasslands (Chapter 8). As soil organic matter 

 accumulates, the soil becomes more acid, restricting the kinds of bacteria 

 and further constraining decomposition. In short, the arctic climate re- 

 stricts decomposition, but many of the temperature effects are indirect 

 and complex as consequences of permafrost, low oxygen and acidity. 



The cycling of some nutrients is more directly dependent upon de- 

 composition than the cycling of others. Eighty-two percent of the potas- 



