Carbon and Nutrient Budgets 477 



solved inorganic nutrient pools from vegetation, microflora, animal 

 wastes, etc., is strongly seasonal. The exchangeable inorganic nutrient 

 pools may perform an important buffering function by diminishing the 

 size of soluble pools when concentrations are high and replenishing them 

 as concentrations decrease. Although the net annual flux into and out of 

 the exchangeable pool is small, the flux at any moment may be large. The 

 labile phosphorus pool, which replenishes phosphorus removed by 

 plants, is smaller in coastal tundra and may provide less seasonal buffer- 

 ing of soil solution concentration than in most soils (Brewster et al. 1975, 

 Barel and Barsdate 1978). 



The absorption of phosphorus (and presumably of other nutrients) 

 by vascular plants is much more strongly limited by the soluble nutrient 

 concentration in the soil than by temperature (Chapin and Bloom 1976). 

 Because the soluble nutrient pools are small relative to the annual plant 

 requirement, particularly for nitrogen and phosphorus, uptake by the 

 vegetation must depend upon simultaneous nutrient release by decompo- 

 sition or chemical exchange processes. Nutrient release by decomposition 

 and nutrient absorption by the vegetation are thus apparently closely 

 coupled. 



Nutrient release and nutrient uptake do not occur at constant rates 

 through the season. Microbial populations are characterized by several 

 population increases and crashes each growing season, due to a variety of 

 factors such as changing soil moisture and grazing by invertebrates. One 

 microbial population crash releases enough phosphorus to supply 90% 

 of the annual vascular plant requirement. Such population crashes re- 

 duce the biomass of microbes that might otherwise effectively compete 

 with vascular plants for nutrients. Thus, conditions that cause crashes in 

 microbial populations may be essential for nutrient uptake by vascular 

 plants. Further evidence for this hypothesis is presented elsewhere (Cha- 

 pin et al. 1978). 



Vascular plants of the coastal tundra are relatively conservative with 

 nutrients and replenish only about 20% of their nutrient capital each 

 year (Table 12-5). This estimate ignores losses from leaching, which may 

 be considerable for elements such as potassium. Much of the plant nutri- 

 ent capital invested in leaves is retranslocated to rhizomes during the lat- 

 ter half of the growing season (Chapter 5). 



Mosses play an important role in nutrient cycling as well, although 

 this role has not been documented for the coastal tundra at Barrow. 

 Mosses appear to derive their nutrients from plant leachates as well as 

 from the soil and snowmelt water and may effectively filter these nutri- 

 ents before they become available to vascular plants or microorganisms 

 (Tamm 1964). ,The nutrient concentration of brown moss tissue is very 

 similar to that of green tissue (Rastorfer 1978), and moss decomposition 

 rates are low. Therefore, mosses represent an important avenue by which 



