Carbon and Nutrient Budgets 475 



Input^ 



. Runoff? 



Soil 



organic 



Mg 



(0,561 



Soluble 



magnesium 



Exchangeable 

 magnesium 



93.9 



c. Magnesium. 



FIGURE 12-3. Annual budgets for calcium, potassium and magnesium 

 in the Barrow wet meadow tundra intensive study sites to a depth of 20 

 cm. The area of each box is proportional to its compartment size, which 

 is indicated in g m'\ Values next to arrows indicate annual fluxes in g m~^ 

 yr'\ Assumptions for budgets and sources of data are presented in Fig- 

 ures 12-1 and 12-2. Soil nutrient pools are calculated from Gersper 

 (unpubi). 



slum content of litter is leached during snowmelt, so that decomposition 

 plays only a minor role in the cycling of this element (Figure 12-3). In 

 contrast, almost no calcium is leached from litter, so calcium must recy- 

 cle exclusively through the decomposition process. However, this ele- 

 ment is relatively abundant in the marine sediments from which soils of 

 the Coastal Plain are derived. The soil contains a large exchangeable 

 pool (Figure 12-3), suggesting that calcium does not strongly limit pri- 

 mary productivity. Calcium addition does not stimulate production in 

 cottongrass tundra at Atkasook, 100 km south-southwest of Barrow 

 (McKendrick et al. 1980). Sixty percent of the phosphorus and 80% of 

 the nitrogen contained in litter must be recycled through the decomposi- 

 tion process (Figure 12-2), and both may occur in compounds that are 

 not readily broken down. Both of these nutrients have been found to 

 limit primary productivity in the coastal tundra at Barrow and at other 



