Introduction and Site Description 43 



Organic matter is abundant in the soil at Barrow, but it is not known 

 whether there is a long-term accumulation or loss. In part, the difficulty is 

 caused by the great quantity of organic matter— from 22 to 45 kg m " ^ (to 

 20-cm depth). This is 50 to 400 times the net annual primary productivity. 

 Given this large quantity, small changes are difficult to measure. In part, 

 the difficulty in calculating a long-term energy budget is caused by the 

 spatial variation and by the tremendous changes in such things as climate 

 and lemming effects from year to year. 



Two hypotheses have been proposed: 



1. The entire system is in steady state but the terrestrial system is 

 accumulating organic matter, while aquatic systems (lakes and ponds) are 

 degrading organic matter. Habitats at Barrow can change from meadows 

 to polygons to ponds to lakes and back to meadows. Thus, the long-term 

 effect is no net gain of organic matter as a given area of land moves 

 through the thaw-lake cycle. 



2. The system is not in steady state; accumulation continues until 

 conditions change. This accumulation is deep in the soil where the 

 decomposition decreases sharply with depth. Since the amount 

 accumulated at Barrow is nowhere near as great as is found in peat bogs, 

 this hypothesis requires that either I) the Barrow tundra system is young; 

 thus large peat deposits have not accumulated, or 2) recurrent 

 disturbances reverse the pattern of accumulation in any habitat. 



Nutrients 



Like energy, nutrients are almost all contained in the pool of soil 

 organic matter; less than 1% of both nitrogen and phosphorus is contained 

 in living biomass (Figure 2-7). This contrasts with the rain forest, for 

 example, where living organisms are a significant reservoir of nutrients. 



There is only a small pool of soluble soil nitrogen and phosphorus 

 available to plants; this is taken up or turned over many times during a 

 season. This pool is replenished from a much larger pool of exchangeable 

 nutrients, but the non-exchangeable pools are even larger. To replenish the 

 nutrients absorbed by plants, the pool of soluble plus exchangeable 

 nitrogen must turn over 1 1 times during a growing season; in this same 

 period the pool of phosphorus must turn over 200 times (3 times per day). 

 It is likely that there is a close connection between the rate of supply of 

 nutrients and plant production. Therefore, primary productivity in tundra 

 (as in tropic ecosystems) depends on the rate of decomposition. 



The Barrow ecosystem is very conservative with nutrients. For 

 example, most vascular plants have mechanisms for retaining nutrients, 

 particularly phosphorus, in belowground parts rather than allowing them 

 to be lost to decomposers. In the drier habitats, plants have mycorrhizae to 

 facilitate phosphorus uptake. Overall, nitrogen, calcium, and potassium 



