TERRESTRIAL DETRITUS AND THE CARBON CYCLE 321 



an enhanced microbial respiration. Even the creation of higher frequency of 

 drying— wetting cycles caused by canopy removal may accelerate decomposi- 



5 5 



tion. 



Other influences leading to accelerated decomposition are draining and 

 cultivation of swamps and marshes, fertilization and microclimatic influences of 

 ground fires, and erosion of soil organic matter from the land to aquatic systems. 



In summary, cultural practices invalidate steady-state assumptions, the most 

 important of which is that output equals input. How they change outputs is 

 problematical. Fundamentally, they tend to decrease carbon input into detritus 

 pools and to accelerate losses. In the short run, these effects tend to balance 

 each other out and minimize influences on the rate of carbon flux to the 

 atmosphere. In the long run, however, they will lead to a lower rate of flux. 



Because these influences are so varied, it would be an enormous task to 

 assess their current effects for the world — assuming sufficient data are available. 

 Such an assessment is beyond the limits of this paper. Furthermore, effects of 

 cultural practices probably are less than the range of estimates for carbon 

 turnover in detritus for steady-state systems given here. It would be premature 

 to produce a reasonable figure for cultural effects on carbon output when the 

 range of estimates on steady-state systems is still so broad. 



The best that might be accomplished at this point is to describe a pattern of 

 change that may be presently occurring, using reasonable but subjectively 

 derived figures. Equation 1 describes a widely accepted model for general 

 detritus-pool dynamics: 5 ' 



dX 



dT = A - LX <» 



where X is pool size, A is annual input, and L is the fraction of X decomposed 

 each year. At steady state 



^ = and ^ = X e (2) 



dt L 



where X e is the steady state of the pool. To describe the pattern of changes in 

 contemporary detritus pools, I have assumed that the world pool behaves as a 

 homogeneous unit or that these figures represent average rates. I have set initial 

 detritus input rates at 46 X 10 9 tons and modeled a decrease in rate in 

 proportion to human-population growth to a lower limit of 20 X 10 9 . Decay 

 rate (L) was initially set at 4.6%/year and was modeled to increase in proportion 

 to human-population growth to an upper limit of 10.0%/year. Human effects 

 somehow influence both these processes; a direct proportionality to population 

 seemed to be the simplest possible assumption. Human population follows 

 historical records from 1500 to 1965, after which it continues to grow at the 



