320 REINERS 



developed in this paper, but, since neither author explains the basis for his 

 estimate, it is impossible to evaluate these discrepancies. 



INFLUENCES OF CULTURAL ACTIVITIES ON TERRESTRIAL 

 DETRITUS TURNOVER 



Human activities vary between ecosystem types, but, in general, they tend to 

 divert primary production from detritus pools to human uses and to accelerate 

 decomposition of stored detritus. Whenever harvesting occurs — through haying 

 marshes, cutting timber, grazing savannas and grasslands, or increasing the 

 frequency of burning in forests, shrublands, or grasslands — organic matter is 

 diverted from the organic processes of decomposition. None of these effects 

 influences the carbon cycle in the long term; carbon is ultimately returned to the 

 atmosphere as C0 2 through combustion, respiration of domestic animals, or the 

 rotting and burning of lumber. These effects do decrease carbon input into 

 detritus pools, however, and, in the long run, will decrease pool sizes and net 

 output from these pools to the atmosphere. 



The effects of forest harvesting are somewhat ambiguous. Tree-bole removal 

 substantially decreases carbon return to the soil. Where periodic cutting is 

 practiced, the proportion of net primary production flowing to detritus is 

 decreased roughly one-half. On the other hand, there is some evidence that 

 litterfall, excluding timberfall, is higher in younger than in older stands. 49 

 Nevertheless, it is doubtful that higher litterfall compensates for loss of tree 

 boles, so that the net effect of forestry is to reduce pool size and carbon 

 turnover. 



At the same time that cultural activities divert inputs from detritus pools, 

 they tend to accelerate decomposition of stored carbon. The most obvious of 

 these is conversion of forest land and grassland to cultivation. Such surficial 

 detritus as the L and F layers of forest floors and the mulch of grasslands is 

 rapidly eliminated, and the more refractory humus is slowly decomposed to 

 new, lower steady states with slow turnover rates. Rates of decline of the order 

 of 50% in nitrogen content of agricultural soils have been described. 52-5 Since 

 soil nitrogen is largely organic, it is reasonable to assume that carbon decreases in 

 parallel with nitrogen. Plass estimated that such agricultural activity increased 

 carbon release from soils 0.5 X 10 9 tons C/year above that fixed by photo- 

 synthesis. Humus-decomposition rates are very likely higher under these 

 circumstances than would be estimated from 14 C-dating methods on undis- 

 turbed soils because of the mechanical mixing and enhanced aeration caused by 

 cultivation itself and by the introduction of inorganic fertilizers that may 

 stimulate the microflora. In contrast to cultivation effects, land converted to 

 well-managed pasture may actually accumulate organic matter. 54 



Decomposition of stored organic carbon is accelerated in other ways. 

 Opening of forests tends to raise the temperature of the forest floor, leading to 



