TERRESTRIAL DETRITUS AND THE CARBON CYCLE 309 



The third C method is based on 14 C enrichment in recent organic material 

 due to nuclear explosions. In effect, testing has given a 14 C label to detritus 

 around the world. 6 As Jenkinson 6 has pointed out, dating by this method is 

 most precise in the period below 200 years where natural- 1 4 C-dating methods 

 are least reliable. This method basically requires an integrated measurement of 

 specific activity of total detritus entering the pool over a period of time, plus a 

 measure of the change in specific activity of the detritus. The ratio of 14 C input 

 to increase in * C activity of the detritus pool is an estimate of turnover time. 

 Operationally, measurement of changes in specific activity in the detritus pool 

 would be just as difficult as the measurement of the mass of carbon input itself, 

 especially root input as described under the input method. In addition, several 

 other assumptions must be met: (1) soil organic matter must be uniformly 

 subject to decomposition, (2) recycling cannot occur, (3) steady-state conditions 

 must hold, and (4) addition and decomposition rates cannot change. 6 In spite of 

 these restrictions, this technique may be useful in certain cases, and it is 

 important that long-range experiments be initiated on a wide range of ecosystem 

 types as soon as possible. 



ESTIMATION OF DETRITUS-POOL TURNOVER 



Even if difficulties inherent in direct C0 2 -evolution-measurement and 



C-isotope methods were resolved, extensive data for a wide range of 



ecosystems are not available. The following estimates of detritus-pool turnover 



are therefore based on litter-input data from the literature and, initially, on the 



assumption of steady states of detritus pools for most of the world. 



The First Approximation 



The first approximation is based on estimates of world annual net 

 production for ecosystem types together with my own rough estimates of the 

 percentage of production that enters detritus pathways (Table 2). 



The rationales for percentages are based on the few measures of energy flow 

 in these systems. They should be evaluated with the consideration that primary 

 production in terrestrial systems is usually estimated by the harvest method — a 

 method based on changes in biomass which rarely takes into account cropping 

 between measurements. Thus much of the energy loss to grazing has already 

 been subtracted. 



The swamp and marsh category unfortunately combines wetland forests and 

 herbaceous graminoid systems. It is generally believed that only about 5% of 

 primary production is lost to grazers in steady-state forests, 41 ' 42 and, in fact, 

 grazing losses are apparently low in marshes as well (about 8% in a Spartina 

 marsh ). However, some production is lost to the detritus pool through export 

 to marine systems from coastal marshes and to fires in inland marshes, so that 



