APPENDIX 377 



To appraise regional or global effects of man's activities requires knowledge 

 of the specific effects of toxic compounds. Studies of such effects by smog and 

 heavy metals are under way at present. The acidification of rain over the 

 northeastern United States may present a particularly appropriate opportunity 

 to document the extent of toxification on a regional basis. 



Detritus Pools 



As shown in Fig. 1, both marine and terrestrial detritus pools are in the same 

 size range as terrestrial biomass. Although these pools appear to be relatively 

 easy to measure, estimates are probably not as good as those for biomass. Some 

 fraction of detritus is relatively labile, with a range of turnover time of minutes 

 to decades, while the rest is relatively refractory, turning over in periods of 

 decades to centuries (see Riley, this volume). In terrestrial systems this latter 

 fraction usually consists of amorphous, altered organic material referred to as 

 humus by soil scientists. Humus is an extremely valuable resource which imparts 

 much of the water and nutrient-holding capacities of soils and is very influential 

 in maintaining beneficial physical soil conditions. Stress will be placed, 

 therefore, on the role of humus in the carbon cycle; but it should be emphasized 

 that net losses of humus through land-management practices can generally be 

 regarded as permanent losses to the productivity of terrestrial ecosystems. 



The Work Group on Ecological Effects that contributed to the SCEP 

 Report and other investigators reporting in this volume found it desirable to 

 divide organic carbon into short and long residence times in making a model for 

 the carbon cycle. Short-lived material includes leaves, litter, short-lived animals, 

 and most algae, while long-lived material includes wood, large roots, upper soil 

 humus, and dead organic matter in the oceans. Tables 1 and 2 are from Tables 

 2.A.1 and 2. A. 2 of the SCEP Report; 1 note the first footnote of Table 1 which 

 states that resistant humus and other material have been omitted. Probably a 

 large part of organic carbon has thus been deleted, but it is impossible to tell 

 how much from the brief description in the SCEP Report. 



Terrestrial detritus is conveniently concentrated at the soil surface. It is 

 relatively easy to sample and analyze except for humus that requires careful 

 carbon determination. It would be conceptually simple but logistically burden- 

 some to sample detritus pools on a worldwide basis where data are not complete 

 or available. Considerable data are available in the literature, however, and with 

 thoughtful interpretation of such data, much of the world picture could be 

 compiled. One major failing of the literature is inadequate assessment of deep, 

 dispersed humus. The problem is most apparent in some podsols where organic 

 matter in the B horizon, which has largely been ignored, can be greater than all 

 the organic matter in the A horizon. With these precautions, it would be 

 relatively simple to produce a reasonable estimate of current pool sizes with a 

 well-directed program combining field sampling and literature review. 



Perhaps to a larger degree than for living biomass, it is convenient to view 

 detritus as a mixed pool where steady-state levels are controlled by a balance of 



