308 REINERS 



or suffer from unavoidable alteration of in situ conditions which may cause 

 serious errors. 2 8 



In spite of these problems, data from the more thorough field studies 

 utilizing this method make much biological sense. Predictable correlations have 

 been found in the field between temperature, moisture, bacterial density, and 

 age of litter. ' Direct measurement of C0 2 output is of unquestionable 



value in illuminating responses to environmental and biotic factors, but because 

 of methodological problems and confounding contributions by roots and 

 mycorrhiza, data published to date probably reflect relative rather than absolute 

 rates. 



14 C-lsotope Methods 



Jenkinson has written excellent reviews on the application of 14 C for 

 soil-organic-matter studies. ' Some of these techniques are proving ex- 

 ceedingly valuable for research on humus chemistry, 4 but only techniques 

 applicable to estimation of total soil-organic-matter turnover will be outlined 

 here. 



The first application involves labeling plant material and following changes in 

 activities of detritus fractions over long periods of time. Although this method is 

 extremely useful in analyzing certain soil dynamics, its use for providing 

 estimates for a broad range of turnover rates would be limited by (1) difficulties 

 in uniformly labeling plant materials in such ecosystems as forests and (2) 

 administrative improbabilities in following through a wide range of experiments 

 over long periods of time. Such experiments would be feasible in such 

 short-statured vegetation types as grasslands 35 and at governmental or other 

 institutional field stations. 



The second application lies in dating organic matter in soil profiles by the 



C method. If systems are near steady state, carbon age represents mean 



residence time or turnover time. A number of such analyses have been made on 



humus horizons which have indicated rather high ages in the range of hundreds 



ft ^ ft— "^ R 



to 3000 years. ' Carbon-14 dating is inaccurate for periods less than 200 



years owing to the half -life of 5730 years and to additions of old carbon to the 

 atmosphere by fossil-fuel combustion. 6 Carbon dating will therefore be most 

 useful for measuring turnover of refractory portions of detritus — especially 

 humus, which is the major carbon reservoir in most soils. On the other hand, 

 humus represents a small fraction of carbon turnover in terrestrial detritus pools. 

 Kononova ' estimated that about one-third of detritus becomes humified; the 

 remaining two-thirds is completely mineralized rather rapidly. Jenkinson 33 

 reviewed a series of experiments on decomposition of labeled plant residues in 

 bare-field soils. At the end of 1 year, 68 to 73% of the carbon was lost in 12 

 tests; after 2 years, 70 to 79% was lost in 11 tests; and after 5 years, 79 to 85% 

 was lost in 8 tests. Thus 14 C dating is helpful but in itself inadequate for 

 estimating total detritus turnover. 



