The Detritus-Based Trophic System 423 



aboveground production. In Eriophorum angustifolium, which makes 

 the largest contribution to this total because of its annual root system, 

 new roots occurred throughout the soil profile to a depth of 30 cm. New 

 roots of Carex were concentrated between 10 and 20 cm depth, and roots 

 of Dupontia were concentrated between 5 and 15 cm. Dennis (1977) 

 found 62% of the belowground live plant biomass, including rhizomes, 

 and 38% of the dead biomass in the top 5 cm of the soil. 



The depth distribution of plant-parasitic nematodes resembles the 

 distribution of live plant biomass below the ground. All other inverte- 

 brates show greater confinement to the surface layers than either biomass 

 or production of belowground plant parts. Clearly, a significant part of 

 the annual net primary production appears as growth below 10 cm in the 

 soil and is very little used by soil invertebrates. 



Since soil invertebrates constitute a detritus-based trophic system, a 

 positive relationship might be expected between the quantity of soil or- 

 ganic matter at the base of the food chain and the abundance of organ- 

 isms supported by this base, in much the same way that a rich plant bio- 

 mass may support an abundance of herbivores. In fact the total biomass 

 of soil fauna is inversely related to accumulated soil organic matter to a 

 depth of 20 cm (Figure 11 -3b) and the hypothesis that a large detritus 

 base leads to an abundance of animals in the detritus-based trophic sys- 

 tem must be rejected. Most of the organic matter lies below the layers of 

 abundant fauna. The soil animals are concentrated in the near-surface 

 organic-rich horizons, but they have access to only a small part of the 

 total pool of organic matter. All of the habitats sampled are highly or- 

 ganic in the near-surface horizons, and no relationship between faunal 

 density and organic matter between and 2.5 cm or between and 5 cm 

 is evident. 



Annual net primary production in the coastal tundra at Barrow also 

 shows a strong inverse correlation with accumulated organic matter (Fig- 

 ure 1 l-3c). The most productive plots are characterized by little accumu- 

 lation, hence rapid turnover of organic matter and recycling of nutrients. 

 In the less productive habitats the annual production of organic matter 

 and uptake of nutrients are small relative to the amounts tied up in ac- 

 cumulation. 



Rate of organic matter turnover may be taken as an index of total 

 microbial activity. The fact that it correlates poorly with microbial bio- 

 mass (Chapter 8) indicates that much of the microbial biomass is inactive 

 at any one time. Total invertebrate abundance, then, correlates positively 

 with the input and turnover of dead organic matter and with microbial 

 activity and productivity. 



Much of the annual input of dead organic matter and microbial pro- 

 duction lies below the depth of significant invertebrate density; thus, po- 

 tential food goes uneaten. This does not necessarily indicate that food is 



