The Detritus-Based Trophic System 441 



organisms, based upon decomposition of the entire net primary produc- 

 tion, was given as 75 g m"^ yr''. Using this estimate, the consumption by 

 microbivores of 25 g m'^ yr"' accounts for 33% of the annual production 

 of microorganisms. If we hypothesize an annual accumulation of 10 g 

 m'^ yr'' and reduce the estimate of microbial production accordingly, 

 estimated consumption by microbivores increases only slightly, to 35%. 

 Alternatively, observed rates of decay indicated a maximum value of 90 g 

 m'^ yr"' for microbial production, which sets the level of consumption by 

 microbivores at 28%. 



Animal biomass and activity are strongly confined to the near- 

 surface layers whereas microorganisms are more evenly distributed, at 

 least through the organic layer. Overall, 71 % of the invertebrate biomass 

 occurs in the top 2.5 cm. Assuming 25% of the microbial biomass occurs 

 there (Figure 8-4), the effect of invertebrate feeding activity is magnified 

 nearly three-fold in the top 2.5 cm compared with estimates integrating 

 over all depths. Thus, animals might consume an amount approximately 

 equal to the annual microbial production in the top 2.5 cm. It is clear 

 that, at least in the surface layers of the tundra, animal activity may exert 

 a considerable influence upon microbial function, and hence upon the 

 decomposition process. Below 5 cm depth the impact of animals is prob- 

 ably minimal. 



THE ROLE OF SOIL INVERTEBRATES 

 IN NUTRIENT CYCLES 



Invertebrate production represents nutrients temporarily withdrawn 

 from cycling and unavailable to plants, and biomass represents the 

 amount of nutrients immobilized at any time. Since invertebrate biomass 

 contains a number of important nutrients such as phosphorus and nitro- 

 gen in concentration much greater than either living plant tissue or detri- 

 tus, these nutrients may be immobilized in amounts larger than dry mat- 

 ter or energy content might suggest. Can this amount be significant in 

 ecosystem dynamics? 



The density and surface concentration of soil invertebrates have pre- 

 viously been combined to express number of animals per unit volume of 

 the soil. The maximum concentration, about 125 f^g of biomass cm"', oc- 

 curred in the top 2.5 cm of the polygon trough. The organic matter den- 

 sity (bulk density x percent organic matter) at the same place is 68 mg 

 cm"'. This value includes live plant parts, microorganisms, invertebrates, 

 and dead organic matter. Thus, living invertebrates at their greatest aver- 

 age concentration represent less than 0.2% of the organic matter of the 

 system. Even allowing for the approximately five-fold concentration of 

 nitrogen and a three-fold concentration of phosphorus in animals rela- 



