The Nature of Recycling 

The Faunal Factor From the plant's point of view, nutrient recycling must make available the limiting chemicals incorporated within the forest floor litter and provide a supply of required chemicals from the inorganic geologic substrate. Because plant roots are incapable of secreting enzymes to carry out these chemical transformations (Curl and Truelove 1986), plants are ultimately and completely dependent upon exoenzyme production by bacteria and fungi in the soil, and to a lesser degree upon the enzymes secreted within the digestive systems of soil-inhabiting fauna (from protozoa to vertebrates). Some abiotic chemical degradation occurs in the most extreme environments (deserts, bare rock surfaces), but these are exceptions to the general case of dependence upon microbial enzymatic degradation. Microbial degradation of resources within the soil matrix leads primarily to growth of microbes themselves because the potential absorptive surface area of microbes greatly exceeds that of roots. Thus, microbes can generally compete more efficiently for soluble nutrients. Microbial exoenzymes benefit plants only if some of the enzyme catalysis occurs directly next to a root tip or if mycorrhizal fungi directly share resources with the plant. Bacteria and fungi serve as a biological sponge in the soil, "immobilizing" or "pooling" nutrients from dead organic residues and the inorganic substrate by incorporating them into their own living biomass. Soil fauna facilitate microbial growth and chemical degradation by: • Shredding the dead plant parts in forest litter and burrowing into coarse woody debris. • Moving either the resources or the microbial inocula in a continual homogenization of the soil. • Improving the water-holding and oxygen-penetrating capabilities of the soil through the geometry and chemistry of their fecal pellets, which is soil itself. • Enhancing the expression of the full range of chemical potentials of soil microbes by facilitating the succession of taxa with different exoenzymic capabilities. 10 USDA Forest Service Gen. Tech. Rep. PSW-GTR-178. 2000. The Functional Roles of Forest Soil Arthropods—Moldenke, Pajutee, Ingham • Liberating into the soil solution (also known as "mobilizing" or "mineralizing") the nutrients pooled within the microbial biomass of the rhizosphere by grazing upon the bacteria and fungi currently active around or within the microbial sheathing of physiologically active roots (Moldenke and others 1994, Seastedt 1984, Setala and Huhta 1991, Visser 1985, Webb 1977). Shredding and Burrowing The half-life of different species of coarse woody debris on the forest floor is largely a function of the abundance and diversity of wood-boring insects (largely ambrosia, long-horned or metallic woodboring beetles) (Schowalter and others 1988). Boring insects carry microbial inocula and provide increased access for fungal attack (Ingham and Moldenke 1995). Often these beetles require the presence of actively growing fungi to concentrate limiting nutrients within the wood (Crowson 1981, Martin 1987). Nutrients in dead leaves or needles are largely unavailable to most microbes. A bacterium in the leaf litter is analogous to a person in a pantry without a can opener. Eventually a person can beat a can open, but it is much more effective to have a can opener. The arthropod shredder is the bacterium's can opener. Bacteria and fungi will eventually use all of a dead leaf, but they are far more efficient if the leaf is shredded first. Shredders (i.e., millipedes, earthworms, sowbugs) crush vast quantities of plant cells from which they extract only the most readily available nutrients--the rest enter the normal soil recycling chain as the shredders defecate the crushed fragments (Hopkin and Read 1992). Passage through numerous shredding devices (the mouthparts of even much smaller organisms, i.e., penknife mites) is required before all the resources are finally available for complete enzymatic digestion (Dawod and FitzPatrick 1993). This process of continually refined shredding takes time, which accounts for the persistence of humus layers in the soil. Transportation and Homogenization People take mobility for granted until they become physically disabled. There may be 500 million bacteria in a teaspoon of forest soil, but each one is largely incapable of movement even though some bacteria have flagella that permit limited movement. Each bacterium needs diverse nutrients for growth and reproduction. From a bacterium's point of view, a resource that is several microns away is infinitely far away, because some other competitor is likely to be closer to the resource. There are several solutions: find a method of travel to cross the distance (hitchhike); have someone bring the resource to you; or hope that all your competitors between you and the resource already have enough of the resource and will not steal it before it diffuses back to you. Soil fauna fill the first two roles and provide a better chance of success for the bacterium. Bacterial and fungal inocula can be carried either on the outer body surfaces of invertebrates or in their intestinal tracts (Anderson 1988, Visser 1985). In general, the number of spores carried phoretically is directly proportional to the surface area of an organism. The viability of ingested inocula is generally proportional to the time it requires to pass through the length of the gut (Anderson 1975). Well-fed individual invertebrates have high percentages of viable inocula in their feces, whereas poorly USDA Forest Service Gen. Tech. Rep. PSW-GTR-178. 2000. 11 The Functional Roles of Forest Soil Arthropods—Moldenke, Pajutee, Ingham fed individuals produce feces with minimal viable inocula. The best-known transporters of both organic substrates and microbial inocula are anecic earthworms (species that feed largely on the soil surface, defecate deep in the soil, and construct extensive tunnels in the rooting zone) (Lavelle 1988).