Soil fauna and soil structure

Significant effects of soil fauna on soil structure are achieved mainly by a few groups among the larger soil invertebrates that are widely distributed and generally present in large numbers. Of these groups the most important are earthworms, termites and ants. The review deals mainly with earthworms, which are distributed throughout all but the coldest and the driest regions of the world. The effects of termites and ants on soil structure are also discussed. These groups of soil animals are also widely distributed, but are most common and most effective in influencing soil structure in tropical and warm temperate regions. A brief section deals with the influence of microarthropods, which are commonly found in large numbers, but because of their small size are unable to make large burrows in the mineral soil horizons, and are largely confined to pre-existing voids in litter and surface soil horizons. Their faecal pellets are granular and largely organic, with little included mineral soil material, and they sometimes make up the major proportion of forest litter layers. Quantitative assessment of the influence of earthworms on soil structure is available, but information on other groups is largely qualitative. The burrows of earthworms contribute to macroporosity and so influence water infiltration and aeration. Anecic species, that live in semi-permanent burrows opening to the soil surface and feed at the surface, provide more or less vertical channels for water infiltration and gas exchange. Endogeic species, that burrow continuously in search of food within the soil, provide more horizontally oriented, frequently extensive and intersecting networks of macropores that promote water movement and gas diffusion. Burrows that penetrate soil surface crusts are particularly important for water entry to the soil. Water movement through pores of the dimensions of earthworm burrows is important only when rainfall or irrigation supplies water at rates that exceed the capacity of the soil surface for capillary uptake. The combination of increase in surface area available for capillary uptake through the burrow walls and of hydraulic pressure resulting from the column of water in a water-filled burrow increases infiltration. Occupied burrows of anecic species may be sealed with soil or plant litter by the resident earthworm when water is ponded on the soil surface, or blocked by the earthworm's body, so as to be ineffective for water infiltration. When burrows are air-filled they provide surfaces that penetrate below ground and facilitate gas exchange.