Summary

Decomposition and pedogenesis are major ecosystem processes that affect bio-geochemical cycling, trace gas fluxes, soil fertility, and primary production. Decomposition of organic matter involves four component processes: photooxi-dation, leaching, comminution, and mineralization. Arthropods are key factors influencing comminution and mineralization.

Functional groups involved in decomposition include coarse comminuters that fragment large materials and fine comminuters that fragment smaller materials, often those produced by large comminuters. In aquatic ecosystems, scrapers and shredders represent coarse comminuters, whereas gatherers and filterers represent fine comminuters. Xylophages represent a specialized group of com-minuters that fragment woody litter. Carrion feeders reduce carcasses, and copro-phages feed on animal excrement. Fungivores and bacteriovores fragment detrital material while grazing on microflora. Fossorial functional groups include subterranean nesters that excavate simple burrows; gatherers that return detrital or other organic materials to nesting areas; and fossorial feeders that consume organic material, soil, or both and mix biotic and abiotic materials in their wake.

Evaluation of detritivore and burrower effects on decomposition and pedogenesis requires appropriate methods for measuring animal abundances and process rates. Abundances of detritivores or burrowers can be manipulated using exclusion and microcosm techniques, and detritivory can be measured as the product of detritivore abundance and individual consumption rate or as the rate of disappearance of substrate. Decomposition most commonly is measured as respiration rate, as the ratio of litter input to litter standing crop, or as the rate of litter disappearance. Isotopic tracers also provide data on decomposition rate.

Decomposition rate usually is higher in mesic than in arid ecosystems. Different functional groups dominate different ecosystems, depending on availability and quality of detrital resources. For example, shredders dominate headwater streams where coarse detrital inputs are the primary resource, whereas filterers dominate larger streams with greater availability of suspended fine organic material. Xylophages occur only in ecosystems with woody residues. Decomposition generally can be modeled as a multiple negative exponential decay function over time, with decay constants proportional to the quality of litter components. Usually, an initial large decay constant represents rapid loss of labile materials and successively smaller decay constants represent slower losses of recalcitrant materials (e.g., lignin and cellulose). Most studies have been relatively short term. Recent long-term studies suggest that mixing of recalcitrant materials and soil in arthropod guts may create stable aggregates that decay very slowly.

Detritivores affect decomposition in three ways: through comminution, effects on microbial biomass, and effects on mineralization. Comminution increases detrital surface area and facilitates colonization and decay by microflora. Low to moderate levels of grazing on microflora stimulate microbial productivity and biomass, maximizing microbial activity and respiration. High levels of grazing may reduce microbial biomass and decomposition. Grazers also disperse fungi and bacteria to new substrates. Not all organic material is converted to CO2.The low oxygen concentrations characterizing warm, humid termite colonies favor reduction of organic molecules to methane and other trace gases. Arthropod detritivores affect mineralization in different ways, depending on the chemical characteristics and biological use of the element. Detritivores often increase mineralization of nitrogen, but nitrogen released from detritus may be immobilized quickly by microorganisms.

Burrowers affect soil development by redistributing soil and organic matter. Ants and termites, in particular, excavate large volumes of soil and accumulate organic material in their centralized nests, mixing soil with organic material and influencing the distribution of soil nutrients and organic matter. Surrounding soils may become depleted in soil carbon and nutrients.

Detritivore and burrower effects on mineralization and soil composition can affect primary production and vegetation dynamics. Elevated moisture and nutrient concentrations in, or adjacent to, ant and termite nests support distinct vegetation and may facilitate succession following colony abandonment. A few studies have demonstrated increased plant growth, altered vegetation structure, and increased herbivory resulting from detritivore-induced nutrient mobilization.

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