Effects Of Fauna On Litter Breakdown Rates

The association of soil fauna with litter decomposition is an ancient one. Labandeira et al. (1997) reviewed the evidence concerning associations of soil fauna in the geologic record. The incidence of oribatid mite feeding in coal deposits from Illinois and Appalachian sedimentary basins occurred in all major plant taxa in Pennsylvanian coal swamps. Virtually every type of plant litter tissue was used by the mites. Evidence for termites and holometabolous wood-boring insects dates to the early Mesozoic. The illustrations published by Labandeira et al. (1997) provide striking evidence of the importance of detritivores in these primitive forests.

In more modern times, the Russian soil scientist Galina Kurcheva (1960, 1964) found that naphthalene (an insecticide) applied to oak leaf litter would drastically reduce the rate of breakdown. In the succeeding decades, various biocides and other techniques have been used to suppress various components of the soil biota, which is a measure of their importance in leaf litter breakdown (e.g., Parker et al., 1984; Beare et al., 1992). The upshot of these experimental manipulations has been to demonstrate that bacterial, fungal, and faunal members of the soil biota all have significant effects on litter breakdown (Fig. 5.11). Given that actual breakdown and decomposition rates are a function of the interaction among the various biota and with substrate quality and climate, the rate estimates derived from manipulations must be accepted with caution. The main effects, however, seem clear.

Seastedt (1984b) suggested that the equation describing litter breakdown might be partitioned into components, so that the constant k could be considered as the sum of several ks:

dX/dt = — kX = — (kbacteria + kfungi + kfauna)X

Seastedt reviewed studies in which microarthropods had been suppressed and found that a variable percentage of breakdown rates could be attributed to microarthropod activities. Table 5.4 shows the results of the Seastedt equation applied to forest tree litter in a floodplain forest in Athens, Georgia, in the United States. Litterbags with a 1-mm mesh size were used so that macrofauna were excluded from the bags. Naphthalene applications were used to reduce microarthropod populations in some of the litterbags. The results show that the importance of microarthropods varied with litter quality. Microarthropod activities were least significant for the more rapidly decomposing litter species (dogwood, tulip-poplar) and were most important for the slowest, most recalcitrant litter type (water oak). In a carefully controlled experiment, Couteaux et al. (1991) measured decomposition of litters of various qualities, namely with carbon-nitrogen ratios of 75 (low quality) versus 40 (higher quality). The soil fauna contributed more to the decomposition of the low-quality substrate, and the effect was significantly greater at later stages of incubation in the 24-week experiment, with greater fau-nal complexity accounting for greater amount of dry mass loss and total CO2 evolution per unit time.

TABLE 5.4. Percent of Leaf Litter Decomposition (Mass Loss) Attributable to

Soil Fauna"

Leaf species




Percent due to fauna


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