D

FIGURE 6.5 Patterns of change in temperature and pH over time in a closed composting system. (Adapted from Gray, K. R. and A. J. Biddlestone. 1974. Biology of Plant Litter Decomposition. C. H. Dickinson and G. J. F. Pugh (eds.). Academic Press, London.)

levels. Under these conditions decomposition is carried out by mesophilic microbes at moderate temperatures and psychrophilic microbes at lower temperatures. An exception is decomposition in muskrat mounds (see Chapter 2). These mounds are the most obvious construction feature of muskrats in temperate zone marshes and act like compost piles (Figure 6.6) in accelerating the decomposition rate (Berg and Kangas, 1989; Wainscott et al., 1990). The mounds are layered with mud and vegetation similar to a classic Indure compost pile (Martin and Gershuny, 1992). The vegetation used to construct the mound is cut while fresh and alive, and thus it has higher nutrient content than vegetation not used in mound construction, which undergoes physical leaching in a standing dead stage before decompositon. The vegetation, which is used in mound construction, is also "shredded" to some extent by the muskrat and by macroinvertebrates that live inside the mound, which may facilitate colonization by microbial decomposers. Finally, the mound itself is moist but aerobic with some insulation effect as in a compost pile. Other examples of compost pile analogs are the nests built by megapode birds in Australasia (Collias and Collias, 1984). (Megapode refers to the big feet, which the birds use to construct large piles of plant materials for their nests.) Alligators in the southeastern U.S. also build similar nests. An open question is who designs the best compost piles: human sanitary engineers or animals such as the muskrat? Perhaps this question could be resolved by careful analysis with heat transfer equations from conventional engineering.

Composting is basically a natural process that is controlled by humans. Most attention has been given to managing or engineering for microbial decomposition of organic wastes. This emphasis is reflected in the standard texts (Golueke, 1977; Insam et al., 2002), which only consider the roles of microbes (see, however, the children's book by Lavies, 1993). Microbes also are the main driving force in decomposition in natural ecosystems but much more biodiversity is involved. Inver-

Layer of earth

The process:

The heap's ingredients decompose rapidly-"cooking" at temperatures near150°F-and break down into soft, easily rotted material high in nutrients. The final product is rich, dark, highly fertile humus.

Layer of earth

Layer of organic material, manure and topsoil alternate from bottom to top of heap.

Topsoil Manure

Organic layer: Leaves, grass clippings, sawdust, organic garbage (eggshells, coffee grounds, vegetable peels, etc.)

Bottom layer of branches and twigs lets air filter in from beneath the heap.

The process:

The heap's ingredients decompose rapidly-"cooking" at temperatures near150°F-and break down into soft, easily rotted material high in nutrients. The final product is rich, dark, highly fertile humus.

FIGURE 6.6 Comparison of a muskrat mound (above) with a compost pile (below). The top part of the figure is from Hodgson, R.G. 1930. Successful Muskrat Farming. Fur Trade Journal of Canada, Toronto. The bottom part of the figure is from United Press International. With permission.

tebrate animals dominate the complex detritus food webs in natural ecosystems, in contrast to the relatively short detritus food chains of microbes found in most human-designed composting systems. Detritus food webs occur primarily in the soil for terrestrial ecosystems and in sediments for wetland and aquatic ecosystems (Anderson and MacFadyen, 1976; Brussaard et al., 1997; Palmer et al., 1997; Snelgrove et al., 1997). Waring and Schlesinger (1985) illustrate the range of invertebrate animal diversity involved in the breakdown of leaf litter detritus with a graph of 50 taxa which spans three orders of magnitude in size — from protozoans at the small end to crayfish and earthworms at the large end of the spectrum. The work of detritus food webs has been called detritus- or leaf-processing which includes physical breakdown, mixing, and consumption of organic matter (Boling et al., 1975; Maltby, 1992; Petersen and Cummins, 1974; Petersen et al., 1989). Successions of different organisms are involved in detritus processing, each with different functional roles (Anderson, 1975; Frankland, 1966; Visser and Parkinson, 1975; Watson et al., 1974). Cousins (1980) also has referred to this kind of processing as a detritus cascade with emphasis on the different sizes of organisms that are involved. Cummins (1973;

Leaf Processing Sequence

Leaf Falls and Blows in

Microbial Colonization, Invertebrate Colonization

Wetting in

Physical Abrasion, and Softening

Leaf Falls and Blows in

Microbial Colonization, Invertebrate Colonization

Wetting in

Physical Abrasion, and Softening

Process Leaching of Soluble Components to DOM

Mineralization by Microbial Respiration to CO2

Process Leaching of Soluble Components to DOM

Cumulative 20% Weight Loss |_

Continued Microbial Activity and Breakdown

Conversion to FPOM

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Organic Gardeners Composting

Organic Gardeners Composting

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