Convection and diffusion are the two mechanisms by which oxygen moves from the open air into a composting matrix and ultimately to the microorganisms themselves. Oxygen transport is intimately related to the moisture content of the compost, as both convection and diffusion are reduced by water saturated pores. Capillary theory and matric potential relationships provide a theoretical framework for evaluating the effects of moisture content on air filled porosity.
Convection
Convection can be categorized as either "forced" (driven by mechanical means) or "natural" (caused by the bouyancy of hot air). In a system with mechanical aeration, blowers move air through the larger pores at relatively high velocities. In a passive system, hot air can often be seen slowly rising out of the tops of piles, and natural convection pulls cool oxygen rich air in to replace it.
Water filled pores create a major impediment to uniform convective air movement, by creating zones of high resistance. As air short-circuits through unsaturated zones, the aerobic regions generate more heat and become even drier, while the wetter regions become anaerobic. One of the principal functions of mixing and turning compost is to redistribute moisture, to minimize this preferential airflow and the nonuniform decomposition that results.
Diffusion
While these convective mechanisms are important for the pile overall, oxygen diffusion through the smaller pores and into the aqueous film surrounding compost particles is essential to maintaining aerobic conditions for the active microorganisms. A saturated matrix also dramatically reduces oxygen diffusion, which is 6000 to 10,000 times greater in air than in water. Diffusion can be mathematically modeled for the range of conditions in a composting matrix, as described in the pages below:
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