Composting is the biological decomposition of organic matter. Although a naturally occurring process, composting can be accelerated by active management to balance the ratios of organic inputs, moisture, oxygen, mechanical mixing, and heat. This balance ensures the diversity and abundance of organisms to optimize the degradation of organic substances during composting. Microorganisms such as bacteria, including actinomycetes, and fungi underlie the decomposition process. Composting is organic recycling that reduces our ecological footprint by diverting waste from landfills where it would break down anaerobically producing the greenhouse gas methane, and instead provides rich organic nutrients for regenerative agriculture, gardening, water filtration, and erosion control. This laboratory exercise is a multi-week project during which upper division microbiology students contribute household compostable materials, record weights and types of those inputs, and compost them in tumbler-style bins. This part of the activity adds relevance for the students, gives them an overall picture of the composting process, and introduces them to food waste and sustainability. For the laboratory portion, students hypothesize relative types, diversity, and abundance of microorganisms in typical household soil and two different compost samples. Extracts of microbial cells from these substrates are serially diluted and grown on selective agar plates to determine the number and diversity of colony-forming units (CFUs). At the end of the project, students visually inspect the compost outputs and reflect on the reduction in materials, ecological cycling, and impacts of composting food waste on their ecological footprints.
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