ABSTRACT
Soil contains a virtual cornucopia of living organisms that obtain energy, structure, and function from organic compounds. Living organisms in soil and their associated organic compounds are collectively called the soil biomass. The living organisms consist of macrofauna (e.g., earthworms and insects), mesofauna (e.g., nematodes), microfauna (e.g., protozoa and archezoa), microorganisms (e.g., algae, fungi, actinomycetes, and bacteria), burrowing animals, and plant roots. These organisms produce a diverse abundance of biochemical compounds required for cellular function that collectively constitute the fraction of organic carbon in soil called cellular or biomass carbon (Figure 4.1). Living organisms also produce and excrete extracellular substances, such as enzymes and other organic compounds that facilitate nutrient uptake and microbial mobility (e.g., microbial mucilages), as well as metabolic byproducts (wastes). Biochemical compounds are also released to the extracellular environment through the death and decay of organisms (cell lysis), and material derived from higher plant residues (leaf litter), sloughing cells from plant roots, and animal excrement. All of these mechanisms add to the innumerable array of biochemical compounds in the soil environment that make up the soil biomass carbon pool.
