Functional Interactions between Earthworms, Microorganisms, Organic Matter, and Plants - Earthworm Ecology - page 221

or even greater amounts enter the soil as dead roots or root exudates. Between 60 and 75% of the
introduced C is lost through respiration during the first year in the soil. The microbial biomass in
these systems is commonly on the order of 0.5 to 5.0 tons ha −1 , and this comprises only 1 to 2% of
the total soil carbon pool.
The microalgae and the primary decomposers (bacteria, actinomycetes, fungi) provide the basis
of the food web in soils and provide food for a wide diversity of secondary consumers (predators),
including protozoa and earthworms. Earthworms also feed selectively on soil rich in organic
materials (organic fragments and the rhizosphere) and gain some nutrients by digesting microor-
ganisms or their by-products associated with the decomposition of organic materials. Furthermore,
earthworms selectively consume some microbial species and digest fungal hyphae, trophozoite
protozoa, algae, and some bacteria and disperse the survivors (spores and resistant structures)
throughout the soil. Such dispersed survivors include bacteria (pseudomonads and rhizobia), fungi,
and protozoa. As an additional adaptive strategy to live in poor soil environments, earthworms have
developed a mutualistic digestive system with gut bacteria that allows them to obtain nutrients from
the soil organic matter, their main source of food.
Earthworms create (through selective feeding, casting, and burying organic residues) zones in
soil of high microbial activity (e.g., plant roots and residue patches) and aggregate in these zones.
In addition, their activity has been shown to decrease the severity of plant root diseases as well as
disperse and increase populations or activity of some beneficial soil microorganisms such as
Rhizobia and plant growth-promoting rhizo-bacteria (see Chapter 2 ). Because of these activities,
earthworms, if present in moderate populations, are likely to have a substantial influence on the
distribution, composition, and activity of the microbial communities responsible for the decompo-
sition of organic residues and for the regulation of plant growth, and thus play a key role in regulating
the processes that maintain soil health.
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