ABSTRACT
The general role of earthworms in cycling plant residues into soil organic matter and CO2 is well known. Earthworms were collected under various cropping conditions in order to identify the carbon isotope signature of their tissue. The earthworms were fed either corn or soybean residues for several weeks. Casts produced when earthworms were fed corn residues had higher carbon contents than when they were fed soybean residues which reflects less ingestion of soybeans than of corn in relation to equal quantities in soil. Considerable quantities of carbon are thus incorporated into earthworm casts and, in some cases, may double the carbon content of the cast compared to that of the surrounding soil. Natural-abundance-carbon-isotope analysis shows the amount of residue as distinct from soil organic matter, because of the unique carbon isotope ratios of fresh residue material due to variations in carbon isotope abundance in photosynthetic products between so called C4 and C3 type plants.
