SOIL ORGANIC-MATTER ASSIMILATION BY A GEOPHAGOUS TROPICAL EARTHWORM BASED ON DELTA-C-13 MEASUREMENTS

Assimilation of soil organic matter and fresh plant debris by Millsonia anomala, a tropical geophagous earthworm, was investigated by measuring changes in the C-13/C-12 ratio of their tissues when fed on organic matter naturally labelled by C-13. Individuals collected from a soil colonized by C4 plants (C4 soil) that were fed on C3 soil had > 50% of tissue C derived from the C3 Soil after 33 d. Assimilation of organic matter associated with different particle size fractions was investigated with individuals fed for 25 d on C4 soil with C4 particle size fractions substituted in turn for the equivalent C3 soil particle fraction. A significant labelling of earthworms was observed with both the 250-2000-mu-m and the 0-20-mu-m particle size fractions. Addition of fresh C3 plant debris to the C4 soil also resulted in a significant change in earthworm C isotope ratio. Assimilation of fresh plant material was greater than that of soil organic matter. This shows that young M. anomala are able to assimilate young organic matter (fresh plant debris, coarse soil organic matter) as well as fine soil organic matter, both of which classically have been regarded as strongly resistant to decomposition in models of soil organic matter dynamics.