Quantifying dung carbon incorporation by earthworms in pasture soils

The effect of different earthworm functional groups on the incorporation of maize (C4 plant) dung into a soil (C3 organic matter background) sown with ryegrass (C3 plant) was explored by using differences in the carbon (C) isotope ratios (12C and 13C) between plant and soil samples in a field mesocosm study. The abundance of earthworms increased with dung inputs, reaching over 4000 earthworms per m2, presumably because of the increased food resources used. The amount of dung C incorporated into the soil profile in the presence of earthworms was dependent on the amount of organic matter deposited on the soil surface (925–4620 g C m−2) and reached rates of 1200 g C m−2 annually in the treatment receiving repeat dung applications. Dung incorporation was largely concentrated in the surface 0–75 mm, although small amounts of dung-derived C were observed to a depth of 300 mm. This was especially so in the presence of anecic earthworms, equating to an extra 70 g C m−2 annually for the 150–300 mm depth increment. It is important to note, in calculating C incorporation rates from earthworms, that only 10–20% of the soil surface in grazed pastures is covered by dung. After 444 days, less than 32% of the applied dung was detected within the upper 300 mm of the soil profile. This study emphasized the need for all three earthworm functional groups to be present within the soil in order to maximize the amount of surface dung that could be incorporated into soil organic matter.