Endogeic earthworms alter carbon translocation by fungi at the soil–litter interface

Abstract

The effect of endogeic earthworms ( Octolasion tyrtaeum (Savigny)) on the translocation of litter-derived carbon into the upper layer of a mineral soil by fungi was investigated in a microcosm experiment. Arable soil with and without O. tyrtaeum was incubated with 13C/ 15N-labelled rye leaves placed on plastic rings with gaze (64 μm mesh size) to avoid incorporation of leaves by earthworms. The plastic rings were positioned either on or 3 cm above the soil surface, to distinguish between biotic and chemical/physical translocation of nutrients by fungi and leaching. Contact of leaves to the soil increased 13C translocation, whereas presence of O. tyrtaeum reduced the incorporation of 13C into the mineral soil in all treatments. Although biomass of O. tyrtaeum decreased during the experiment, more 13C and 15N was incorporated into earthworm tissue in treatments with contact of leaves to the soil. Contact of leaves to the soil and the presence of O. tyrtaeum increased cumulative 13CO 2-C production by 18.2% and 14.1%, respectively. The concentration of the fungal bio-indicator ergosterol in the soil tended to be increased and that of the fungal-specific phospholipid fatty acid 18:2 ω6 was significantly increased in treatments with contact of leaves to the soil. Earthworms reduced the concentration of ergosterol and 18:2 ω6 in the soil by 14.0% and 43.2%, respectively. Total bacterial PLFAs in soil were also reduced in presence of O. tyrtaeum, but did not respond to the addition of the rye leaves. In addition, the bacterial community in treatments with O. tyrtaeum differed from that without earthworms and shifted towards an increased dominance of Gram-negative bacteria. The results indicate that litter-decomposing fungi translocate litter-derived carbon via their mycelial network in to the upper mineral soil. Endogeic earthworms decrease fungal biomass by grazing and disruption of fungal hyphae thereby counteracting the fungal-mediated translocation of carbon in soils.