Carbon and nitrogen mobilisation by earthworms of different functional groups as affected by soil sand content

SummaryEarthworms play an important role in organic matter processing and nutrient cycling in temperate ecosystems. It is known that earthworms preferentially ingest sand grains mixed with organic material and it has been suggested that the mixture of sand and organic material during the gut passage may play an important role in litter degradation and nutrient release, which may accelerate assimilation of nutrients by earthworms and likely enhance plant growth. In a greenhouse experiment, we investigated the effect of the anecic earthworm species Lumbricus terrestris and the endogeic earthworm species Octolasion tyrtaeum separately and in combination on carbon and nitrogen mobilisation from surface applied rye litter labelled with 13C and 15N. By mixing arable soil with 25% sand, we investigated the effect of the availability of sand. To quantify the mobilisation of 15N, three rye seedlings were planted in each microcosm and analysed for isotope signature after 3 months of incubation. Mobilisation of 13C was quantified by analysing the incorporation of label into the soil and earthworm tissue.Irrespective of the addition of sand the biomass of L. terrestris decreased during the experiment, whereas that of O. tyrtaeum increased in single species treatment and slightly decreased in the combined treatment with L. terrestris. The concentration of 13C decreased while that of 15N increased in the tissue of both earthworm species, with the effect being more pronounced in L. terrestris for 13C and in O. tyrtaeum for 15N. Both earthworm species increased shoot biomass, with the effect of L. terrestris (+80%) exceeding that of O. tyrtaeum (+28%) and maximum plant biomass in the combined treatment (+92%). Earthworms did not affect the 15N concentration of rye plants, but sand significantly increased 15N concentration of plants, presumably due to improving soil structure. Overall, the incorporation of 13C into the soil was low and was significantly increased in presence of sand, with the highest enrichment in treatments without earthworms.The results indicate that the availability of sand does not increase effects of earthworms on litter degradation, nutrient release and plant growth. Rather, independent of soil sand content earthworms increase plant growth, whereas the presence of sand itself enhances the uptake of nitrogen from plant litter and the incorporation of litter carbon into the soil.