Impact of different earthworm ecotypes on water stable aggregates and soil water holding capacity

We carried out mesocosm experiments using either the anecic earthworm Lumbricus terrestris or the endogeic earthworm Allolobophora chlorotica and loam, silt loam and sandy loam soils to investigate the differing impact of these ecotypes on aggregate formation (percentage water stable aggregates, %WSA) and soil water holding capacity (WHC), two soil properties that underpin many of the ecosystem services provided by soils. Earthworms significantly increased %WSA (by 16-56 % and 19-63 % relative to earthworm free controls for L. terrestris and A. chlorotica, respectively). For L. terrestris this increase was significantly greater in the upper 6.5 cm of the soil where their casts were more obviously present. A. chlorotica treatments significantly increased WHC by 7-16 %. L. terrestris only caused a significant increase in WHC (of 11 %) in the upper 6.5 cm of the sandy loam soil. Linear regression indicated a consistent relationship between increases in %WSA and WHC for both earthworm species. However, for a given %WSA, WHC was higher for A. chlorotica than L. terrestris likely due to the known differences in their burrow structure. Overall, earthworms increased soil %WSA and WHC but the significant species / ecotype differences need to be considered in discussions of the beneficial impacts of earthworms to soil properties.