Earthworm functional traits and interspecific interactions affect plant nitrogen acquisition and primary production

We performed a greenhouse experiment to test how the functional diversity of earthworms, the dominant group of soil macro-invertebrates in many terrestrial ecosystems, affects nitrogen cycling and plant growth. Three species were chosen to represent a range of functional traits: Lumbricus terrestris (large, mainly detritivorous, makes vertical permanent burrows open at the surface), Aporrectodea longa (medium-large, feeds on both detritus and soil, makes burrows more branched than L. terrestris), and Allolobophora chlorotica (small, geophagous, makes ephemeral burrows below the soil surface). Mesocosms with ryegrass (Lolium perenne) were inoculated with none to all three species (similar total biomass), using an experimental design (Simplex) suited to partition single species and diversity effects. Two contrasting N sources, urea or mammalian dung, were labelled with 15N so that the acquisition by plants and earthworms and recovery of applied 15N could be estimated.Over 3 months, plant production was higher with urea applications, but there were also species-specific earthworm effects: A. chlorotica and, to a lesser extent, A. longa increased shoot biomass, whereas L. terrestris increased root biomass. Earthworms did not affect soil N concentrations or leaching losses, whereas more N was leached under urea. A. chlorotica tended to increase dung-15N recovery in grass shoots, but in interaction with A. longa had the opposite effect, possibly through increased N immobilization in the microbial biomass. Earthworms assimilated negligible amounts of urea-15N but a substantial proportion (17% on average) of the dung-15N, with no clear-cut differences between species. Our findings show that earthworm species may have similar trophic response to N sources and yet different effects on plant N uptake and primary production, and that inter-specific earthworm interactions can result in non-additive diversity effects.