Earthworms modify plant biomass and nitrogen capture under conditions of soil nutrient heterogeneity and elevated atmospheric CO₂ concentrations

Earthworms modify the way roots respond to soil nutrient patchiness. However, few studies have evaluated the joint effects of earthworms and soil heterogeneity on plant community biomass and species dominance, and none of them have assessed the influence of different patch features and environmental conditions on such effects. We evaluated how soil nutrient heterogeneity, earthworms (Eisenia fetida), organic material quality (¹⁵N-labelled leaves and roots of contrasting C: N ratios) and elevated atmospheric CO₂ concentrations (phytotron chambers) affected the resource-use strategy, biomass and species dominance of mixtures formed by Lolium perenne L. and Plantago lanceolata L. Soil heterogeneity decreased N capture from the organic material, especially in the presence of earthworms. Mixtures experienced a 26 and 36% decrease in shoot and root biomass when earthworms were added to the heterogeneous microcosms, but only with high quality organic material. The dominance of L. perenne was lower under conditions of elevated CO₂, nutrient heterogeneity and earthworms. Our data suggest that earthworms can neutralize positive plant growth responses to soil heterogeneity by exacerbating decreases in the supply of N to the plant. Specifically, earthworms foraging for high quality patches may stimulate microbial N immobilization, translating into lower N capture by plants. Increases in casting activity under elevated CO₂, and hence in microbial N immobilization, may also explain why earthworms modulated the effects of soil heterogeneity and CO₂ concentrations on plant community structure. We show that earthworms, absent from most soil nutrient heterogeneity studies, mediate plant biomass responses to nutrient patchiness by affecting N capture. Future plant-foraging behaviour studies should consider the roles played by soil engineers such as earthworms, so that results can be better extrapolated to natural communities.