Modification by earthworms of effects of soil heterogeneity and root foraging in eight species of grass

Spatial heterogeneity of soil nutrients and earthworm activity can each increase the performance of plant species, but their interactive effects have been little studied. The ability of plants to forage for nutrients by concentrating roots where nutrients are concentrated can partly explain the positive effects of nutrient heterogeneity, but whether root foraging can help explain the positive effects of earthworm activity is untested. We conducted a greenhouse experiment in which we grew eight species of Poaceae in homogeneous and heterogeneous soils with or without the earthworms Eisenia fetida and Metaphire guillelmi and measured net accumulation of plant mass and tillers. Effects of heterogeneity and earthworms on plant performance were positive in most species. The presence of earthworms reduced the directly measured effect of heterogeneity on total mass in some grass species. Most species showed root foraging ability. Ability showed no relationship to effects of heterogeneity or earthworms on final total dry mass. However, earthworms reduced foraging in some species, possibly by lessening heterogeneity. Earthworm activity in heterogeneous soil may thus reduce the benefits of root foraging for nutrients in plants.