Plant and soil biodiversity can have significant effects on herbivore
resistance mediated by plant metabolites. Here, we disentangled the
independent effects of plant diversity and soil legacy on constitutive
and herbivore-induced plant metabolomes of three plant species in two
complementary microcosm experiments. First, we grew plants in sterile
soil with three different plant diversity levels. Second, single plant
species were grown on soil with different plant diversity-induced soil
legacies. We infested a subset of all plants with Spodoptera exigua
larvae, a generalist leaf-chewing herbivore, and assessed foliar and
root metabolomes. Neither plant diversity nor soil legacy had
significant effects on overall foliar, root, or herbivore-induced
metabolome composition. Herbivore-induced metabolomes, however, differed
from those of control plants. We also detected 139 significantly
regulated metabolites by comparing plants grown in monocultures with
conspecifics growing in plant or soil legacy mixtures. Moreover,
plant-plant and plant-soil interactions regulated 141 metabolites in
herbivore-induced plants. Taken together, plant diversity and soil
legacy independently alter the concentration and induction of plant
metabolites, thus affecting the plant's defensive capability. This is a
first step towards disentangling plant and soil biodiversity effects on
herbivore resistance, thereby improving our understanding of the
mechanisms that govern ecosystem functioning.