Although biotic legacies of past agricultural practices are widespread and increasing in contemporary ecosystems, our understanding of the mechanisms driving such legacies is still poor. Forest understories on former agricultural land show low frequencies and abundances of typical woodland species when compared with ancient forests. These community shifts have been ascribed to the effects of dispersal limitation. A rarely considered mechanism is that post-dispersal processes driven by plant-associated communities determine the poor performance and recruitment of woodland indicators. Given the strong alterations in soil conditions due to former agricultural practices, we hypothesized that (abiotic) plant–soil feedbacks could be a major factor in community shifts. We addressed this hypothesis by comparing plant-associated communities in soil and aboveground in ancient and post-agricultural alluvial forests; then, we experimentally tested whether changes in biotic and biotic soil properties could affect aboveground herbivore abundance and pressure, and plant performance. Ancient and post-agricultural communities clearly differed in composition at different levels of the food-web. Besides the plant community, we also observed differences in the microbial and nematode community with increased abundances of root-feeding nematodes in post-agricultural soils. The composition of the aboveground invertebrate community did not differ in ancient and post-agricultural forest parcels; however, plants growing in post-agricultural sites showed higher abundances of invertebrate herbivores and suffered more herbivory. Nutrient analyses of soil and plants showed that increased levels of phosphorus (and to a lesser extent, nitrogen) made plants more nutritious for insect herbivores. Laboratory experiments further pointed to this mechanism as an explanation of the poorer performance of woodland indicators in post-agricultural woodlands. Our results point to biotic and abiotic plant–soil feedbacks coupled with herbivory as a new mechanism to explain legacy effects in temperate forests. The modification of the belowground community and soil abiotic characteristics by previous agricultural activity affect not only plant growth but also plant nutrient content in the compared understory species; making them more susceptible to aboveground herbivory. Our results provide one of the first examples of integrating plant–soil feedback and above- and belowground interactions to explain land-use legacies.
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