Plant-litter-soil feedbacks driven primarily by litter type and plant compartment, but not modified by litter exposed to insect herbivory

Abstract Purpose Insect herbivory affects plant growth, nutrient and secondary metabolite concentrations and litter quality. Changes to litter quality due to insect herbivory can alter decomposition, with knock on effects for plant growth mediated through the plant-litter-soil feedback pathway. Methods Using a multi-phase glasshouse experiment, we tested how changes to fast- and slow-growing grass shoot and root litter quality caused by insect herbivores affect the performance of response plants in the soil in which the litter decomposed. Results We found that insect herbivory resulted in marginal changes to litter quality and did not affect growth when plants were grown with fast- versus slow-growing litter. However, there was a strong interactive effect between litter type (i.e., root versus shoot) and response plant compartment (i.e., root versus shoot growth): shoot litter resulted in a marginally negative effect on both roots and shoots, while the addition of root litter had a strong negative effect on root growth and a positive effect on shoot growth. Further, shoot litter exposed to insect herbivory interacted with response plant identity to affect root growth. Conclusions Our results suggest that whether litter originates from plant tissues exposed to insect herbivory or not and its interaction with fast- versus slow-growing grasses is of little importance. In contrast, litter origin (root versus shoot) effects on allocation to root versus shoot growth and species-specific responses to litter exposed to insect herbivory appear to play a critical role in the plant-litter-soil feedback pathway. Taken collectively, plant-litter-soil feedbacks likely affect broader ecosystem processes.