Aboveground antagonists mitigate belowground plant–antagonist interactions but not affect plant–mutualist interactions

Species interactions exert important influences on biodiversity and ecosystem stability. In complex natural communities, species interactions have gone beyond pairwise mechanisms, as interactions between two species can be regulated by one or more other species (higher-order species interactions). However, few studies consider higher-order interactions among organisms that are indirectly contacted, particularly under high soil nutrient conditions. Here, we performed a common garden experiment to investigate how natural herbivory (aboveground weevil) and simulated herbivory (leaf clipping) affect plant (Triadica sebifera) interactions with soil antagonists (root-knot nematodes) and mutualists (arbuscular mycorrhizal fungi; AMF) under nitrogen and phosphorus addition. We also tested the effects of nitrogen, phosphorus, and herbivory-stimuli on T. sebifera leaf extrafloral nectary (EFN) production. We found that T. sebifera can compensate for biomass loss caused by clipping or weevil feeding, moreover, high nitrogen availability caused plant biomass to outpace herbivory-stimuli. Plant–antagonist (root-knot nematodes) interactions were not affected by clipping or weevil feeding under ambient nitrogen condition but were reduced by clipping or weevil feeding under high nitrogen supply, however, we did not find the same pattern under phosphorus addition. Aboveground herbivory-stimuli did not affect plant–mutualist (AMF) interactions, whether fertilized or not. In addition, nitrogen addition stimulated plants to secrete more EFN against clipping but did not increase EFN production against weevil feeding. Clipping and weevil feeding exhibited consistent effects on both plant–antagonist (root-knot nematodes) interactions and plant–mutualist (AMF) interactions. These results suggest that aboveground antagonists mainly mitigate belowground plant–antagonist interactions but not affect plant–mutualist interactions, and higher-order species interactions depend on nitrogen addition but not phosphorus addition.