Community-wide effects of below-ground rhizobia on above-ground arthropods

Abstract

1. Plants take nutrients for their growth and reproduction from not only soil but also symbiotic microbes in the rhizosphere, and therefore below-ground microbes may indirectly influence the above-ground arthropod community through changes in the quality and quantity of plants. 2. Rhizobia are root-nodulating bacteria that provide NH4+ to legume plants. We examined bottom-up effects of rhizobia on the community properties of the arthropods on host plants, using a root-nodulating soybean strain (R+) and a non-nodulating strain (R−) in a common garden. 3. R+ plants grew larger and produced a greater number of leaves than R− plants. We observed 28 species of herbivores and three taxonomic groups of predators on R+ and R− plants. The herbivorous species were classified into sap feeders (12 species) and chewers (16 species). 4. The species richness of overall herbivores, sap feeders, and chewers on R+ plants was greater than that on R− plants. Rhizobia positively affected the abundance of chewers. 5. The community composition of herbivores was significantly different between R− and R+ plants, although species diversity and evenness did not differ. 6. Rhizobia-induced bottom-up effects were transmitted to the third trophic level. The abundance, taxonomic richness, and diversity of the predators on R+ plants were greater but evenness was lower than those on R− plants. The community composition of predators was not affected by rhizobia. 7. These results indicate that the below-ground microbes initiated bottom-up effects on above-ground herbivores and predators through trophic levels.