Legumes, particularly soybeans, are among the most important agricultural food crops, providing food security, biological nitrogen (N) fixation, and economic value. However, soybean production can be environmentally costly due to intensive pesticide use. These costs are projected to increase in the near future as climate change results in more frequent and intense droughts and associated alterations in herbivore pest densities. Thus, identifying factors that buffer soybean production against decreased yield is needed. Soybeans associate with mutualistic, N-fixing rhizobial bacteria. Although there is extensive evidence that legumes simultaneously associate with multiple rhizobial strains, the consequences of these diverse rhizobial associations on production remains unknown. Biodiversity-Ecosystem Function (BEF) theory predicts that biological diversity is linked to important ecosystem functions and services. Here we assess the interactive effects of rhizobial diversity and drought on soybean resistance to herbivores (insect and mammalian). We performed a pot experiment within which we manipulated rhizobial diversity (all possible combinations of 1, 2, or 3 rhizobial strains from a 4-strain pool) and drought (ambient, +3°C and 2-week drought). We examined the effects of these experimental treatments on leaf damage by insect and mammalian herbivores, plant growth, and total yield at the end of the growing season.
Results/Conclusions
We found that increasing rhizobial strain diversity led to linear decreases in foliar damage by leaf-chewing invertebrate pests. This effect was particularly strong in pots droughted and warmed to 3°C above ambient temperatures. Rabbit damage showed the opposite trend, with the number of leaves consumed per plant increasing with rhizobial strain diversity, and no direct or interactive effect with drought. Finally, aphid abundances were 100-fold higher on droughted plants than undroughted plants, however rhizobial strain diversity had no direct or interactive effects on aphid abundances. These differences suggest that these herbivores (leaf chewing invertebrate, mammalian, and sap-sucking invertebrate) may be utilizing different cues in determining their feeding preferences. Overall, plant growth rates did not differ among the rhizobial strain diversity treatments. However, drought treatment did impact plant growth, with the droughted plants experiencing delayed phenology. In the end, this delayed phenology did not result in differences in final yield, as the droughted plants eventually caught up to the undroughted plants in their phenology. Together, these results highlight the need to examine the effects of rhizobial diversity in modulating soybean resistance and resilience to drought and herbivore pests in a large-scale observational and experimental context.