Phosphorus folly: Soil phosphorus rewires top-down control of herbivores in agroecosystems

Phosphorus is an essential nutrient for crop growth and development, but in excess may exacerbate weed and insect pest pressure in agroecosystems. While a high level of phosphorus is a soil characteristic of the vast majority of cropping systems, remarkably little is known about how surplus phosphorus alters agroecological communities via top-down forces, such as insect predation. To explore how surplus phosphorus affects pest pressure in agroecosystems, we manipulated six combinations of two compost fertilizers to create a gradient of phosphorus on a production area within the Southeastern United States, where we grew zucchini (Cucurbita pepo cv. Yellowfin). We found that surplus phosphorus increased herbivorous pest abundance and decreased omnivorous natural enemy abundance, disrupting pest suppression. Surplus phosphorus also increased the abundance of fire ants (Solenopsis invicta), which further decreased natural enemy abundance. However, fire ants also decreased herbivore abundance, which provides a net positive effect on pest control in a cropping system where they are a key predator. Although not significantly correlated with surplus phosphorus, weeds significantly decreased crop yield, in part by reducing the nitrogen-to-phosphorus ratio of the crop plants, suggesting competition between weeds and crops for nitrogen in the soil. Our results indicate that soil phosphorus is an important mediator of top-down control of herbivores and, in excess, can decrease crop yields. Further understanding of phosphorus-mediated multi-trophic interactions in agroecosystems could be critical for farmers to develop sustainable fertility management plans while promoting biocontrol of weeds and insect pests.