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

In many agroecosystems, regular application of fertilizers to meet nitrogen requirements inadvertently leads to excessive buildup of phosphorus in the soil. In turn, over-abundant phosphorus can hinder crop growth and benefit weeds. Less well studied, but perhaps as impactful, surplus phosphorus might also cascade up the food chain to help or harm herbivorous arthropods and their predators. Here, we explore how soil phosphorus impacts crop yield through both bottom-up and top-down channels. While holding nitrogen applications roughly constant, we manipulated phosphorus application on field plots of Cucurbita pepo to create a gradient of low to high soil phosphorus levels. We then measured weed and crop biomass, and abundance of herbivorous and predatory arthropods, before ultimately determining crop yield. Excessive soil phosphorus directly reduced yield by reducing the nitrogen-to-phosphorus ratio of the crop. Although weeds also significantly decreased yields, weed biomass was not correlated with soil phosphorus. Excessive soil phosphorus also indirectly impacted yields through its effects on arthropods. Herbivorous squash bugs (Anasa tristis), the key pest, were more abundant and did more crop damage with increasing soil phosphorus. However, the negative effects of herbivory were at least partly counterbalanced by an increase in the abundance of invasive fire ants (Solenopsis invicta) in high-phosphorus plots, heightening suppression of pests by the ants. This demonstrates that fertility and pest management practices on farms are linked. Overall, our results indicate that excessive phosphorus in the soil, a nearly-inevitable result of fertilizer applications to meet nitrogen targets, harms crop yields.