Associations between mycorrhizal fungi and plants occur within approximately 90% of all plant species and are considered a classic example of a mutualism in which both partners benefit from the interaction. Through the association, fungi serve as an extension of the plant root system and supply the plant with nitrogen and phosphorus in exchange for carbohydrates from the plant. Most plants associate with either ectomycorrhizal (EM) fungi that live outside of plant roots or arbuscular mycorrhizal (AM) fungi that penetrate root cells, but some plants associate with both types of fungi. Association with both AM fungi and EM fungi is a common strategy for plants that inhabit riparian areas, for reasons that are not understood. Strong gradients in nutrient availability occur in riparian zones due to natural and anthropogenic transport of nutrients through these areas. It is hypothesized that the availability of nutrients in riparian zones affects the chemistry of cottonwood trees and the colonization of cottonwood roots by mycorrhizal fungi. The study investigates the relationship between nutrient content of cottonwood trees (Populus deltoides ssp. wislizenii), the dominant species along rivers in central New Mexico, USA, and their associated mycorrhizal fungi. The research involves experiments in greenhouses in which cottonwoods are fertilized with nitrogen and phosphorus, and tree growth, leaf chemistry, and mycorrhizal colonization are characterized over a growing season. The results will contribute fundamental information on nutrient exchange between mycorrhizal fungi and cottonwoods. The results of this research may inform decisions regarding river management and protection and rehabilitation of cottonwood gallery forests that are currently in decline throughout much of their range.