Human activity has fundamentally altered the global nitrogen (N) and phosphorus (P) cycles—even more profoundly than the carbon cycle—leading to massive environmental consequences. Although P is necessary for food production, excess P leads to environmental consequences, such as eutrophication and harmful algal blooms, which can influence recreation, transportation, fisheries, human health and water treatment costs. Nutrient stoichiometry (N:P) can play an important role in regulating ecosystem function, and the N:P associated with agricultural inputs, processing losses and waste warrants further attention. An integrated perspective on the release of both elements to the environment from human actions would allow for a better understanding of the management of these drivers in concert. In this project, we developed virtual P factors, which describe the P lost to the environment during the food production process for major food categories in the United States. The methods match those of existing virtual N factors. The virtual P factors will contribute to the creation of an integrated N and P footprint tool to help consumers understand and reduce their contribution to N and P pollution.
Virtual P factors (VPF), as may be expected by stoichiometric relationships, were smaller than virtual N factors (VNF). For example, corn has a VPF of 2.7 g P / kg food and a VNF of 15 g N / kg food. We found that like with the N footprint, the consumption of animal products in US diets has a disproportionately large impact on an individual’s P footprint. In fact dietary choices, specifically the consumption of meat, contributes to the majority of an individual’s P footprint and represents a larger faction of the P footprint than N footprint. For N footprints, in part because of N emissions related to fossil fuel combustion, travel decisions and energy consumption can be used as leverage points for change, but this is not the case of P because negligible amounts of P are emitted from fossil fuel combustion. In summary, individual U.S. consumer integrated N and P footprints can inform the public about ways to decrease their environmental impact linked to nutrient losses. Importantly, this combined N-P footprint tool will facilitate future work to integrate with existing carbon accounting tools.