Abstract
Phosphorus (P) and nitrogen (N) are essential nutrients for food production but their excess use in agriculture can have major social costs, particularly related to water quality degradation. Nutrient footprint approaches estimate N and P release to the environment through food production and waste management and enable linking these emissions to particular consumption patterns. Following an established method for quantifying a consumer-oriented N footprint for the United States (U.S.), we calculate an analogous P footprint and assess the N:P ratio across different stages of food production and consumption. Circa 2012, the average consumer’s P footprint was 4.4 kg P capita−1 yr−1 compared to 22.4 kg N capita−1 yr−1 for the food portion of the N footprint. Animal products have the largest contribution to both footprints, comprising >70% of the average per capita N and P footprints. The N:P ratio of environmental release based on virtual nutrient factors (kilograms N or P per kilogram of food consumed) varies considerably across food groups and stages. The overall N:P ratio of the footprints was lower (5.2 by mass) than for that of U.S. food consumption (8.6), reinforcing our finding that P is managed less efficiently than N in food production systems but more efficiently removed from wastewater. While strategies like reducing meat consumption will effectively reduce both N and P footprints by decreasing overall synthetic fertilizer nutrient demands, consideration of how food production and waste treatment differentially affect N and P releases to the environment can also inform eutrophication management.
Highlights
Fundamental changes to the global nitrogen (N) and phosphorus (P) cycles, largely driven by increased nutrient use and loss along the food chain, have major social and environmental consequences (Steffen et al 2015)
We address the following questions related to nutrient stoichiometry: (a) What is the difference in the magnitude of N and P food footprints? Do different food groups contribute disproportionately to N versus P footprints?
The largest difference in proportional contributions to the footprints comes from wastewater. 5% of the annual per capita P footprint can be attributed to surface water release following wastewater treatment; in comparison, N removal via wastewater treatment is less efficient than wastewater P removal, contributing 9% of the N footprint
Summary
Fundamental changes to the global nitrogen (N) and phosphorus (P) cycles, largely driven by increased nutrient use and loss along the food chain, have major social and environmental consequences (Steffen et al 2015). N and P loading are key drivers of surface water eutrophication and associated harmful algal blooms (Carpenter et al 1998, Diaz and Rosenberg 2008). These blooms can negatively impact recreation, transportation, fisheries, human health, and water treatment costs (Anderson et al 2002). As with the far-reaching social costs of the N ‘cascade’ as reactive N moves through multiple environmental pools (Galloway et al 2003, Billen et al 2013), excess reactive forms of P associated with human activities can negatively affect multiple ecosystem services that are vital to human well-being (MacDonald et al 2016). Many actions to reduce N loading likely reduce P loading, but specific crops, animal products, and management practices have different effects on downstream relative abundance of N and P in the environment (Jobbagy and Sala 2014, Cease et al 2015, Bouwman et al 2017, Springmann et al 2018, Penuelas et al 2020)
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