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−1yr−1compared to 22.4 kg N capita−1yr−1for 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.
Through the trade of products and services, cities indirectly depend on distant water sources to function, prosper, and grow. To fully account for indirect (virtual) water dependencies, virtual water flows need to be known along complex supply chains. To this purpose, we build a new environmental multiregional input–output model for U.S. regions. The model is used to quantify the domestic, blue virtual water flows and analyze the water footprints of 69 major U.S. cities. Our results show a large inequality in the urban water consumed for economic production: just 7 out of the 69 cities included in this study account for 35% of the U.S. national water footprint of production. This is due to the production of water‐intensive agricultural products in the metropolitan areas of western cities. The inequality reduces for the urban water footprint of consumption because, through the supply chains of industrialized food sectors, western virtual water is partially transferred to eastern cities as final demand. The water embodied in industrial products and services tends to be higher in western cities than in eastern cities; that is, the water embodied in food services could be several times higher in Los Angeles than in New York City. Trade hub cities attract large inflows of products which are mostly transformed for consumption elsewhere. Thus, the omission of product interdependencies within trade hub cities can increase by several times their water footprints of consumption. Overall, the proposed model is able to enhance subnational estimates of U.S. virtual water flows.
more » « less- Award ID(s):
- 1639529
- NSF-PAR ID:
- 10445759
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Water Resources Research
- Volume:
- 56
- Issue:
- 4
- ISSN:
- 0043-1397
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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