Citation:

MacDonald, G., A. Leach, G. Metson, R. Clarke, A. Tseng, J. Compton, J. Galloway, AND E. Bennett. Linking the budget to the footprint: Assessing phosphorus flows in United States agriculture from two contrasting approaches. AGU, Washington, DC, December 10 - 14, 2018.

Impact/Purpose:

This paper brings a team working on phosphorus cycling together with scientists who have created an individual nitrogen footprint calculator, in order to develop a phosphorus footprint tool. Previous EPA publications on P inputs and watershed retention will be used as the P budget information to compare with an approach scaling up from individual phosphorus footprint calculations. This work will inform efforts within SSWR 4.03C on management of nutrients by providing options for addressing nutrient-related decisions at different scales, including watershed scales, regional scales and the scale of individuals. This work connects efforts that have historically been somewhat isolated - the footprint perspective and the watershed perspective - to identify the commonalities and lessons learned for the other approach. Emerging from this work will be a perspective on nutrient management that merges these different scales, which can be useful for state and federal groups tackling the challenges associated with increased nutrients in aquatic systems.

Description:

Phosphorus (P) use and management in agriculture has important implications for both agricultural productivity and environmental quality. Diet has a particularly strong influence on the amount of P used in food production, with livestock products typically entailing disproportionately large use of non-renewable mineral P fertilizers compared to crop products. Nutrient budget approaches offer important insight on management outcomes by accounting for nutrient inputs and outputs within a given system, typically emphasizing a food production perspective. Nutrient footprint approaches, such as those developed for nitrogen at different scales, are particularly relevant for helping consumers to understand how consumption patterns affect the magnitude of nutrient release to the environment from the food system. As part of the development of a P footprint calculator tool directed at U.S. consumers, we developed a set of virtual phosphorus factors (VPFs) to estimate the P lost to the environment during the food production process for twelve crop and five livestock commodities. These VPFs (g P lost per kg of food product or g P lost per kg P consumed in foods) are central to the calculation of the average P footprint of food production nationally and facilitate consumption-based estimates of per capita P footprints (P losses capita-1 year-1). We compare and contrast the crop and livestock product VPF results with a series of simple production-related P flows and mass-balance P budgets at the national scale to explore sensitivity in the results to methodological assumptions and decisions about system boundaries. We also assess estimates of P release nationally under different diet scenarios based on the food production P footprint approach and a national food system model that accounts for international imports and exports of agricultural commodities. The VPFs we describe provide a useful approach for bridging a typical P budget focussed on P flows in production with perspectives on how consumers interact with that system through their diets.

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