Nitrogen surplus: An environmental performance indicator for sustainable food supply chains

Eileen L. McLellan1, Ken Cassman2, Shai Sela3, Harold van Es3, Rebecca Marjerison3, Rod Venterea4, Christina Tonitto3 and Peter Woodbury3.

1 Environmental Defense Fund, 1875 Connecticut Avenue, Washington, D.C. 2009, www.edf.org, [email protected]

2 University of Nebraska, Plant Science Hall, Lincoln, NE 68583

3 Soil and Crop Sciences, Bradfield Hall, Cornell University, Ithaca, NY 14853

4 USDA-ARS Soil and Water Management Research Unit, Borlaug Hall, 1991 Upper Buford Circle, St. Paul MN 55108

Abstract

Nitrogen pollution and its negative impacts on human and environmental health are embedded in commodities traded domestically and internationally; we focus on grain because of its importance as a feedstock for food, feed and fuel. Food supply chain companies, in particular retailers and food processors, can play a catalytic role in reducing that burden through sustainable sourcing of grain and grain-derived products. We describe how such sourcing commitments might work to reduce N losses and how progress towards them can be tracked using a simple, robust and scalable indicator: N surplus. Using model simulations and empirical data on U.S. maize production, we show that N surplus, the difference between annual N inputs (fertilizer, manure, biological nitrogen fixation) and grain N outputs, is strongly related to N losses at field to regional scales. This analysis suggests that supply chain companies can set performance goals related to reductions in N surplus, which in turn could translate into large improvements in environmental outcomes. Recognizing that individual producers will need guidance and motivation on how to reduce N surplus, we present a conceptual model for using farmer-derived data in a social learning context to identify combinations of management practices that most effectively reduce N losses and improve crop yield or profit.