Nitrogen Footprint: A Novel Indicator to Quantify Nitrogen Loss to the Environment

Abstract

Emissions of anthropogenic reactive nitrogen have led to the emergence of serious threats to both humanity and the environment. A nitrogen footprint is a novel indicator that quantifies, on a consumption basis, the loss of reactive nitrogen to the environment through human activity. We describe three recently proposed methods to determine a nitrogen footprint: N-Calculator; N-Input; and N-Multi-region. N-Calculator applies a bottom-up analysis, which is based on per-capita food and energy consumption, to describe how each of a consumer’s actions impacts their nitrogen footprint. N-Input uses a top-down analysis—which is based on agricultural nitrogen inputs and the production, importing, and exporting of food—to accurately assess nitrogen flows for countries that import food from multiple foreign states. N-Multi-region employs input–output analysis using a global, multi-region input–output table that is extended to cover sectoral reactive nitrogen emissions of each country/region. This method can estimate nitrogen footprints of many countries while considering complex international supply chains and different reactive nitrogen types. We also review related research projects, and current and potential measures to decrease nitrogen footprints; these include food choices, decreasing household waste, footprint labeling（for nitrogen, carbon, and water）, institutional footprinting（for nitrogen and carbon）, and nitrogen-footprint offsetting.