Nutrient-extended input-output (NutrIO) method for the food nitrogen footprint

Agro-food systems require nutrient input from several sources to provide food products and food-related services. Many of the nutrients are lost to the environment during supply chains, potentially threatening human and ecosystem health. Countries therefore need to reduce their nutrient/nitrogen footprints. These footprints are importantly affected by links between sectors. However, existing assessments omit the links between sectors, especially between the agriculture, manufacturing, and energy sectors. We propose a novel approach called the nutrient-extended input-output (NutrIO) method to determine the nutrient footprint as a sum of direct and indirect inputs throughout the supply chains from different sources of nutrients. The NutrIO method is based on a nutrient-based material flow analysis linked to economic transactions. Applying this method, we estimated the nitrogen footprint of Japan in 2011 at 21.8 kg-N capita-1yr-1: 9.7 kg-N capita-1 yr-1 sourced from new nitrogen for agriculture and fisheries, 7.0 kg-N capita-1 yr-1 from recycled nitrogen as organic fertilizers, and 5.1 kg-N capita-1 yr-1 from industrial nitrogen for chemical industries other than fertilizers. A further annexed 55.4 kg-N capita-1 yr-1 of unintended nitrogen input was sourced from fossil fuels for energy production. The nitrogen intensity of the wheat and barley cultivation sector, at 1.50 kg-N per thousand Japanese yen (JPY) production, was much higher than that of the 0.12 kg-N per thousand JPY production for the rice cultivation sector. Industrial nitrogen accounted for 2%-7% of the nitrogen footprint of each major food-related sector. The NutrIO nitrogen footprint sourced from new nitrogen for agriculture and fisheries, at 8.6 kg-N capita-1 yr-1 for domestic final products, is comparable to the food nitrogen footprint calculated by other methods, at 8.5-10.5 kg-N capita-1 yr-1. The NutrIO method provides quantitative insights for all stakeholders of food consumption and production to improve the nutrient use efficiencies of agro-food supply chains.