Assessing nitrogen load on a trade-oriented subtropical island in Japan by the concept of food nitrogen footprint

Various human activities generate reactive nitrogen (Nr, all forms of nitrogen except di-nitrogen [N₂]); food production is one of the primary emission sources. Chemical fertilizer, which we can generate artificially, is indispensable to meet the world population's demand. However, excessive fertilizer use leads to nitrogen leaching into water bodies and N₂O emissions harming the environment. To overcome the problems, we should reuse organic resources such as manure instead of chemical fertilizers. Moreover, it is known that the present meat-dominant food style produces more Nr load on the environment than a plant-based diet. Therefore, customers’ food choice also significantly affects the nitrogen balance. To explore measures of Nr load mitigation both on the produce and customer sides, we applied the concept of food nitrogen footprint to a subtropical island in Japan—Ishigaki Island, as a case study. Agriculture and tourism are the primary industries on the island. The main products are sugarcane, pineapple, beef, and calf; most of them are exported. The food for the inhabitants relies on the import. We used the statistical data from 2022 for the calculation. The results showed that Nr loss from the island’s food system was 41.7 kgN per capita; 58% and 33% of the Nr load were related to the exported and imported food, respectively, indicating trade-oriented characteristics. Most of the Nr influx was chemical fertilizer and imported food and feed. The results indicated that reducing chemical fertilizer use and importing food and feed would effectively mitigate the Nr loss in the Island’s food system. By conducting scenario analyses, it was revealed that manure use reduced Nr loss on the island (13% reduction), and changing import food from a meat-dominant into a plant-dominant reduced mainly the Nr loss in overseas, where imported food produced (26% reduction). This indicated that both production and consumers’ choices are necessary to reduce Nr loss not only on the island but also in overseas. These findings contribute to maintaining the global nitrogen balance.