Assessing the Impact of Dietary and Feed Self-Sufficiency Changes on Nitrogen Load and Water Quality in the Kasumigaura Watershed, Japan

In recent years, dietary changes towards reducing animal-based proteins was recognized as a nitrogen pollution-mitigating strategy. This is because producing animal protein generates higher nitrogen emissions compared to its plant-based alternatives. In Japan, there is a switch towards an animal-based diet, potentially leading to degraded water quality. While national-scale studies are common, watershed-level scale dietary changes are not researched, even though nitrogen pollution is often localized. This study aims to evaluate whether dietary and feed self-sufficiency changes can reduce nitrogen load and improve water quality in the Kasumigaura watershed. Firstly, nitrogen load was quantified and spatially distributed. Then, the estimated nitrogen concentration was compared with observed data. Finally, the impact of dietary and feed self-sufficiency changes on nitrogen load and water quality was assessed. Results estimated that nitrogen loading for year 2020 was 4403 tons/N/year, correlating with previous research. Results further showed that switch from livestock to legume protein would significantly improve water quality, up to 0.27 mg N/L. On the other hand, increasing feed self-sufficiency would negatively affect the water quality, up to 0.32 mg N/L. The results emphasize the importance of dietary patterns in mitigating nitrogen pollution. This method can be generalized on other watersheds.