Study analyzes 60 years of agricultural nitrogen load changes in the Kumamoto area and calls for region-specific nitrogen management

Key points: 1. Before 1985, changes in nitrogen load were driven mainly by the scale of agriculture; after 1985, changes in agricultural structure became the main factor. 2. As agriculture became regionally specialized, disparities in nitrogen load productivity shifted from “within-region disparities” to “between-region disparities.” 3. The study classified six development patterns by administrative division and proposed nitrogen management strategies based on empirical evidence and regional characteristics. Overview: Groundwater contamination by nitrate nitrogen from agricultural activities is a global issue with serious implications for drinking water safety and ecosystems. Previous studies have mainly relied on aggregate analyses at the national or prefectural scale, while detailed analysis of drivers at the administrative-division level within watersheds and quantitative evaluation of regional disparities have remained insufficient. Uniform policy approaches can obscure region-specific issues and delay effective countermeasures. Zhuolin Li, a doctoral student at Nagasaki University’s Graduate School of Integrated Science and Technology; Professor Kei Nakagawa of Nagasaki University; Professor Hidemichi Fujii of Kyushu University; Professor Takahiro Hosono of Kumamoto University; and a professor from Lund University in Sweden analyzed agricultural statistical data from 1960 to 2020 across 10 administrative divisions in the Kumamoto area. The study clarified the spatiotemporal variation of nitrogen loads and their underlying factors. The researchers built an original analytical framework combining the Logarithmic Mean Divisia Index decomposition method and the weighted Theil index. For both crop farming and livestock farming, they quantitatively decomposed changes in nitrogen load into three factors: nitrogen intensity, agricultural structural change, and agricultural scale. The results showed that multiple external factors, including agricultural policy shifts, market fluctuations, and natural disasters, affected nitrogen loads in different ways. The study also identified six development patterns by administrative division and made specific policy recommendations for nitrogen management strategies suited to local realities. The findings were published online on April 27, 2026, Japan time, in the international academic journal Agriculture Systems. Conclusions and future outlook: This study provides new insight by systematically clarifying the drivers of agricultural nitrogen load changes and the structure of regional disparities over 60 years at the administrative-division scale within a watershed. Its main policy implications are: first, uniform agricultural policies and nitrogen reduction targets risk overlooking regional disparities within watersheds, making region-specific nitrogen management strategies necessary; second, pursuing only higher agricultural productivity, meaning increased output value, may concentrate nitrogen loads in groundwater recharge areas, so policy design must account for the trade-off between efficiency and equity; and third, because nitrogen infiltration into groundwater involves delays on the scale of decades, early policy intervention that anticipates current changes in agricultural activity is essential. Future work is expected to expand the method to other regions and watersheds and to refine causal analysis by integrating long-term monitoring data on groundwater nitrogen concentrations. The analytical framework can be applied not only in Japan but also in international regions with similar agricultural and hydrological conditions, contributing to evidence-based policymaking for sustainable agricultural management and groundwater conservation.