農業由来の窒素負荷の時空間変動要因を解明～熊本地域を対象とした60年間のデータ解析による持続可能な農業管理への提言～

2025-05-15 長崎大学

長崎大学、九州大学、熊本大学、スウェーデン・ルンド大学の共同研究チームは、熊本地域における1960～2020年の農業由来窒素負荷を解析し、その時空間変動要因と地域格差構造を明らかにした。研究では、10行政区画の農業統計データを用い、LMDI分解法と加重タイル指数を組み合わせた分析を実施した。その結果、1985年以前は農業規模拡大が窒素負荷増加の主因だったが、1985年以降は農地縮小や畜産構造変化など農業構造転換が主要因へ移行したことが判明した。また、窒素負荷生産性の格差は、地域内格差中心から地域間格差中心へ変化し、農業地域専門化の進行が示された。さらに、各行政区画を6つの発展パターンに分類し、4Rニュートリエント管理や高付加価値農業導入など地域特性に応じた窒素管理戦略を提案した。研究チームは、地下水汚染対策には一律政策ではなく、地域特性を踏まえた長期的・先行的介入が不可欠だと指摘している。

図１分析フレームワーク

＜関連情報＞

日本の熊本地域における窒素負荷の時間的・空間的分解分析および窒素負荷生産性の地域格差：対数平均ディヴィジア指数と加重タイル指数を用いた行政区分に基づくアプローチ Temporal and spatial decomposition analysis of nitrogen load and regional disparity of nitrogen load productivity in the Kumamoto area in Japan: An administrative-division-based approach using the Logarithmic Mean Divisia Index and the weighted Theil index

Zhuolin Li, Kei Nakagawa, Hidemichi Fujii, Takahiro Hosono, Ronny Berndtsson
Agricultural Systems Available online: 27 April 2026
DOI:https://doi.org/10.1016/j.agsy.2026.104766

Highlights

Nitrogen load and nitrogen load productivity were analyzed at the administrative division level.
The Logarithmic Mean Divisia Index was applied to identify drivers for nitrogen load.
The weighted Theil index was used to trace regional disparity shifts in nitrogen load productivity.
Six agricultural development patterns were classified for tailored nitrogen management strategies.

Abstract

Context

Nitrate contamination of groundwater, driven by human activities such as agriculture and urbanization, poses a significant global challenge.

Objective

We aimed to identify the major drivers of nitrogen load (nitrogen pollution generation potential) and quantify spatial disparities in nitrogen load productivity across administrative divisions in the Kumamoto area from 1960 to 2020 under evolving agricultural policies and market dynamics.

Methods

We applied the Logarithmic Mean Divisia Index decomposition and weighted Theil index to systematically examine nitrogen load dynamics and regional disparities in the Kumamoto area from 1960 to 2020. Focusing on crop production and farm animals, the analysis identified nitrogen intensity, scale, and structural changes as key driving indicators of nitrogen load, tracing their temporal evolution under shifting agricultural policies and market forces.

Results and conclusions

We show that (1) from 1960 to 1985, scale was the dominant driver of changes in staple crop production and farm animals. After 1985, farmland contraction and a decreasing number of farm animals allowed structural changes to become the primary determinant, indicating a shift toward high-quality agriculture. (2) Simultaneously, the weighted Theil index reveals that as regional specialization in crop production intensified, disparity shifted from within-to between-region. (3) Finally, by categorizing six development patterns at the administrative division level for a small watershed, this study exposes the limitations of one-size-fits-all policy and stresses the need for tailored strategies to enhance nitrogen load productivity.