For a better understanding of the effects of regional groundwater management for different land-use types on nitrogen content in groundwater, we investigated the Miyakonojo River Basin in south of Japan where the Nitrate Directive Plan has been in practice since 2004. For this purpose, we used nitrogen concentrations from 420 wells between 2000 and 2017 together with 8 different land-use categorizations. The data were analyzed using self-organizing maps (SOM) and results showed that forest recharge areas have lowest mean nitrogen concentrations of about 2.9 mg/L. Urban areas displayed a mean nitrogen concentration of about 4.4 mg/L. Agricultural land such as paddy fields had a mean nitrogen concentration of about 5.1 mg/L. Groundwater discharge and residential areas had mean groundwater nitrogen concentrations of 8.2 and 7.1 mg/L, respectively. Intensive agricultural land-use and wastewater discharge from urban areas caused the main groundwater nitrogen contamination in these areas. About 70% of the wells had a decreasing trend of groundwater nitrogen concentration (p < 0.05, p ≥ 0.05) during the period 2009–2017. About 20% of the wells displayed a trend reversal from 2000–2008 (increasing) to 2009–2017 (decreasing). In general, the Nitrate Directive Plan appears to have had positive effects for mitigating groundwater nitrogen problems. However, 30% of the wells still do not display a decreasing trend and some wells exceed maximum permissive level for drinking water. Thus, management needs to continue to improve groundwater conditions regarding nitrogen content.

为了更好地了解不同土地利用类型的区域地下水管理对地下水中氮含量的影响，我们调查了自 2004 年以来实施硝酸盐指令计划的日本南部的宫古城河流域。为此，我们使用了 2000 年至 2017 年间 420 口井的氮浓度以及 8 种不同的土地利用分类。使用自组织图 (SOM) 分析数据，结果显示森林补给区的平均氮浓度最低，约为 2.9 mg/L。城市地区的平均氮浓度约为 4.4 毫克/升。水田等农业用地的平均氮浓度约为 5.1 mg/L。地下水排放和居民区的平均地下水氮浓度分别为 8.2 和 7.1 毫克/升。集约化的农业用地和城市地区的废水排放导致这些地区的主要地下水氮污染。约70%的井地下水氮浓度呈下降趋势（p  < 0.05, p  ≥ 0.05) 在 2009-2017 年期间。从 2000-2008 年（增加）到 2009-2017 年（减少），大约 20% 的井显示出趋势逆转。总的来说，硝酸盐指令计划似乎对缓解地下水氮问题产生了积极影响。但仍有30%的水井未出现下降趋势，部分水井超过饮用水最大允许水位。因此，管理层需要继续改善地下水中氮含量的条件。