Controlled Drainage as a Mitigation Strategy for N Leaching in Northern Ostrobothnia, Finland

This study examines the effects of controlled drainage compared to conventional drainage on nitrogen (N) concentrations, N loads, and hydrological conditions in a boreal agricultural setting in northern Finland. The research was conducted at an experimental field in Sievi over
five consecutive growing seasons (2020–2024). The aim was to evaluate whether controlled drainage can reduce nitrogen losses to water bodies while also enhancing water retention capacity under varying climatic conditions. Nitrogen concentrations (total N) and loads were
measured from collector drains, and groundwater levels were continuously monitored. Statistical tests were conducted to determine significant differences between the two drainage systems.

Results showed that total nitrogen concentrations and nitrogen loads were generally lower in the controlled drainage system, despite occasional anomalies in 2020 and 2024. Controlled drainage also maintained higher and more stable groundwater levels and moderated peak discharges more effectively than the conventional system. A moderate positive correlation was found between nitrogen concentration and load in the controlled system. However, results indicate that controlled drainage may be less effective during weather extremes or specific crop and fertilization regimes.

Overall, the findings support the use of controlled drainage as a mitigation strategy for nutrient leaching in agricultural fields. It can reduce nitrogen transport to adjacent water bodies and serve as a climate-adaptive water management tool, particularly under increasingly variable weather conditions. The study also highlights the importance of long-term data and suggests including additional indicators like phosphorus and organic matter in future evaluations of drainage systems.