Mitigating Diffuse Nitrogen Loading in Tile-Drained Agricultural Catchments Through Controlled Drainage and Constructed Wetland Integration: A Field-Scale Mass Balance Assessment
Keywords:
diffuse nitrogen pollution, tile drainage management, constructed wetland denitrification, agricultural catchment hydrology, nitrate mass balance, controlled drainage hydraulics, hypoxic zone mitigation, stable isotope tracing δ¹⁵N, agroecosystem water qualityAbstract
Diffuse nitrogen (N) loading from tile-drained croplands constitutes a primary driver of hypoxic zone expansion in receiving water bodies. This study quantifies the nitrogen removal efficiency of a hybrid controlled drainage–constructed wetland (CD–CW) system installed across a 340-hectare tile-drained maize–soybean rotation catchment in the Danube lowland plain. Using continuous water quality sensors, stable isotope tracing (δ¹⁵N–NO₃⁻), and mass balance modelling over three hydrological years (2021–2023), we demonstrate a 58.4% reduction in annual nitrate-N export compared with conventional free-drainage controls. Denitrification accounted for 71% of total N removal within the CW cells during peak spring discharge events. The integrated CD–CW configuration proved cost-effective at USD 4.2 kg⁻¹ N removed. Findings provide actionable design thresholds for wetland-to-catchment area ratios applicable to temperate lowland agroecosystems.
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