Vegetation, drainage and peat depth control ecosystem scale nitrogen loading from a cultivated peatland

Abstract. Organic soils are productive for cultivation, but leaching loss to water bodies and the atmosphere emissions causes high environmental and climatic costs. Nutrient losses are not constant in time or space; instead, they are formed by the interplay between soil stocks, farmer management actions, hydrological processes, and weather drivers. Monitoring systems that capture these aspects are necessary to better integrate cultivated peatlands into broader hydrological and climate systems and assess the effectiveness of farm-level interventions to mitigate harmful emissions.

We conducted a two-year study with hourly monitoring of the water table depth, drain discharge, drainage nitrate (NO₃), and atmospheric nitrous oxide (N₂O) concentrations in a 26-ha cultivated peatland on top of an acid sulfate affected subsoil. The field was divided into six blocks with an organic horizon varying from 15 to 75 cm. During the monitoring period, the grass sward was renewed and the drainage depth was adjusted from 80 to 30 cm.

Drainage flow and NO₃-N concentrations showed high seasonal variability, with the highest drainage flow occurring together with a high NO₃-N concentration and an elevated water table during spring and autumn, and the lowest concentration and drain discharge during the growing season. Adjustment of the drainage depth drastically reduced the generation of drain discharge; however, the drainage NO₃-N concentration was only slightly reduced. Annual export of NO₃-N through drainage systems from deep and shallow peat portions was 4.75 kg NO₃-N h^-1 year ^-1 and 3.81 kg NO₃-N h^-1 year ^-1 with majority of NO₃-N leaching occurring during single spring melt event during period with bare soil without plant cover. Atmospheric N₂O concentrations co-varied with drainage NO₃-N only during bare soil periods when elevated soil temperatures coincided with high NO₃ availability. During vegetated periods, this coupling was absent despite similar NO₃ levels, highlighting the critical role of plant uptake and environmental constraints in regulating N₂O emissions.

In addition to process insights, this study demonstrates the feasibility and limitations of high-frequency optical NO₃-monitoring in iron-rich, organic drainage waters. Together, the results highlight the importance of hydrological event timing, vegetation cover, and drainage management in controlling nitrogen losses and provide guidance for both mitigation strategies and monitoring system design in peatland and agricultural ecosystems.