In search of control points: A study on the interaction of hydrological fluxes and nitrogen in the boreal landscape

Sammanfattning: Nitrogen (N) export from terrestrial ecosystems is strongly influenced by hydrological flux, but losses of reactive N forms through surface waters are generally thought to be under strong biological control. However, losses from terrestrial ecosystems may be fostered by catchment processes that allow N to circumvent biological uptake. The aim of my research was to investigate how boreal landscape attributes interact with hydrological forcing to regulate N delivery to streams. I explored these controls through detailed study of key land-water interfaces, in combination with broader assessments of N concentration and export in the Krycklan Catchment Study (KCS), located in northern Sweden. I found, contrary to the prevailing view, forest riparian zones represented a net source of dissolved organic N (DON) and dissolved inorganic N (DIN) to streams. This pattern is connected to organic-rich riparian soils that promote local N production, potentially decoupling stream N exports from upland forest processes. Mires also acted as important N sources as reduced N in deep peat layers was delivered to streams through preferential flowpaths. At network scales, unique headwater biogeochemical patterns became more synchronous downstream, as mire, forest, and lake signals mixed. Nonetheless, mire and wet riparian areas emerged as important explanatory variables for stream N composition and concentration across the KCS. Finally, the relative importance of hydrological flux as the driver of N export in this system differed according to N form, the temporal scale considered, and catchment structure. DON formed close and consistent relationships with discharge among sites, years, and seasons, while DIN export was more weakly and variably related to flow. Still, surprisingly strong flow-controlled DIN export was observed during snow-free seasons, highlighting the importance of local, riparian N sources. Overall, my results suggest that long term peat accumulation – both in riparian zones and in mires – has created control points that regulate N losses through different mechanisms. The strong influence of these patches may overwhelm the detection of upland forest influences on N export in relatively pristine boreal landscapes.