Långsiktiga geokemiska förändringar kopplade till skoglig markanvändning i boreala sjösediment

Sammanfattning

Human land use in boreal forest landscapes, including clearcutting, ditching, and agricultural conversion, has substantially altered forested catchments. These disturbances modify hydrology and increase the transport of mineral particles and nutrients from soils to lakes, yet their long-term effects on boreal lake sediments remain insufficiently understood. This study investigated how land use has influenced erosion, inorganic geochemistry, and nutrient dynamics over time in three small boreal forest lakes in northern Sweden. Sediment cores from Ottjärnen, Aldertjärnen, and Klintsjön were analyzed using X-ray fluorescence spectroscopy and loss on ignition to quantify lithogenic material, geochemical variability, and organic matter content. Elemental ratios of titanium to rubidium (Ti:Rb) and silicon dioxide to aluminum oxide (SiO2:Al2O3) were used to infer changes in erosion intensity and biogenic silica. The results revealed distinct geochemical zones in all lakes, indicating temporal shifts in sediment sources and transport processes. Elevated Ti:Rb ratios indicated enhanced minerogenic input. This was observed particularly in Aldertjärnen and Klintsjön, where repeated clearcutting, agriculture, and possible ditching in the catchment area likely intensified erosion. Ottjärnen instead showed evidence of an older disturbance phase followed by partial recovery. Phosphorus dynamics differed among the lakes: Ottjärnen displayed strong coupling between phosphorous, iron, and minerogenic transport, whereas Aldertjärnen and Klintsjön showed patterns more consistent with internal nutrient cycling and altered redox conditions in recent sediments. Overall, the findings demonstrate that land-use practices in boreal forest landscapes can leave long-lasting geochemical signatures in lake sediments and influence both erosion dynamics and nutrient conditions over time.