36 years of water quality data in agricultural runoff : Insights on trends and hydrological drivers in a cold climate

Sammanfattning

Carbon, nitrogen and phosphorous in runoff play a crucial role for biota and water quality in agricultural systems. Boreal landscapes have been subject to environmental changes, and it is unknown how these, and other factors, have affected its agricultural systems. Here, I aimed to unravel the chemical signatures and mobilisation processes of surface and subsurface runoff and potential changes over time and compare these to a nearby stream. To do this, I analysed a 36-year long dataset from an agricultural field in northern Sweden looking at differences between soil and surface runoff regarding export and concentration of total organic carbon (TOC), total phosphorous (TP), nitrate (NO3), their ratios (TOC:TP, TOC:NO3) seasonally and over time, also using concentration-discharge (c-Q) analysis for determining solute mobilisation patterns. While chemistry of surface and drainage runoff differed, seasonality was limited for both sources and solute exports followed annual water fluxes. Several trends in concentration and ratios for different solutes and runoff types were observed, including a long-term increase in TOC concentrations. Mostly constant c-Q relationships were observed but also changes in c-Q behaviour throughout the study period. Stream runoff showed a more pronounced c-Q behaviour when compared to soil runoff, with lower solute concentrations and areal exports despite similar water yields. This study shows the importance of shallow and deep runoff for field exports, but also the impact of long-term fertilisation, management practices, climate and soil characteristics on seasonality, trends and mobilisation of solutes in an agricultural field and the disparity to adjacent stream conditions.