Biochar addition changed the nutrient content and runoff water quality from the top layer of a grass field during simulated snowmelt

•Biochar temporarily improved growing conditions in the soil (pH, P and K content).•Biochar increased PO4-P losses in surface runoff during snowmelt.•Biochar systematically retained cattle slurry/mineral fertilizer-originated NO3-N in the soil.•Biochar decreased N losses in surface runoff during snowmelt. Biochar has the potential to decrease the leaching of nutrients from agricultural systems to watercourses. We applied cattle slurry (0 or 40 t ha−1) and different amounts of spruce biochar (0, 10, 20 or 40 t ha−1) to the surface of a boreal grass field in autumn and analyzed the nutrient content in the soil and the regrowth of grass (Phleum pratense) in the autumn over three years. In addition, four grass mats from each treatment were lifted each autumn and stored for 2–3 months outdoors (between −36 °C and 12 °C) for simulated snowmelt experiments. In the following winter, 30 kg snow was added to the grass mats and slowly melted with infra-red heaters over a 7-day period. All surface runoff water was collected and the concentrations of nitrate nitrogen (NO3-N), ammonium nitrogen (NH4-N), total nitrogen (TN), dissolved phosphate phosphorus (PO4-P), total phosphorus (TP), dissolved total potassium (K), dissolved organic carbon (DOC) and pH were measured. Biochar temporarily improved growing conditions by increasing pH values and the content of plant available acid ammonium acetate extractable P (PAc) and K (KAc) in the soil, which also increased losses in surface runoff during the first snowmelt. In contrast, the biochar retained slurry- and mineral fertilizer-originated NO3-N on the soil during the three year experiment and prevented NO3-N losses in surface runoff during the first snowmelt after autumnal slurry addition. The biochar-induced increase in phosphorus (P) loss in surface runoff during the first snowmelt after slurry application was considerable (0.1–0.5 kg P ha−1) and the decrease in nitrogen (N) loss was moderate (0.2–0.6 kg N ha−1) compared to annual P and N losses in surface runoff in an adjacent field. The surface application of biochar can be recommended for the inhibition of nutrient losses to watercourses via surface runoff only if the capacity of used biochar to retain both N and P can be verified before application.