Nitrogen migration paths and source areas at different snowmelt periods in a seasonal freezing agricultural watershed

•The snowmelt period was divided into 2 stages by the relationship between δ18O and N.•PFP, RAP, AS and SAP (<2°), SAP (6–15°) affect water discharge and N concentration/yield.•N in lower slope areas was flushed into rivers by water from steeper areas (slope>6°).•N reserves in lower slope areas was abundant and discharge controlled nitrogen yield.•Snow and domestic sewage were N sources in rural areas during early/late snowmelt stages.