Landscape properties and precipitation as determinants for high summer nitrogen load from boreal catchments

Context Hydrological conditions are among the most important factors influencing nutrient concentrations in rivers and their fluxes out of the catchments. In the boreal area extreme hydrological conditions are typical with intense floods during the snow-melt period in spring and the base-flow conditions during winter and summer. Objectives In this study we compared nitrate–nitrogen (NO 3 –N), ammonium–nitrogen (NH 4 –N) and dissolved organic nitrogen (DON) concentrations and fluxes among summer seasons (June–August) with contrasting hydrology in four small boreal rivers with differing land-use in southern Finland. Methods For the analysis we selected 3 years of the lowest summer discharge (1999, 2010, 2011) and 3 years of the highest summer discharge (1996, 1998, 2004). Results During high discharge summers NH 4 –N and DON concentrations were on average 187 and 240 % higher than during low discharge summers. Because of large differences in discharge between the summers the flux values of the different N fractions were at maximum 10–20 times higher during high discharge summers. The effect of heavy rains on N loading was clearly demonstrated in summer 2004 when two consecutive floods transported 42 % of the annual NO 3 –N flux, 44 % of the NH 4 –N flux and 57 % of the DON flux out of the catchment. Conclusions Available nitrogen storages in the studied catchment areas were probably in excess especially during the wet summers when the plant uptake was presumably lower compared to dryer and warmer summers. When the hydrological conditions were suitable for surface and subsurface runoff, the concentrations and fluxes of NO 3 –N, NH 4 –N and DON increased substantially.