Projected climate change might increase the deposition of nitrogen by about 10% to seminatural ecosystems in southern Norway. At Storgama, increased precipitation in the growing season increased the fluxes of total organic carbon (TOC) and total organic nitrogen (TON) in proportion to the water flux. In winter, soil temperatures near 0°C, common under a snowpack, induced higher runoff of inorganic nitrogen (N) and lower runoff of TOC. By contrast, soil temperatures below freezing, caused by little snow accumulation (expected in a warmer world), reduced runoff of inorganic N, TON, and TOC. Long-term monitoring data showed that reduced snowpack can cause either decreased or increased N leaching, depending on interactions with N deposition, soil temperature regime, and winter discharge. Seasonal variation in TOC was mainly climatically controlled, whereas deposition of sulfate and nitrate (NO₃) explained the long-term TOC increase. Upscaling to the river basin scale showed that the annual flux of NO₃ will remain unchanged in response to climate change projections.

A high-resolution chemical transport model, driven by meteorology representing current and future climate, was used to investigate the effects of possible future changes in climate on nitrogen deposition in northwestern Europe. The model system was able to resolve the climatology of precipitation and chemical properties observed in northern Europe during the 1980s, albeit with some underestimation of the temporal and spatial variability of meteorological parameters and chemical components. The results point toward a substantial increase (30% or more) in nitrogen deposition over western Norway as a consequence of increasing precipitation but more moderate changes for other areas. Deposition of oxidized nitrogen will increase more than the deposition of reduced nitrogen. Over Sweden, oxidized nitrogen will increase only marginally and reduced nitrogen will decrease, although annual precipitation is expected to increase here as well. This is probably because more reduced nitrogen will be removed further west in Scandinavia because of the strong increase in precipitation along the Norwegian coast. The total deposition of oxidized nitrogen over Norway is expected to increase from 96 Gg N y⁻¹ during the current climate to 107 Gg N y⁻¹ by 2100 due only to changes in climate. The corresponding values for Sweden are more modest, from 137 Gg N y⁻¹ to 139 Gg N y⁻¹.

To provide baseline data for climate manipulation experiments in 11 small (30–268 m²) headwater catchments at Storgama, Telemark County, Southern Norway, we assessed the natural variability in site characteristics and runoff quality. Annual average concentrations in runoff at the sites have coefficients of variation between 26–61%, with the smallest values for total organic carbon (TOC) and carbon to nitrogen (C/N) ratios and the largest for inorganic nitrogen (N). The catchments have between two and five times higher concentrations of inorganic N, TOC, and total phosphorus than the larger (0.6 km²) Storgama watershed nearby. Concentrations of TOC and TON in runoff tend to increase with soil C and N content and with the volume of soil in the catchment. For nitrate (NO₃) and ammonium in runoff, the reverse is true. In wet years the proportion of bare rock is a major predictor for the annual average NO₃ concentration in runoff.

We have manipulated the winter-time soil temperature regime of small headwater catchments in a montane heathland area of southern Norway to study the possible effects on concentrations and fluxes of inorganic nitrogen in runoff. The experiments included extra insulation of soils in two catchments to prevent subzero temperatures during winter, and removal of snow in two other catchments to promote soil frost. Increased soil temperatures during winter increased the springtime concentrations and fluxes of ammonium (NH₄) and nitrate (NO₃) in runoff. By contrast, snow removal with development of significant soil frost showed no systematic effects on mean concentrations or fluxes of inorganic N. The results from our experiments suggest that warmer soils during winter caused by exceptionally mild winters, or alternatively a heavy snowpack, imply a greater risk for inorganic N leaching in this region than a possible increase of soil frost events because of reduced snow cover.

Projected increases in winter temperature due to future climate change may cause decreased snow accumulation at lower and intermediate altitudes in northern temperate regions. The resulting changes in soil temperature and water regime may affect the leaching of total organic carbon (TOC) and total organic nitrogen (TON). We manipulated the snow cover of small headwater catchments in a montane heathland area of southern Norway to quantify its effect on concentrations and fluxes of TOC and TON in runoff. Manipulations included snow removal, to promote soil frost, and insulation, to prevent soil frost. Snow removal resulted in increased TOC and TON concentrations, but decreased fluxes. Insulation caused a slight decrease in concentrations and fluxes of TOC. Our experiments show that a change in snow depth, and thus soil temperature, is not likely to have serious effects on TOC and TON leaching in the montane heathland area studied.

Projected changes in climate in Southern Norway include increases in summer and autumn precipitation. This may affect leaching of dissolved organic matter (DOM) from soils. Effects of experimentally added extra precipitation (10 mm week⁻¹) during the growing season of 3 years (2004–2006) to small headwater catchments at Storgama (59°0′N, 550–600 m a.s.l.) on leaching of total organic carbon (TOC) and total organic nitrogen (TON) were assessed. Extra precipitation did not have a significant effect on average TOC and TON concentrations in runoff. Thus, fluxes of TOC and TON increased nearly proportionally with water fluxes. This suggests that a store of adsorbed and potentially mobile TOC and TON in catchment soils buffers the concentration of DOM in runoff. The size and dynamics of the pool of TOC and TON depends on the balance between production and leaching rates. Infrequent short droughts had only small effects on TOC and TON fluxes in runoff from the reference catchments.

Fluctuations in the 20-year record of nitrate (NO₃) and total organic carbon (TOC) concentrations and fluxes in runoff at the small headwater catchment Storgama, southern Norway, were related to climate and acid deposition. The long-term decline in NO₃ related to reduced NO₃ deposition and increased winter discharge, whereas the long-term increase in TOC related to reduced sulfur deposition. Multiple regression models describing long-term trends and seasonal variability in these records were used to project future concentrations given scenarios of climate change and acid deposition. All scenarios indicated reduced NO₃ fluxes and increased TOC fluxes; the largest projected changes for the period 2071–2100 were −86% and 24%, respectively. Uncertainties are that the predicted future temperatures are considerably higher than the historical record. Also, nonlinear responses of ecosystem processes (nitrogen [N] mineralization) to temperature, N-enrichment of soils, and step-changes in environmental conditions may affect future leaching of carbon and N.

The mass transport model TEOTIL was used to project nitrate (NO₃) fluxes from the Tovdal River basin, southernmost Norway, given four scenarios of climate change. Forests, uplands, and open water currently account for 90% of the NO₃ flux. Climate scenarios for 2071–2100 suggest increased temperature by 2–4°C and precipitation by 3–11%. Climate experiments and long-term monitoring were used to estimate future rates of nitrogen (N) leaching. More water will run through the terrestrial catchments during the winter but less will run in the spring. The annual NO₃ flux from the Tovdal River to the adjoining Topdalsfjord is projected to remain unchanged, but with more NO₃ delivered in the winter and less in the spring. Algal blooms in coastal waters can be expected to occur earlier in the year. Major sources of uncertainty are in the long-term fate of N stored in soil organic matter and the impacts of forest management.