The impact of atmospheric nitrogen deposition on nitrogen losses from forested areas is discussed within this report. Special emphasize is placed on the nitrogen leakage to surface waters in the Nordic countries. Atmospheric nitrogen deposition, nitrogen pools, internal nitrogen cycling and nitrogen losses in coniferous forests are discussed in connection with potential effects of atmospheric nitrogen deposition on the forest ecosystem. The influence of atmospheric nitrogen deposition on the terrestrial ecosystem and thereby the leakage of nitrogen to surface water is far from fully understood, but som conclusions can be drawn. Atmospheric nitrogen deposition on water surfaces and probably saturated discharge areas will contribute more or less instantly and to almost 100% to the nitrogen loading during flow events. Decreased atmospheric nitrogen deposition on these areas would reduce the load immediately. Atmospheric nitrogen deposition and mobilized nitrogen in the soil in unsaturated discharge areas might rapidly be laterally flushed through the soil and into the surface water. This process is especially important for the leakage of nitrate and organically bound nitrogen and it os probably the most important path for the nitrogen loading from forests. Atmospheric nitrogen deposition on recharge areas will contribute to the load in proportion to the net loss from the unsaturated zone. Net loss will be low from naturally growing forest with high nitrogen retention, while any change limiting the growth or soil adsorption capacity or favouring nitrification will increase the leakage. Groundwater transit times will determine when the nitrogen reaches the surface water and fractions might be delayed for considerable time. Denitrification or chemical reduction will diminish the nitrate flux wherever it occurs in the system. Areas with high concentrations of organic carbon of ferrous iron in the soil or bedrock seem to be the most probable sites. Long groundwater transit times favour nitrate reduction. Seasonal variations in climate and biological uptake and release largely influence the nitrogen loss and a considerable range on the nitrogen leakage will be found in forests growing under natural conditions even if the atmospheric deposition is constant. Hypothetic estimations of the short-term nitrogen loading to the sea caused by excess atmospheric nitrogen deposition on Sweden showed that only forests in the south-west part of the country would increase the nitrogen loss. It is probable that the increase would occur mainly in forests growing on nitrogen rich soils and at the most severely polluted areas close to shore. Increased nitrogen leakage from the forests in northern Sweden are unlikely to occur. It could be expected that the nitrogen leakage from forested areas only would play a minor role in the total nitrogen loading of the sea if ...
