Human impacts on the nitrogen cycle: a global problem judged from a local perspective

This chapter revisits the major problems relating to man-made disturbances in the global nitrogen (N) cycle, and links the local findings from the project to the large-scale effects. The human transformation of atmospheric N2to chemically and biologically more reactive species causes a number of environmental effects. The focus of this project has been budget estimates of N for two large watersheds with a set of subcatchments in southern Norway, accompanied by process studies to explain patterns of retention and runoff of N. Atmospheric inputs were close to 2.5 mg N m-2yr-1and by far the dominant source of N for the sparsely populated, heathland dominated watershed. Low retention and apparent N saturation could be accredited to high atmospheric inputs of N, but also hydrology, poorly developed soils and terrestrial P limitation. In this acidified watershed, nitrate contributed nearly 50% to surface water acidification, underlining the importance for strong cuts in NOxand${\rm NH}_{{\rm x}}$emissions. P concentrations were low, frequently $