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

This chapter revisits the major problems relating to manmade 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 N-2 to 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(-2) yr(-1) and 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 NOx and NHx emissions. P concentrations were low, frequently < 1 mu g L-1, causing high N:P ratios in runoff, and promoting P limitation both in freshwater and the nearshore marine recipient. For the agriculturally and forested watershed, atmospheric N deposition was < 1.5 mg N m(-2) yr(-1), and agricultural activities were the totally dominant source of N, yielding generally high but fluctuating concentrations of N and P in runoff water. Forestry and agricultural practices were major determinants of N runoff. The high concentrations of both N and P in the major outlet of this watershed would be a major source of eutrophication for the recipient fjord. For both watersheds, climatic fluctuations strongly affected annual runoff patterns of N. The fate of ammonium relative to nitrate and long-term climatic changes with a possible mineralization of soil stores of organic N are unpredictable determinants to future effects of N.