ABSTRACT
The Earth is undergoing rapid changes due to human disturbance, es pecially in the concentrations of the trace gases in its atmosphere and global land cover (Jenkinson, 2001; Vitousek, 1994; Wuebbles et al., 1999). The annually averaged C 02 concentration in the atmosphere has increased by almost one third since the beginning of the Industrial Rev olution (Whorf and Keeling, 1998), and is projected to double by the end of the 21st century due to increased combustion of fossil fuels and changes in land use (IPCC, 1996). At the same time, soluble nitrogen (N)-deposition has increased two to several fold, as a result of combus tion processes and fertilizer inputs in agriculture (Korner, 2000). Dur ing the past four decades, we have witnessed historically unprecedented additions of reactive N to terrestrial systems (Vitousek et al., 1997): anthropogenic N production has increased from about 40 Tg N year-1 in 1961 to about 160 Tg N year-1 in 1995 (Smil, 1999). Moreover, in creasing evidence supports the notion that the Earth’s average surface temperature has increased by about 0.5°C over the past century (Gaffen et al., 2000; Santer et al., 2000) and is predicted to increase by 1.4-5.8°C within this century (Houghton, 2001).
