Overall, fertilizer production, legume crops, and
fossil fuel burning deposit approximately 140 Tg of new
nitrogen into land-based ecosystems each year, a figure
that equals the upper estimates for nitrogen fixed naturally
by organisms in these ecosystems. Other human
activities liberate and make available half again that much
nitrogen. From this evidence, it is fair to conclude that
human activities have at least doubled the transfer of
nitrogen from the atmosphere into the land-based biological
nitrogen cycle.
This extra nitrogen is spread unevenly across the
Earths surface: Some areas such as northern Europe are
being altered profoundly while others such as remote regions
in the Southern Hemisphere receive little direct input
of human-generated nitrogen. Yet no region remains
unaffected. The increase in fixed nitrogen circulating
around the globe and falling to the ground as wet or dry
deposition is readily detectable, even in cores drilled from
the glacial ice of Greenland.