This important difference in the retention of N and P
by soils is very important because humans are now
having a major impact on the N budget of the Earth.
Before human alterations to the global N cycle, the terrestrial
N cycle had an annual rate of N fxation from
natural sources of 90 to 140 Tg year-1. Three human-controlled
processes now release similar amounts of additional N,
doubling the rate of N input to terrestrial ecosystems.
The industrial production of nitrogenous fertilizers used
in agriculture now results in the release of about 80 Tg
year-1. Combustion of fossil
fuels results in the fxation of about an additional 20 Tg
year-1
, and the farming of legume crops such as peas,
alfalfa, soybeans, and vetch, fxes about an additional
40 Tg year-1 . Because of its
mobility via atmospheric transport , and via water, this added N is
redistributed around the Earth. The pre-industrial rates
of N deposition in terrestrial habitats are not well
known, but these inputs likely were less than 1 or 2 kg N
ha-1 year-1
. Human impacts on the terrestrial N cycle
have dramatically increased rates of N deposition in
some ecosystems, such as those of the Netherlands, to
rates as high as 40±90 kg ha-1 year-1
. This represents a
massive increase in the effective N supply rates to these
ecosystems because the annual rate of N mineralization
in most native forest soils ranges only from 10 to 60 kg
ha-1 year-1
.