Principles of terrestrial ecosystem ecology.pdf

Terrestrial N fixation (Tg yr -1 )
150
100
50
Total anthropogenic
N fixation
Range of estimates of
natural N fixation N fertilizer
Fossil fuel
Legume crops
1960 1970 1980 1990
Time
Figure 15.5. Anthropogenic fixation of nitrogen in
terrestrial ecosystems over time compared with the
range of estimates of natural biological nitrogen
fixation on land. (Redrawn with permission from
Globol Biogeochemical Cycles; Galloway et al. 1995).
developing nations, with 40% of this total being
applied in the tropics and subtropics. Much
of the projected increase in fertilizer use is
expected to occur in the less-developed nations
of the world.
Cultivation of nitrogen-fixing crops such as
soybeans, alfalfa, and peas adds fixed nitrogen
over and above that which is added via biological
fixation in natural ecosystems. Some nitrogen
fixation is also carried out by free-living
and associative nitrogen fixers like Azolla that
Table 15.3. Global sources to
and sinks from the atmosphere
of nitrogen trace gases.
Sources and sinks NOx-N a
The Global Nitrogen Cycle 345
commonly occur in rice paddies. Annual fixation
rates in crop systems are about 32 to
53Tgyr -1 , 20 to 40% of total biotic fixation that
occurs on land.
Human activities account for most of the
nitrogen trace gases transferred from Earth to
the atmosphere (Table 15.3). In addition to
the large pool of relatively unreactive N2, the
atmosphere contains several nitrogen trace
gases, including nitric oxides (NO and NO2),
nitrous oxide (N2O), and NH3. Although the
pools and fluxes of these nitrogen trace gases
are much smaller than those of N2 (Fig. 15.4),
they play a much more active role in atmospheric
chemistry and have been more strongly
affected by human activities (see Chapter 9).
N2O, which is increasing at the rate of 0.2 to
0.3% yr -1 (Fig. 15.3), is an inert gas that is 200fold
more effective than CO2 as a greenhouse
gas and contributes about 6% of the greenhouse
warming (Ramaswamy et al. 2001). Nitrification
and denitrification in the oceans and
in tropical soils are the major natural sources
of N2O (Schlesinger 1997). Human activities
have nearly doubled N2O flux from Earth to
the atmosphere, primarily through agricultural
fertilization (Fig. 15.3). Other anthropogenic
N2O sources include cattle and feedlots,
biomass burning, and various industrial sources.
N2O-N a
NH3-N b
Total N
Natural biogenic sources 7.8 10 23 40.8
Oceans 0 4 c
13 17
Soils 2.8 d
6 10 18.8
Lightning 5 0 0 5
Anthropogenic sources 44.2 8.1 52.2 104.5
Cultivated soils 2.8 d 4.2 9 16
Biomass burning 7.1 0.5 5 12.6
Domestic animals 0 2.1 32 34.1
Fossil fuels/industrial 33 1.3 2.2 36.5
Other 0.7 0 4 4.7
Sinks
Atmospheric destruction ?? 12.3 1
Deposition on land 40 e ?? 0 57
Annual accumulation 0 3.9 0
a Data from Prather et al. (2001).
b Data from Schlesinger (1997).
c Data from Karl et al. (in press).
d Soil NOx flux uncertain; assumed to be 50% natural, 50% anthropogenic.
e Data from Holland et al. (1997).