Effects of global change during the 21st century on the nitrogen cycle
The global nitrogen (N) cycle at the beginning of the 21st century has been shown to be strongly influenced by the inputs of reactive nitrogen (N<sub>r</sub>) from human activities, including combustion-related NO<sub><i>x</i></sub>, industrial and agricultural N...
| Main Authors: | , , , , , , , , , , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2015-12-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/15/13849/2015/acp-15-13849-2015.pdf |
| Summary: | The global nitrogen (N) cycle at the beginning of the 21st century has
been shown to be strongly influenced by the inputs of reactive nitrogen
(N<sub>r</sub>) from human activities, including combustion-related NO<sub><i>x</i></sub>,
industrial and agricultural N fixation, estimated to be 220 Tg N yr<sup>−1</sup> in
2010, which is approximately equal to the sum of biological N fixation in
unmanaged terrestrial and marine ecosystems. According to current
projections, changes in climate and land use during the 21st century
will increase both biological and anthropogenic fixation, bringing the total
to approximately 600 Tg N yr<sup>−1</sup> by around 2100. The fraction contributed
directly by human activities is unlikely to increase substantially if
increases in nitrogen use efficiency in agriculture are achieved and control
measures on combustion-related emissions implemented.
<br><br>
Some N-cycling processes emerge as particularly sensitive to climate change.
One of the largest responses to climate in the processing of N<sub>r</sub> is the
emission to the atmosphere of NH<sub>3</sub>, which is estimated to increase from
65 Tg N yr<sup>−1</sup> in 2008 to 93 Tg N yr<sup>−1</sup> in 2100 assuming a change
in global surface temperature of 5 °C in the absence of increased
anthropogenic activity. With changes in emissions in response to increased
demand for animal products the combined effect would be to increase NH<sub>3</sub>
emissions to 135 Tg N yr<sup>−1</sup>. Another major change is the effect of
climate changes on aerosol composition and specifically the increased
sublimation of NH<sub>4</sub>NO<sub>3</sub> close to the ground to form HNO<sub>3</sub> and
NH<sub>3</sub> in a warmer climate, which deposit more rapidly to terrestrial
surfaces than aerosols. Inorganic aerosols over the polluted regions
especially in Europe and North America were dominated by
(NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> in the 1970s to 1980s, and large reductions in
emissions of SO<sub>2</sub> have removed most of the SO<sub>4</sub><sup>2−</sup> from the
atmosphere in these regions. Inorganic aerosols from anthropogenic emissions
are now dominated by NH<sub>4</sub>NO<sub>3</sub>, a volatile aerosol which contributes
substantially to PM<sub>10</sub> and human health effects globally as well as
eutrophication and climate effects. The volatility of NH<sub>4</sub>NO<sub>3</sub> and
rapid dry deposition of the vapour phase dissociation products, HNO<sub>3</sub> and NH<sub>3</sub>,
is estimated to be reducing the transport distances,
deposition footprints and inter-country exchange of N<sub>r</sub> in these
regions.
<br><br>
There have been important policy initiatives on components of the global N
cycle. These have been regional or country-based and have delivered
substantial reductions of inputs of N<sub>r</sub> to sensitive soils, waters and
the atmosphere. To date there have been no attempts to develop a global
strategy to regulate human inputs to the nitrogen cycle. However,
considering the magnitude of global N<sub>r</sub> use, potential future increases,
and the very large leakage of N<sub>r</sub> in many forms to soils, waters and the
atmosphere, international action is required. Current legislation will not
deliver the scale of reductions globally for recovery from the effects of
N<sub>r</sub> deposition on sensitive ecosystems, or a decline in N<sub>2</sub>O
emissions to the global atmosphere. Such changes would require substantial
improvements in nitrogen use efficiency across the global economy combined
with optimization of transport and food consumption patterns. This would
allow reductions in N<sub>r</sub> use, inputs to the atmosphere and deposition to
sensitive ecosystems. Such changes would offer substantial economic and
environmental co-benefits which could help motivate the necessary actions. |
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| ISSN: | 1680-7316 1680-7324 |