Definition (Encyclopedia of Global Warming)
Nitrogen fertilization is a phenomenon in which plant growth is unusually stimulated by the addition of reactive nitrogen compounds, such as ammonia and nitrate. Nitrogen compounds are usually applied as fertilizers to soil for uptake by plant roots. They are also often applied through air deposition from the atmosphere to foliage for uptake through leaves and to surface soil for plant-root uptake. Nitrogen fertilizers are usually manufactured through chemical processes, such as the Haber-Bosch process, to produce ammonia. This ammonia is applied directly to the soil or used to produce other compounds, including ammonium nitrate and urea. Reactive nitrogen compounds in the atmosphere are side-products of fossil fuel combustion.
Nitrogen is an essential element of all proteins, enzymes, and metabolic processes for the synthesis and transfer of energy. Nitrogen is also an element in chlorophyll, the green pigment of plants that facilitates light harvest and photosynthesis. Thus, nitrogen is essential for plant survival, growth, and reproduction. Many experiments have shown that photosynthesis is linearly correlated with nitrogen concentration in leaves. Increased nitrogen availability via fertilization or deposition generally increases photosynthetic carbon fixation and stimulates plant growth.
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Significance for Climate Change (Encyclopedia of Global Warming)
Nitrogen is very abundant in the Earth’s atmosphere but mostly not active for plants to use. Most plants take up only reactive nitrogen compounds, mostly from soil, for photosynthesis and growth. In natural ecosystems, inactive nitrogen in the atmosphere is converted to biologically useful forms mainly via nitrogen fixation by lightning or by a limited number of plant and microbial species. anthropogenic activities, such as manufacture of nitrogen fertilizer, cultivation-induced nitrogen fixation, and combustion of fossile fuels, have accelerated the addition of reactive nitrogen to ecosystems, roughly doubling the rate of introduction. It is expected that conversion of inactive nitrogen in the atmosphere and in fossils to reactive nitrogen will increase another two- to threefold in the future.
Nitrogen addition, particularly through fertilization and deposition, has profound implications for climate change and global warming. Nitrogen deposition has been suggested to be a major mechanism underlying terrestrial ecosystem carbon sequestration. Extensive experimental evidence supports the theory that plant growth is limited by nitrogen in almost all ecosystems, and nitrogen addition to ecosystems often stimulates plant biomass growth and increases carbon storage in plant pools. The nitrogen limitation is usually persistent in ecosystems, largely because of the transient nature of biologically available forms...
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Further Reading (Encyclopedia of Global Warming)
Galloway, J. N., et al. “Transformation of the Nitrogen Cycle: Recent Trends, Questions, and Potential Solutions.” Science 320 (2008): 889-892. Comprehensive overview of the nitrogen cycle, including its impacts on climate change and global warming.
Hungate, B. A., et al. “Nitrogen and Climate Change.” Science 302 (2003): 1512-1513. Critically examines nitrogen’s effects on carbon processes and climate change. Concludes that projections of the global carbon cycle and its influence on climate change must consider nitrogen regulation.
LeBauer, D. S., and K. K. Treseder. “Nitrogen Limitation of Net Primary Productivity in Terrestrial Ecosystems Is Globally Distributed.” Ecology 89 (2008): 371-379. Synthesizes experimental results from many studies and concludes that nitrogen addition increases plant production in all types of ecosystems except deserts. Illustrates that nitrogen limits plant growth even in tropical regions, where it is traditionally thought to be limited by phosphorus.
Reay, D. S., et al. “Global Nitrogen Deposition and Carbon Sinks.” Nature Geoscience 1 (2008): 430-437. Focuses on impacts of nitrogen deposition on carbon sequestration. Demonstrates that nitrogen deposition likely increases carbon sequestration in plant pools. Nitrogen’s effects on soil carbon pools are controversial.
(The entire section is 178 words.)