In the context of climate change, carbon dioxide (CO2) fertilization is the stimulation of biospheric carbon uptake by rising atmospheric CO2 concentrations. That is, as the CO2 level increases, plants absorb more carbon and sequester it in biomass, removing it from the atmosphere and increasing their own size and productivity. This fertilization is realized by photosynthetic enzymes. It involves multi-scale processes, from the level of individual leaves and plants all the way up to regional and global ecosystems, and it is regulated by many other factors.

CO2 and water (H2O) are the two basic substrates of photosynthesis, which is driven by solar energy and regulated by an enzyme, Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). When all other factors are constant, photosynthesis increases as CO2 concentration increases. Thus, photosynthesis is sensitive to CO2 concentration.

The photosynthetic sensitivity to CO2 concentration is universal to all carbon 3 plants, because all carbon 3 plants share the same enzyme, RuBisCO, to catalyze photosynthesis. The sensitivity is also virtually independent of light and nutrient levels, but it declines with CO2 concentration. This sensitivity can be measured as the percentage increase in carbon fixation caused by a 1 part per million increase in CO2 concentration. In other words, a CO2 sensitivity of 0.2 percent means that if CO2 concentration increases by 1 part per million, plants will fix 0.2 percent more carbon than they did at the previous concentration.

When atmospheric CO2 concentration increases from 280 parts per million (the preindustrial level) to 440 parts per million (a level projected to be reached in the 2030's), photosynthetic CO2 sensitivity declines from 0.183-0.352 percent to 0.077-0.183 percent. Assume that atmospheric CO2 concentration increases by 2 parts per million per year and that global photosynthetic carbon fixation is 120 billion metric tons per year and is accomplished entirely by carbon 3 plants. Under such conditions, Earth's land ecosystems will fix roughly an additional 600 million metric tons of carbon per year as a result of photosynthetic sensitivity. Carbon 4 plants, however, are less sensitive to CO2 concentration than are carbon 3 plants.

When plants grow under different CO2 concentration, they adjust their leaf structures and biochemical properties, resulting in photosynthetic acclimation. Photosynthetic acclimation is regulated by sugar signals at the biochemical level and is usually related to nutrient supply. Acclimation may increase or decrease photosynthetic sensitivity. Many studies indicate that photosynthetic acclimation greatly varies with species and environmental conditions. However, on average across all studies conducted in natural ecosystems, photosynthetic acclimation is not very substantial.

Photosynthetically fixed carbon is stored in plant biomass and soil organic matter (SOM). Carbon storage may last for years, decades, or centuries in plant wood pools and for up to thousands of years in soil pools, but it is very ephemeral in leaf and fine root pools. Carbon storage in wood pools is regulated by plant allocation and species. Carbon storage in soil pools is regulated by nutrient availability. Many CO2 experiments in natural ecosystems indicate that rising atmospheric CO2 concentration results in carbon storage in plant and soil pools. However, theCO2 fertilization may not occur under conditions in which some other growth factor is severely limiting, such as low temperature or low nutrient availability.

CO2 fertilization is a major mechanism of terrestrial carbon sequestration and a negative feedback mechanism protecting ecosystems from climate change. CO2 is a major greenhouse gas (GHG). Buildup of CO2 in the atmosphere results in climate warming. Since terrestrial ecosystems absorb approximately 120 billion metric tons of carbon from the atmosphere every year, a small stimulation of photosynthetic carbon uptake by CO2 fertilization can substantially influence global carbon balance and reduce the likelihood or degree of climate change. Several global analyses indicate that nearly 30 percent of the CO2 emitted by human activities is absorbed by land ecosystems. CO2 fertilization is one of the major mechanisms responsible for land carbon sequestration.

Because photosynthetic enzymes are sensitive to CO2 concentration, the CO2 fertilization factor should gradually decline as atmospheric CO2 concentration increases. However, as the world continues to consume fossil fuels, the yearly increase in atmospheric CO2 concentration becomes larger over time. CO2 fertilization will remain a major mechanism in the regulation of atmospheric CO2 concentration.

Most experimental and modeling studies demonstrate that nitrogen deposition acts synergistically with atmospheric CO2 concentration to stimulate carbon sequestration in land ecosystems. Nitrogen deposition is expected to increase by another two- to threefold in the future. This increase is likely to enhance the effects of CO2 fertilization on plant growth and carbon sequestration. CO2 fertilization effects are also regulated by other global change factors, such as climate warming and altered precipitation regimes.

In addition to regulation of CO2 concentration in the atmosphere, stimulation of food production by CO2 fertilization helps mitigate climate change impacts on developing countries. Although CO2 fertilization's effects on food production are relatively small in comparison to those of nitrogen fertilization and genetic breeding, food production still can increase by 10-20 percent when atmospheric CO2 concentration increases by 200-300 parts per million.

CO2 fertilization-based increases in plant growth and carbon sequestration are fundamentally driven by rising atmospheric CO2 concentrations. If those concentrations level off as a result of effective climate-mitigation activities, the effects of fertilization will also level off. If atmospheric CO2 concentration declines, as hypothesized in some scenarios of the Intergovernmental Panel on Climate Change, CO2 fertilization will also decrease, and land ecosystems will likely release some CO2 from plants and soils into the atmosphere. Thus, any analysis of climate dynamics and climate change mitigation must take into account CO2 fertilization effects, whether positive or negative.

Overall, CO2 fertilization stimulates carbon storage in plants and soil, reduces buildup of GHGs in the atmosphere, and determines the airborne fraction of anthropogenic carbon emissions.

Bibliography:

1)         Curtis, P. S., and X. Z. Wang. "A Meta-analysis of Elevated CO2 Effects on Woody Plant Mass, Form, and Physiology." Oecologia 113 (1998): 299-313.

2)         Luo, Y., D. Hui, and D. Zhang. "Elevated Carbon Dioxide Stimulates Net Accumulations of Carbon and Nitrogen in Terrestrial Ecosystems: A Meta-Analysis." Ecology 87 (2006): 53-63.

3)         Luo, Y., and H. A. Mooney. "Stimulation of Global Photosynthetic Carbon Influx by an Increase in Atmospheric Carbon Dioxide Concentration." In Carbon Dioxide and Terrestrial Ecosystems, edited by G. W. Koch and H. A. Mooney. San Diego, Calif.: Academic Press, 1996.

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