Both are greenhouse gases that affect the global

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).Both are greenhouse gases that affect the global climate. Anthropogenic release of C to the atmosphere from the terrestrial pool results from land use change, mostly deforestation (20%); and from burning fossil fuels (80%).Before the mid-nineteenth century, deforestation was the main anthropogenic flux. Major pools of C: Atmosphere, oceans, land surface (includes soils and vegetation), sediments and rock. 99% of global C is in sediments and rock, the most stable pool; fluxes occur on geological time scales. Terrestrial pool: Soils contain twice as much C as plants.CO 2 is exchanged with the atmosphere mostly by photosynthesis and respiration.Prior to the Industrial Revolution, these two fluxes were roughly equal, with no net change in atmospheric CO 2 . Removing the forest canopy warms the soil, increasing rates of decomposition and respiration. Burning trees releases CO 2 , and small amounts of CO and CH 4 . In the 20 th century, major deforestation shifted from the mid-latitudes to the tropics. Anthropogenic emissions of CO 2 more than doubled from 1970 to 2011.About half is taken up by the oceans and terrestrial biota. But this proportion will decrease because terrestrial and ocean uptake will not keep pace with the rate of atmospheric increase. Higher concentrations of CO 2 may stimulate photosynthesis. But experiments have shown that increased photosynthetic rates may be short lived, and plants will acclimate to higher concentrations. For forest trees, increased CO 2 uptake may be sustained longer. One method of testing effects of elevated CO 2 uses free-air CO 2 enrichment, or FACE.CO 2 is injected into the air through vertical pipes that surround a stand of trees. Rate of injection is controlled to achieve a particular concentration of CO 2 . A FACE experiment with loblolly pines measured tree basal area to estimate aboveground NPP and soil cores to estimate belowground NPP (DeLucia et al. 1999).Elevated CO 2 levels increased the overall NPP of the forest by 25%. Input of C into the soil increased, both from litter and fine root turnover. Thus, forests may be an important sink for anthropogenic CO
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  • Spring '11
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