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Changes in Carbon Content
Figure 2 shows changes in carbon reserves for each land-use
category between 1850 and 1980. In this table, positive numbers indicate a
decrease in the carbon content in that particular land use category. Note that
some areas show an increase in total carbon because the area of this category
has increased and not because the carbon per unit area has increased. For
example, cultivated lands in temperate zones have doubled their carbon content
between 1850 and 1980, but this is due to a doubling of the area in this land
classification. The amount of carbon per unit area in this land-use type has
been essentially constant.
Agricultural and forestry management practices can have a significant effect on the earth's global carbon cycle. Soil tillage practices, crop choices, and plantation and forestry management practices all impact the global carbon cycle. High latitude continental areas have vast boreal forests and frozen tundra that store carbon for longer periods of time. Many biological and physical processes depend on temperature: higher temperatures tend to accelerate these processes. For example, cooking food speeds up the physical transformation processes. In tropical areas where the temperature is high but moisture is not limiting, growth and decay processes occur very rapidly, whereas at high latitudes and high altitudes (mountainous areas) they proceed very slowly. Disturbances to plant ecosystems that occur in areas of where transformations proceed slowly take much longer to recover than in warmer climates.
Agricultural soils have lost about a third to one half of their native carbon. Agriculture, by use of alternative tillage practices and also by eliminating excessive use of nitrogen fertilizers, may help to reduce atmospheric carbon dioxide by allowing more carbon to be stored in the soil. Nitrogen fertilizers stimulate soil processes and accelerate the transformation of soil carbon to carbon dioxide: reducing excessive use of nitrogen fertilizer reduces the amount of soil carbon lost to the atmosphere.
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