Page 648 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

612 part IV Soils, ecosystems, and Biomes
The atmosphere, which is the integrating link be- tween photosynthesis (fixation) and respiration (release) in the carbon cycle, contains only about 700 billion tonnes of carbon (as CO2) at any moment. This is far less carbon than is stored in fossil fuels and oil shales (13200 billion tonnes, as hydrocarbon molecules) or in living and dead organic matter (2500 billion tonnes, as carbohydrate molecules). In addition to being re- leased through the respiration of plants and animals, CO2 is released into the atmosphere through burning of grasslands and forests, volcanic activity, land-use changes, and fossil-fuel combustion by industry and transportation.
The carbon released into the atmosphere by human activity constitutes a vast, real-time geochemical exper- iment, using the Earth’s actual and only atmosphere as a laboratory. Annually, we are adding carbon to the atmo- spheric pool in an amount 400% greater than we were
in 1950. Global emissions of carbon from the burning of fossil fuels continue to increase. The removal of for- ests eliminates a portion of Earth’s carbon “sinks” (areas where carbon is stored). Land-use changes in which forest (which stores more carbon in a greater amount of biomass) is converted to agriculture (which stores less carbon) is causing the release of several million tonnes of carbon into the atmosphere annually; the loss of soil organic matter is another significant contribution. All of this enhances Earth’s natural greenhouse effect and as- sociated global warming.
Nitrogen Cycle Nitrogen, which accounts for 78.1% of each breath we take, is the major constituent of the at- mosphere. Nitrogen also is important in the makeup of organic molecules, especially proteins, and therefore is essential to living processes. A simplified view of the nitrogen cycle is portrayed in Figure 19.6.
Secondary consumers
Primary consumers
Producers
Wastes, death, and decay
Atmospheric nitrogen
Fertilized farm fields
Nitrates
and ammonia
Electrical storms produce nitrates
Nitrogen-fixing bacteria in the roots of legumes
and in the soil
Soil bacteria and detritus feeders
▲Figure 19.6 The nitrogen cycle. The atmosphere is the reservoir of gaseous nitrogen. atmospheric nitrogen gas is chemically fixed by bacteria in producing ammonia. lightning and forest fires produce nitrates, and fossil-fuel combustion forms nitrogen compounds that are washed from the atmosphere by precipitation. Plants absorb nitrogen compounds and incorporate the nitrogen into organic material.