Material Cycles in Nature

Biogeochemical Cycles
In this section, we examine three material cycles of nature: cycles of water, carbon, and nitrogen--substances central to maintaining life on Earth. The three material cycles consist of the transfer of chemicals from biological systems to geological systems and are therefore called biogeochemical cycles. Processes that affect these transfers are biological processes such as respiration, transpiration, photosynthesis, and decomposition, as well as geological processes such as weathering, soil formation, and sedimentation. As materials cycle through, we note that the total quantity (mass) of matter remains the same, and energy that is put in changes to work (often to rearrange forms of matter) and is eventually lost to the surroundings.

Figure 3:Map of Schematic Material Cycles.

Figure 3 shows several of the cycles that determine the balance between life (biosphere), the Earth (lithosphere), and air and water (atmosphere and hydrosphere). All of us are familiar with the water cycle. The major elements cycled in nature are carbon, phosphorus, nitrogen, and sulfur, along with oxygen which forms part of all the cycles. The diagram includes general material and energy flows. Nutrients of smaller systems also cycle--with carbon and oxygen being the main components. Figure 1 shows the interactions between material cycles, energy input and transfers. Various aspects such as the water cycle, state of the oceans, and the climate are all interrelated and the rate of human activities disturbs the natural flows of materials and energy. When the rates of the disruptions are larger than the capacity of the entire system to bounce back, the system begins to shift, affecting all levels of the ecosystems through local and global changes.

Materials are transferred between the atmosphere, hydrosphere (oceans), lithosphere (land), and the biosphere. These various "spheres" act as "reservoirs" that keep materials for different amounts of time, called residence times. Each cycle forms a complicated system and the systems then interact with each other to produce weather and climate as well as the periodic fluctuations that maintain the dynamic balance on Earth, including all life. These cycles have evolved to the present rate over billions of years. Interruptions of these cycles at much larger rates by human endeavors such as fossil fuel burning produce several of the environmental problems we face.

Four elements form the main components of biogeochemical cycles - S, N, O, and C. Table 2 shows the chemical species in terms of where these elements primarily occur and the relative amounts in the four major reservoirs - atmosphere, oceans, biosphere, and crust. Phosphorus is another element that is cycled in nature. We do not describe the phosphorus cycle here.

In this unit, we describe three cycles in detail - the water (hydrologic) cycle, the carbon cycle, and the nitrogen cycle, and give a brief descriptions of the sulfur and oxygen cycles. As we describe each cycle. We also describe the chemical and its sources.

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