Materials recycling

4.3.1 Materials recycling

As is evident from the above discussion, the natural recycling process starts with the formation of plant tissues through the processes of photosynthesis and biosynthesis. At this early stage, some oxygen is released into the environment. In many ecosystems, the second major stage of recycling occurs when animals, in their effort to metabolize the stored energy from plant tissue, release carbon dioxide and organic wastes. Major recycling (decomposition), however, is done by microorganisms. The microorganisms ultimately break down dead organic matter into its simpler (inorganic) molecular components. This recycling is particularly important because the amount of mineral elements found in the ecosystem (especially N and P) is finite, and limiting to the growth and reproduction of organisms.

However, decomposition may not always be complete. The oxidation process involved in decomposition depends on the availability of oxygen and the energy circulation of a given environment. For example, oxidation takes place at a much faster pace in a tropical forest than at the bottom of a lake. Thus, in nature,

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material recycling is not 100 percent efficient, and some amounts of organic matter may remain, only partially decomposed. This incompletely decomposed organic matter, accumulated and aged over a period of time, forms peat, coal and petroleum—that is, fossil fuel. This is the origin of the sources of energy so crucial to the modern human economy. It is also a large reserve of carbon that gets released rapidly when

fossil fuels are burned and contributes to global warming by releasing CO 2 to the atmosphere at an unprecedented rate. Recycling of materials is not limited to the biological and material cycles in an ecosystem as discussed above. The well-known atmospheric cycles (such as those of carbon, nitrogen and sulfur) contribute to the circulation of these elements within the various media of the ecosystem. Furthermore, it is through atmospheric cycles that the concentration of these elements in a given environmental medium is maintained or regulated. For example, the atmosphere is composed of approximately 20 percent oxygen, 79 percent nitrogen, 0.9 percent argon (which is not significant biologically) and 0.03 percent carbon dioxide. It is very important to note, when the concern is the functioning of an ecosystem, that the atmospheric cycles cannot

be viewed in isolation from other cycles (that is, geologic and biological cycles). For example, there is a large reserve of nitrogen in the atmosphere, and a variety of microorganisms are responsible for converting atmospheric nitrogen to a form that plants can use through a process called nitrogen fixation, whereas there is no large reserve of nitrogen in rocks. Thus, nitrogen fixation is the critical process of converting unavailable gaseous nitrogen from the atmosphere to available (inorganic) nitrogen for plants. Furthermore, physical and chemical processes associated with volcanic activities and the combustion of fossil fuels also can increase the availability of useful nitrogen to ecosystems.

In addition to the atmospheric cycles, geological processes also contribute to the constant recycling of materials in the ecosystem. For example, it is through erosion and water movement that nitrates, sulfates and phosphates in the soil, rock and sediments can be freed and reintroduced at the roots of plants. This process is particularly important for the recycling of phosphate as there is a large reserve of phosphorus in rocks and virtually none in the atmosphere. Thus, the process of converting available (inorganic) phosphorus in rock to available phosphate for plants is primarily a physical and chemical process (erosion).

Therefore, on the basis of the above discussions, the recycling process of the ecosystem is all- encompassing and demands the interaction of every facet of the ecosystem. Strictly speaking, then, the decomposition and recirculation of materials in the ecosystem is facilitated by these biogeochemical cycles (Miller 1991; Pearce 1978).