In contrast to the flow of energy in the ecosystem, materials can be and are recycled and reused. The patterns in which the elements are used, stored, made available again, and reused are called biogeochemical cycles. We have previously met biogeochemical cycles in Section 2.5 with reference to the effects of living things on the nonliving world, and in Chapter 13, since many of the important pathways in the cycles are catalyzed by microbes. Whereas Chapter 13 described the details of the microbial transformations involved, here we describe the overall cycles in quantitative terms as well as the abiotic parts of the cycles.
Each cycle consists of a network of compartments or reservoirs, which store forms of the element. The major compartments are the atmosphere, the biosphere, the hydrosphere, and the lithosphere. The hydrosphere is the liquid water on Earth, including rivers, lakes, groundwater, and the ocean. The lithosphere is the mineral part of the Earth, the crust with its soil, rocks and sediments, and mantle and core. The amount contained in each compartment is measured in units of mass or moles and is called the standing stock. The compartments are linked by flows or fluxes of the element, called cycling rates, which represent the rate at which the element moves from one compartment to the next. The units of the cycling rates are in mass per unit time or moles per unit time. The cycling rates may be the result of chemical or biochemical transformations, such as the fixation of carbon by photosynthesis, or simply the transport between phases such as the absorption of CO2 by the oceans.
The elements constitute the basic nutrient requirements of the ecosystem. Of interest is the major storage compartment from which living things obtain each nutrient most directly. In the case of carbon and nitrogen, the inorganic source for living things is the atmosphere. Most of the other nutrients, such as phosphorus, sulfur, and potassium, originate in the lithosphere. However, weathering of rocks does not provide nutrients at a sufficient rate to replace losses of phosphorus and sulfur, and nitrification does not replace fixed nitrogen fast enough. Thus, the ecosystem must recycle its nutrients. This is one function of the detritus-based soil food chain. The decomposers release the nutrients in the dead organic matter. The partially decomposed organic matter increase the adsorptive capacity of the soil, preventing the nutrients from being leached out and lost to the ecosystem. Thus, a healthy soil environment is a key component of an ecosystem.
However, the soil is being compromised by human activities. Erosion causes a loss of organic-rich topsoil. Agricultural practices do more to replace lost nutrients than to replace soil organic matter, which contributes to favorable physical soil properties as well as chemical ones. Increased water runoff reduces infiltration, thereby reducing weathering of bedrock and the consequent liberation of minerals. As a result, agricultural fertilizers now need to include trace minerals in some areas.
When you examine biogeochemical cycles, an important consideration is whether or not the cycle is at steady state. Steady state is defined as the condition in which none of the variables are changing with time. A cycle is at steady state if all of its compartments are at steady state, which can be determined by a simple mass balance around each compartment (units of each term are mass or moles per unit time):
accumulation = X inputs — X outputs ± reactions (14-3)
A compartment is at steady state if there is zero accumulation. The only reactions that create or destroy elements are nuclear ones, which we can ignore. (If we were doing mass balances on compounds, such as ammonia, we would have to consider reactions.) Thus, for a compartment to be at steady state, equation (14.3) reduces to
Natural systems tend to be close to steady state, called the "balance of nature.'' An exception is when there is a net import or export from the ecosystem. Other exceptions operate at long time scales, such as when a long-term storage compartment is being formed. Examples of this are the deposition of carbonate sediments on the ocean floor, the formation of peat deposits in marshes, and burial of organic matter being converted over geologic time into coal or oil deposits. A system could also be disturbed from a steady-state condition by human activities.
Another important consideration is the size of various compartments relative to the total fluxes in or out of them. This leads to the concept of turnover time, 0, for a steady-state compartment without reaction:
y standing stock in a compartment X input rates to that compartment
For example, all the plants on Earth store about 600 Tg organic carbon (1 Tg = 109 kg). Photosynthesis forms about 120 Tg/yr, which is approximately balanced by respiration. Thus, the turnover time for organic carbon in the biosphere is five years. Compartments with short turnover times, on the order of days or weeks, respond more rapidly to disturbances and therefore may be more susceptible to pollution. Compartments with long turnover times change more slowly. However, it may take longer to recognize that a change is occurring, and it will also be slow to recover if a significant impact does occur.
Some cycles, such as phosphorus, tend to be local in scale. Others, such as carbon and nitrogen, include the atmosphere and the ocean as major compartments. These link all local cycles into a global cycle. Figure 14.3 shows some of the major compartments and fluxes associated with global cycles. In this chapter we discuss mainly global cycles. In the next chapter we look at local cycles for particular ecosystems.
Geological processes Figure 14.3 Generalized global biogeochemical cycle.
All of the elemental cycles except for that of nitrogen are also linked to the sedimentary cycle, in which elements are cycled through Earth's crust (Figure 14.4). The cycling rates associated with this are small for most elements, but this only means that the turnover time is geologic in scale. Each of the cycles except that for nitrogen shows losses to
the ecosystem as minerals are washed to sea and eventually buried in sediments. The losses are made up for by gains from volcanic activity or weathering of rocks. This part of the cycle is closed by processes driven by geologic rock-forming activity associated with continental drift. According to the theory of continental drift, Earth's crust is divided into sections called tectonic plates, which move around relative to one another on Earth's surface and upon which the continents ride. The motion is driven by thermal convection in Earth's molten mantle. The plates are formed continually at one edge by volcanic activity. The other edge, known as the subduction zone, is slowly pushed down into the mantle, carrying sediments with it. For example, the North American Plate is formed by volcanic activity at the mid-Atlantic ridge. Along the west coast of North America it is pushing over the Pacific Plate, forming a subduction zone there.
In the sedimentary cycle, buried minerals may follow several pathways to reenter the surface cycles. In the longest path they are carried into, and become part of, Earth's mantle. Eventually, they may resurface at a plate formation site. Other substances, especially the volatile sulfur and carbon dioxide, are expelled to the atmosphere by volcanic activity near the subduction zone. A third route is the crustal pathway. Buried sediments eventually consolidate to form sedimentary rocks, such as shale and limestone. Heat and pressure from deep burial may transform these into metamorphic rocks, such as slate and marble. Tectonic motion can cause parts of the continental masses to fold and crumple, forming mountains. This may raise their rocks above the surface, exposing them to erosion and making them available to the biosphere.
Living things act to reduce the rate at which minerals liberated by weathering return to the sedimentary cycle. Plants take up minerals faster than weathering makes them available. When plants and other living things die, the detritus food chain helps recycle them within the ecosystem. The saprotrophs release the minerals in close association with plants, so they can be reused immediately before they are lost. In this way most of the minerals in some ecosystems, such as the tropical rain forest, are stored in living things. Little is held in the soil.
Carbon forms the backbone of biochemical compounds, and its fixation by primary producers coincides with the first biological step in the energy pyramid. Furthermore, the carbon cycle is at the center of one of the most important environmental impacts of human activities. Figure 14.5 illustrates some of the major parts of the global carbon cycle. The largest compartment shown is the ocean, which contains carbon mostly as dissolved carbonates. The ocean stores about 50 times as much CO2 as the atmosphere does and has a very large turnover time of about 350 years. Terrestrial plants have a turnover time of less than five years. However, soil organic matter turns over about every 25 years. The biomass and soil organic matter form the major reservoir of reduced carbon, other than fossil fuels.
The importance of the carbon cycle reactions was dramatized by the experience of the Biosphere II project in Oracle, Arizona (the Earth is Biosphere I). Biosphere II is a 1.27-ha (3.15-acre) closed structure originally designed to support a crew of eight for several years without any food or other material supplies from the outside. It contained simulated ocean, desert, and forest ecosystems, as well as agricultural areas, plus an interconnected atmosphere. Thus, it was a highly visible, if only semiscientific, examination of a closed ecosystem. However, once the crew of eight was sealed inside in September 1991 for an initial two-year mission, it soon became apparent that oxygen was disappearing
Deforestation, land use change and burning 0 -2
Deforestation, land use change and burning 0 -2
Figure 14.5 Global carbon cycle. Reservoirs units are 1015 g C, flux units are 1015 g C/yr. (From Odum, 1987.)
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