Chapter 21: Nutrient Cycling PDF

# All Nutrients Follow Biogeochemical Cycles The living world depends on a flow of energy and the cycling of nutrients through the ecosystem. Energy and nutrients are tightly linked in organic matter. One cannot be separated from the other. Their linkage begins in photosynthesis, in which plants...

# All Nutrients Follow Biogeochemical Cycles The living world depends on a flow of energy and the cycling of nutrients through the ecosystem. Energy and nutrients are tightly linked in organic matter. One cannot be separated from the other. Their linkage begins in photosynthesis, in which plants use solar energy to fix CO₂ into organic carbon compounds. Carbon, together with a variety of essential nutrients, makes up organic matter, the tissues of plants and animals. Because of this linkage between energy and nutrients, the general model of energy flow through an ecosystem presented in Chapter 20 provides a basic framework for examining the flow of matter through ecosystems. All nutrients flow from the nonliving to the living and back to the nonliving components of the ecosystem in a more or less cyclic path known as a biogeochemical cycle. The important players in all nutrient cycles are: - green plants, which organize the nutrients into biologically useful compounds - decomposers, which return them to their simple elemental state - air and water, which transport nutrients between the abiotic and living components of the ecosystem. Without these components, no cyclic flow of nutrients would exist. There are two basic types of biogeochemical cycles: 1. **Gaseous cycles**: the main reservoirs of nutrients are the atmosphere and the oceans. Gaseous cycles are pronouncedly global. The gases most important for life are nitrogen, oxygen, and carbon dioxide. 2. **Sedimentary cycles**: the main reservoir is the soil, rocks, and minerals. The mineral elements that living organisms require come initially from inorganic sources. They occur as salts dissolved in soil water or in lakes, streams, and seas. The mineral cycle varies from one element to another, but essentially it consists of two phases: 1. **The rock phase** 2. **The salt solution phase** Mineral salts come directly from Earth's crust through weathering. The soluble salts then enter the water cycle. With water, they move through the soil to streams and lakes and eventually reach the seas, where they remain indefinitely. Other salts return to Earth’s crust through sedimentation. They become incorporated into salt beds, silts, and limestone. After weathering, they enter the cycle again. There are many different kinds of sedimentary cycles: 1. **The sulfur cycle**: a hybrid between the gaseous and the sedimentary because they have reservoirs not only in Earth’s crust but also in the atmosphere. 2. **The phosphorus cycle**: wholly sedimentary; the element is released from rock and deposited in both the shallow and deep sediments of the sea. Both gaseous and sedimentary cycles involve biological and nonbiological agents. Both are driven by the flow of energy through the ecosystem, and both are tied to the water cycle. Water is the medium by which elements and other materials move through the ecosystem. Without the cycling of water, biogeochemical cycles would cease. Although the biogeochemical cycles of the various essential nutrients required by autotrophs and heterotrophs differ in detail, from the perspective of the ecosystem, all biogeochemical cycles have a common structure, sharing three basic components: - inputs - internal cycling - outputs. # Nutrients Enter the Ecosystem via Inputs The input of nutrients to the ecosystem depends on the type of biogeochemical cycle. For example: - **Nutrients with a gaseous cycle** (e.g., carbon and nitrogen) enter the ecosystem via the atmosphere - **Nutrients with sedimentary cycles** (e.g., calcium and phosphorus) have inputs dependent on the weathering of rocks and minerals. The process of soil formation and the resulting soil characteristics have a major influence on processes involved in nutrient release and retention. Many soil materials are deficient in nutrients on which plants depend, affecting both plants and herbivores. Supplementing nutrients in the soil are nutrients carried by rain, snow, air currents, and animals. Precipitation brings appreciable quantities of nutrients, called **wetfall**. Some of these nutrients (e.g., tiny dust particles of calcium and sea salt) form the nuclei of raindrops; others wash out of the atmosphere as the rain falls. Some nutrients are brought in by airborne particles and aerosols, collectively called **dryfall**. # The Gaia Hypothesis Earth's atmosphere is extremely different from that predicted for a nonliving Earth and from that of other planets in the solar system. Those atmospheres are dominated by carbon dioxide and possess only a trace of oxygen. There are two views of the formation of Earth's atmosphere. One is that physical forces interacted to form life-sustaining conditions, and life then evolved to adapt to those conditions. The other is that organisms evolved in partnership with the physical environment. From the beginning, these organisms helped to control geochemical cycles. For example, photosynthetic algae in the early oceans first released O₂ into the atmosphere. Photosynthetic organisms in the oceans still provide 70 percent of our atmosphere's oxygen. The constancy of Earth's atmosphere over 3.6 billion years, with its high O₂ and low CO₂ content and moderate temperatures, suggests some feedback system. It prompted James Lovelock - physical scientist, engineer, and inventor of instruments to measure the Martian environment - and microbiologist Lynn Margulis to postulate the Gaia hypothesis (Gaia is the Greek word for "Earth goddess") of global biogeochemical homeostasis. The theory says that Earth's biosphere, atmosphere, oceans, and soil together make up a feedback system that maintains an optimal physical and chemical environment for life on Earth. This feedback system could not have developed nor could be maintained without the critical buffering activity of early life forms and continued coordinated activity of plants and other photosynthetic organisms. Together they damp the fluctuations of the physical environment that would occur in the absence of a well-organized living system. No control mechanisms have been discovered, but microorganisms are the only life forms that could function like a chemostat, making Earth one large cybernetic system. For example, maintenance of 21 percent O₂ in the atmosphere - which maximizes aerobic metabolism just below the level that would make Earth’s vegetation flammable -is possibly the outcome of microbial activity. Microbial production of CH₄ from the small amount of carbonaceous living matter buried each year might keep oxygen in check. Not all ecologists and atmospheric scientists accept the Gaia hypothesis, but it does help us understand the behavior of ecosystems and the interactions of biogeochemical cycling. Evidence seems to indicate that organisms do play a dynamic role in determining the composition of many chemicals in the soil, water, and atmosphere. We need to look no farther than the tremendous impact we humans have had on the physical aspects of Earth. # Nutrients Are Recycled Within The Ecosystem Primary productivity in ecosystems depends on the uptake of essential mineral (inorganic) nutrients by plants and their incorporation into living tissues. Nutrients in organic form, stored in living tissues, represent a significant proportion of the total nutrient pool in most ecosystems. As these living tissues senesce, the nutrients are returned to the soil or sediments in the form of dead organic matter. Various microbial decomposers transform the organic nutrients into a mineral form, a process called mineralization, and the nutrients are once again available to the plants for uptake and incorporation into new tissues. This process is called internal cycling and is an essential feature of all ecosystems. It represents a recycling of nutrients within the ecosystem. Only a small fraction of the nutrient pool is involved in short-term annual cycling of nutrients in the forest ecosystem. Nutrients taken up by trees are returned to the forest floor by litterfall, throughfall, and stemflow. A significant portion of the nutrient uptake is stored in tree limbs, trunk, bark, and roots as accumulated biomass. This portion is effectively removed from short-term cycling. Some of the nutrients accumulate in the litter and in the living biomass of consumer organisms, including decomposers of the forest floor, from which they are recycled at various rates. Nutrients accumulated in soil organic matter have a key role in recycling because they prevent rapid losses from the ecosystem. Large quantities of nutrients are bound tightly in this organic matter structure; they are not readily available until released by activities of decomposers. Open-water ecosystems, such as lakes and ponds, lack the long-term biological retention of nutrients typical of forested systems. Nutrient availability depends heavily on the turnover of nutrients in phytoplankton and zooplankton. Major long-term storage takes place in deep bottom sediments, where nutrients may be unavailable for a long time. Retention of nutrients in flowing-water ecosystems is difficult, but it is aided by logs and rocks that hold detritus in place, by algal uptake of nutrients, and by aquatic invertebrates. # Key Ecosystem Processes Influence the Rate of Nutrient Cycling You can see from Figure 21.1 that the internal cycling of nutrients through the ecosystem depends on the processes of primary production and decomposition. Primary productivity determines the rate of nutrient transfer from inorganic to organic form (nutrient uptake), and decomposition determines the rate of transformation of organic nutrients into inorganic form (nutrient release). Therefore, the rates at which these two processes occur directly influences the rates at which nutrients cycle through the ecosystem. But how do these two key processes interact to limit the rate of the internal cycling of nutrients through the ecosystem? The answer lies in their interdependence. For example, consider the cycling of nitrogen, an essential nutrient for plant growth. The direct link between soil nitrogen availability, rate of nitrogen uptake by plant roots, and the resulting leaf nitrogen concentrations was presented in Chapter 6. The maximum rate of photosynthesis is strongly correlated with nitrogen concentrations in the leaves because certain compounds directly involved in photosynthesis (e.g., rubisco and chlorophyll) contain a large portion of leaf nitrogen. Thus, availability of nitrogen in the soil will directly affect rates of ecosystem primary productivity via its influence on photosynthesis and carbon uptake. A low availability of soil nitrogen reduces not only net primary production (the total production of plant tissues), but also the nitrogen concentration of the plant tissues that are produced. Thus, the reduced availability of soil nitrogen influences the input of dead organic matter to the decomposer food chain by reducing both the total quantity of dead organic matter produced and its nutrient concentration. The net effect is a lower input of nitrogen in the form of dead organic matter. Both the quantity and quality of organic matter as a food source for decomposers directly relate to the rate of decomposition and nitrogen mineralization (nutrient release). Lower nutrient concentrations in the dead organic matter promote immobilization of nutrients from the soil and water to meet the nutrient demands of the decomposer populations. This immobilization effectively reduces nutrient availability to the plants, adversely affecting primary productivity. You can now appreciate the feedback system that exists in the internal cycling of nutrients within an ecosystem. Reduced nutrient availability can have the combined effect of reducing both the nutrient concentration of plant tissues (primarily leaf tissues) and net primary productivity. This reduction lowers the total amount of nutrients returned to the soil in dead organic matter. The reduced quantity and quality (nutrient concentration) of organic matter entering the decomposer food chain increases immobilization and reduces the availability of nutrients for uptake by plants. In effect, low nutrient availability begets low nutrient availability. Conversely, high nutrient availability encourages high plant tissue concentrations and high net primary productivity. In turn, the associated high quantity and quality of dead organic matter encourages high rates of net mineralization and nutrient supply in the soil. # Both Climate and Plant Characteristics Influence the Rate of Nutrient Cycling In addition to its role in the weathering of rocks and minerals and soil formation, climate directly affects the rate of nutrient cycling in ecosystems by influencing rates of primary production and decomposition. Both increase as conditions become warmer and wetter. The net effect is a faster rate of nutrient cycling in warm, wet environments, such as a tropical rain forest, than in cooler (temperate or boreal forest) or drier (grassland) ecosystems. Nutrient cycling in an ecosystem is also influenced by the nature of its organisms. Organisms such as phytoplankton and zooplankton in aquatic systems are short-lived, grow rapidly, absorb nutrients at a high rate (nutrient uptake), and release nutrients quickly during decomposition (nutrient release). In contrast, terrestrial plants and animals, especially those in forests, have a longer life cycle. Therefore, they cycle nutrients more slowly but efficiently. # Focus on Ecology: Hot Ecology Ecologists have developed a good understanding of the processes controlling the cycling of nutrients within ecosystems. Although time consuming, quantifying the amount of nutrients in various components of the ecosystem at any one time can be accomplished by sampling the soil, plants, and other organisms and determining their nutrient concentrations. Quantifying the rates of exchange between the various components of the ecosystem is infinitely more difficult. During the 1950s and 1960s, an interdisciplinary field of radiation ecology developed. It was the product of the “nuclear age” that emerged with the development of nuclear weapons in the Manhattan Project and their use in World War II. Radiation ecology was concerned not only with studying the effects of ionizing radiation, but also with the development and use of radioactive compounds that could be used as tracers to examine the movement of nutrients through ecosystems. In the early 1960s, J. P. Witherspoon of Oak Ridge National Laboratory conducted a pioneering study using radioisotopes of elements to quantify the cycling of nutrients through an ecosystem. The object was to follow the pathway of a radiolabeled trace element (micronutrient) through a forest ecosystem. Cesium behaves like potassium. It is highly mobile, cycles rapidly in an ionic form, and is easily leached from plant surfaces by rainfall. Moreover, because a known quantity of the element could be traced, the amounts of the element apportioned to wood, twigs, and leaves could be determined. # Figure 21.1: A generalized model of nutrient cycling in a terrestrial ecosystem. The main components of inputs, internal cycling, and outputs are shown in bold. The key ecosystem processes of net primary productivity and decomposition are italicized. # Figure 21.2: Feedback that occurs between nutrient availability, net primary productivity, and nutrient release in decomposition for initial conditions of low and high nutrient availability (After Chapin 1980).