Unit 7: Resources and the Environment Chapter 25: Earth Resources     Earth Resources In addition to living things and sunlight, many other Earth resources are considered renewable. Water, for example, is a renewable resource because it continuously travels through Earth's systems in a process known as the water cycle. Other renewable resources include carbon, nitrogen, and phosphorus, all of which, like water, endlessly cycle through Earth's systems. These resources have one other thing in common-all are crucial to life on Earth. Carbon Carbon is an element that is cycled through the atmosphere, biosphere, hydrosphere, and lithosphere. Carbon dioxide in the atmosphere is taken in by plants and other organisms and used to capture the Sun's energy in the process of photosynthesis. Animals release carbon dioxide as a waste product in the process of respiration. Carbon dioxide from the atmosphere also enters the hydrosphere directly by dissolving in Earth's surface waters. Some marine animals use the carbon dioxide and calcium in ocean water to produce calcium carbonate for shell and bone formation. When these organisms die, their remains settle to the bottom of the ocean and, over time, result in the formation of limestone. Limestone also may form from the direct precipitation of calcium carbonate from water. As part of the rock cycle, limestone at Earth's surface weathers and erodes, releasing carbon into the hydrosphere. There is 50 percent more carbon in the hydrosphere than in the atmosphere. When organisms die, they are decomposed by bacterial activity, which releases some carbon into the atmosphere. When the decaying organisms are buried, they can develop into fossil fuels such as peat, coal, and petroleum. The burning of these fossil fuels releases carbon dioxide into the atmosphere again, and the carbon cycle continues. LINK UP: Find out more information about the carbon cycle. Nitrogen The element nitrogen makes up approximately 78 percent of the atmosphere. Nitrogen is so important to plant growth that it is usually a component of inorganic fertilizers. However, the nitrogen found in the atmosphere, N2, cannot be used directly by plants. Plants take in nitrogen in the form of nitrate, NO3. How do plants obtain nitrogen in this form? Some nitrates form in the atmosphere when lightning strikes, but most of the nitrates used by plants are converted by nitrogen-fixing bacteria that live in the roots of plants such as peas, clover, and alfalfa. In surface waters, algae also convert nitrogen to nitrates. Plants use nitrates to produce proteins, which animals take in when they consume plants. When living things die, bacteria, fungi, and other decomposers break down their remains into simpler substances. Bacteria convert the nitrites, NO2-, in organic matter into nitrates, NO3-, and some nitrogen in the form of ammonia gas, NH3, is released during the process. The nitrates released during decomposition can be used directly by plants, while the nitrogen in the ammonia gas reenters the atmosphere. Phosphorus Like water, carbon, and nitrogen, the element phosphorus is essential to all living things. Phosphorus is found in DNA, in the bones of vertebrates, and in the droppings of fish-eating seabirds, called guano. The phosphorus cycle begins when sedimentary rocks weather. The processes of weathering and erosion remove phosphorus from rocks, making the element available for plants to absorb from soil. Wind also can carry phosphorus for long distances. Animals obtain phosphorus when they eat plants. Phosphorus returns to soil when organisms die and decompose. As water moves through soil, it dissolves phosphorus and carries it to surface waters. Plankton use the phosphorus for their life processes, and eventually it moves along the food chain to fish, birds, and humans. Phosphorus in dead organisms and waste products returns to the ocean bottom as deposits of phosphate rock. When geologic processes push up the seafloor and expose these rocks, the cycle of weathering and erosion begins again. Activity Based on the information in this article, draw an illustration of the carbon cycle. Show how the element cycles through Earth's systems. Be sure to label each part of your diagram.