Biogeochemical cyclesWe have adopted a model of the Earth System as a set of interacting “spheres”, the atmosphere, hydrosphere, biosphere, and lithosphere. Being open systems, energy and mass is constantly cycled between them. The transport and transformation of substances through the Earth system are known collectively as biogeochemical cycles. These include the hydrologic (water), nitrogen, carbon, and oxygen cycles. The hydrologic cycle is covered in “The Hydrosphere” chapter. Nitrogen cycleNitrogen comprises 78.08 % of the atmosphere making it the largest constituent of the gaseous envelope that surrounds the Earth. Nitrogen is important in the make up of organic molecules like proteins. Unfortunately, nitrogen is inaccessible to most living organisms. Nitrogen must be “fixed” by soil bacteria living in association with the roots of particular plant like legumes, clover, alfalfa, soybeans, peas, peanuts, and beans. Living on nodules around the roots of legumes, the bacteria chemically combine nitrogen in the air to form nitrates (NO3) and ammonia (NH3) making it available to plants. Organisms that feed on the plants ingest the nitrogen and release it in organic wastes. Denitrifying bacteria frees the nitrogen from the wastes returning it to the atmosphere.
Figure ES.11 The Nitrogen Cycle Oxygen CycleOxygen is the second most abundant gas in Earth’s atmosphere and an essential element of most organic molecules. Though oxygen is passed between the the lithosphere, biosphere and atmosphere in a variety of ways, photosynthesizing vegetation is largely responsible for oxygen found in the atmosphere. The cycling of oxygen through the Earth system is also accomplished by weathering of carbonate rock. Some atmospheric oxygen is bound to water molecules from plant transpiration and evaporation. Oxygen is also bound to carbon dioxide and released into the atmosphere during animal respiration.
Figure ES.12 The Oxygen Cycle Carbon CycleCarbon is the fourth most abundant element in the Universe and is the building block for all living things. The conversion of carbon dioxide into living matter and then back is the main pathway of the carbon cycle. Plants draw about one quarter of the carbon dioxide out of the atmosphere and photosynthesize it into carbohydrates. Some of the carbohydrate is consumed by plant respiration and the rest is used to build plant tissue and growth. Animals consume the carbohydrates and return carbon dioxide to the atmosphere during respiration. Carbohydrates are oxidized and returned to the atmosphere by soil microorganisms decomposing dead animal and plant remains (soil respiration). Another quarter of atmospheric carbon dioxide is absorbed by the world’s oceans through direct air-water exchange. Surface water near the poles is cool and more soluble for carbon dioxide. The cool water sinks and couples to the ocean's thermohaline circulation which transports dense surface water toward the ocean's interior. Marine organisms form tissue containing reduced carbon, and some also form carbonate shells from carbon extracted from the air.
Figure ES.13 The Carbon Cycle
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