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Looking Back: Biogeochemical Cycles and Future Geographies Assess your understanding of concepts related to this chapter section by answering the questions below before proceeding with this chapter. Click the question to reveal the correct answer.
The hydrologic cycle refers to the movement of water through its various stores within the Earth system. The cycle starts with evaporation from the surface which later condenses into clouds in which precipitation forms. Water falling as precipitation may be intercepted by vegetation or fall directly onto the surface. Water intercepted by plants may ultimately fall to the ground and seep into it. Likewise, water falling directly on the surface may seep into the subsurface or runoff to nearby streams. Water seeping into the ground may become soil water or groundwater. Water in the soil may be taken up by plants then transpired to the air. Groundwater may seep into streams or return to the ocean along along a coast. Water found in streams may also empty into the ocean.
Nitrogen in the atmosphere 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.
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.
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.
A tipping level (point) is a level at which "no additional forcing is required for large climate change and impacts."
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