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	Carbon is the basic building of the carbohydrates, fats, proteins, nucleic acids such as DNA and RNA, and other organic compounds necessary for life. The carbon cycle is based on carbon dioxide gas, wich make up almost 0.036% by volume of the troposphere and is also dissolved in water. 
	Producers absorb CO2 from  the atmosphere or water and use photosymthesis to convert it into complex carbohydrates. Then the cells in oxygen-consuming producers and consumers carry out aerobic respiration, which breaks down glucose and athers complex organic compounds and convert the carbon back to CO2 in the atmosphere or in water for reuse by prodecers. This linkage between photosynthesis in producers and aerobic respiration in producers and consumers circulates carbon in the ecosphere and is amajor part of the gloval carbon cycle.
	Carbon dioxide is the key component of nature’s thermostat. If the carbon cycle removes too much CO2 from the atmosphere, Earth will cool; if the cycle generates too much, Earth will get hotter. Some carbon lies deep in the earth In fossil fuels-caol, pretoleum, and natural gas-and  is released to the atmosphere as carbon dioxide only when these fuel extracted and burmed. CO2 also enters the atmsphere from aerobic respiration and from volcanic eruptions, which free carbon from rocks deep in the earth’s crust.
	The oceans also play a major role in regulating the level of carbon dioxide in the atmosphere. Some CO2 stay in the sea, some is removeed by photosynthesizing producers. As water warms, more dissolved CO2 returns to the atmosphere. In marine ecosystems some organisms take up dissolved CO2 molecules from ocean water and form slightly soluble carbonate compounds such as calcium carbonate (CaCO3 ) to build shells and the skeletons of marine organisms.  In fact, most of the earth’s carbon--10,000 times that in the total mass of all life on Earth--is stored in the ocean floor sediments and on the continents.  This carbon reenters the cycle very slowly when some of the sediments dissolve and form dissolved CO2  gas that can enter the atmosphere.  Geologic processes can also bring bottom sediments to the surface, exposing the carbonate rock to chemical attack by oxygen and conversion to CO2 gas.  
	We have disturbed the carbon cycle in two ways that add more carbon dioxide to the atmosphere than oceans and plants can remove:
	--  Forest and brush clearing, leaving less vegetation to absorb CO2.

	--  Burning fossil fuels and wood, which produces CO2 that flows into the

	Organisms need nitrogen to make proteins, DNA, RNA, and other nitrogen-containing orgain compounds.  The nitrogen gas (N2) that makes up 78% of the volume of the troposphere cannot be used directly as a nutrient by multicellular plants or animals.  Bacteria conver nitrogen gas into water-soluble compounds, which are taken up by plant roots as part of the nitrogen cycle.
	The conversion of atmospheric nitrogen gas into other chemical forms useful to plants is called nitrogen fixation.  Animals get their nitrogen by eating plants or plant-eating animals.  
	We intervene in the nitrogen cycle in the following ways:
	--  Emitting nitric oxide (NO) into the atmosphere when any fuel is burned.  This 
	     nitric oxide combines with oxygen to from nitrogen dioxide(NO2) gas , which can react with vapor to form nitric acid (HNO3). This acid is a component of acid deposition (commonly called acid rain)

-Emitting heat-trapping nitrous oxide (N2O) gas into the atmosphere by the action of bacteria on livestock waste and izers applied to the soil.

-Mining minerals deposits containing nitrate and ammonium ions for fertilizers.

-Depleting nitrate and ammonium ions from soil by harvesting nitrogen-rich crops.

-Adding excess nitrate and ammonium ions to aquatic ecosystems in agricultural runoff and discharge of municipal sewage.  This excess of plant nutrients stimulates rapid growth of algae and other aquatic plants.  The subsequent breakdown of dead algae be aerobic decomposers depletes the water of dissolved oxygen gas, killing great numbers of fish.

	Phosphorus, is an essential nutrient of both plants and animals.  It is a part of DNA molecules, which carry genetic information; other molecules, which store chemical energy for use by organisms in cellular respiration; certain fats in the membranes that encase plant and animal cells; and animal bones and teeth.
  	Phosphorus moves through water, Earth’s crust, and living organisms in the phosphorus cycle.
	Phosphorus released by the slow breakdown, or weathering, of phosphate rock deposits is dissolved in soil water and taken up by plant roots.  Most soils contain little phosphorus.  Thus phosphorus is the limiting factor for plant growth in many soils and aquatic ecosystems.
	Animals get their phosphorus by eating producers or animals that have eaten producers.  Animal wastes and the decay products of dead animals and producers return much of this phosphorus to the soil, to streams, and eventually to the ocean bottom as deposits of phosphate rock.
	Some phosphate returns to the land as guano--the phosphate-rich manure of fish-eating birds such as pelicans and cormorants.  We intervene in the phosphorus cycle chiefly in two ways.
- Mining large quantities of phosphate rock to produce commercial inorganic fertilizers and detergents.
-Adding excess phosphate ions to aquatic ecosystems in runoff of animal wastes.  Much of this nutrient causes explosive growth of cyanobacteria, algae, and aquatic plants, disrupting life in aquatic ecosystems.

	Sulfur circulates through the ecosphere in the sulfur cycle.  Much of the earth’s sulfur is tied up underground.
	However, sulfur also enters the atmosphere from several natural sources.  Hydrogen sulfide (H2S, a colorless, highly poisonous gas with a “rotten-egg” smell, comes from active volcanoes and from the breakdown of organic matter in swamps, bogs, and tidal flats caused by decomposers that don’t use oxygen (anaerobic decomposers).  
	In the atmosphere sulfur dioxide reacts with oxygen to produce sulfur trioxide gas (S03), which in reacts with water vapor to sulfuric acid (H2SO4).  These droplets of sulfuric acid and paricles of sulfate salts fall to Earth as components of acid deposition, which can harm trees and aquatic life.
	About a third of all sulfur, including 99% of the sulfur dioxide, that reaches the atmosphere comes from human activities.  We intervene in the atmospheric phase of the sulfur cycle in two ways:
-Burning sulfur-containing coal and oil to produce electric power, which produces about two-thirds of the human inputs of sulfur dioxide
-Refining petroleum, smelling sulfur compounds of metallic minerals into free metals such as copper, lead, and zinc, and performing other industrial processes

	The main processes in this water recycling and purifying cycle are evaporation, transpiration, condensation, cipitation, infiltration, percolation, and runoff downslope back to the sea to begin the cycle again.
	The water cycle is powered by energy from the sun and by gravity.  About 84% of the moisture comes from the oceans, and the rest from land.  The amount of moisture in the atmosphere at any one time is the equivalent of only about 25 millimeters (1 inch) of rainfall spread over Earth’s entire surface.  However, because this moisture is renewed, the average annual rainfall over Earth as a whole is 1 meter (40inches), although actual rainfall varies widely in different places.
	The amount of water vapor air can hold depends on its temperature, with warm air capable of holding more water vapor than cold air.  Absolute humidity is air (usually expressed as grams of water per kilogram of air).  Relative humidity is a measure (expressed in percentages) of the amount of water vapor in a certain mass of air compared with the maximum amount it could hold at that temperature.  For example, a relative humidity of 60% at 27 degrees C (80 degrees F) means that each kilogram (or other mass unit) of air has 60% of the water vapor it could hold at that temperature.
	Winds and air masses transport this water vapor over various parts of the earth’s surface, often over long distances.  Falling temperatures cause the water vapor to condense into tiny droplets taht form clouds or fog.  For precipitation to occur, air must contain condensation nuclei--tiny particles on which droplets of water vapor can collect.  Volcanic ash, soil dust, smoke, sea salts, and particulate matter emitted by factories coal-burning power plants, and vechicles are sources of such particles.  The temperature at which condensation occurs for a given amount of water vapor is called the dew point.
	It takes millions of tiny water droplets adhering to condensation nucleus to produce a drop or rain or a snowflake that will fall to the earth’s surface.  About 77% of this precipitation falls back into the sea, and the rest over land.  
	Some of the fresh water returning to the earth’s surface as precipitation becomes locked in glaciers.  Much of it collects in puddles and ditches, and runs off into nearby lakes and into streams, which carry water back to the ocearns, completing the cycle.  
	Besides replenishing streams and lakes, surface water running off the land also causes soil erosion, which moves various nutrients through portions of other biogeochemical cycles.  Water is the medium that carries rock debris from the mountains into valleys and eventually down to the ocean floor.  It is the primary sculptor of Earth’s  landscape.  Also, acidic rainwater reacts chemically with metallic minerals on Earth’s surface, forming water-soluble metallic salts that are carried by rivers to the sea.  This is one of the reasons the ocean is so salty.  
	Much of the water returning to the land seeps into or infiltrates surface soil layers.  Some percolates downward into the ground, dissolving minerals from porous rocks on the way.  There this mineral-laden water is stored as groundwater in the pores and cracks of rocks.  This underground water, like surface water, flows downhill and seeps out into streams and lakes or comes out in springs.  Eventually this water evaporates or reaches the sea to begin the cycle again.  The average circulation rate of underground water in the hydrologic cycle is extremely slow (300-4,600 years) compared with that of water in lakes (13 years), in streams (13 days), and in the atmosphere (9 days).  The turnover time for water in the ocean is about 37,000 years.
	Withdrawing large quantities of fresh water from streams, lakes, and underground sources.  In heavily populated or heavily irrigated areas withdrawals have led to groundwater depletion or intrusion of ocean salt water into underground water supplies.
	Clearing vegetation from land for agriculture, mining, roads, construction, and other activities.  This reduces seepage that recharges groundwater supplies, increases the risk of flooding, and speeds surface runoff, producing more soil erosion and landslides.


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