The Nitrogen Cycle
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The Nitrogen Cycle APES So… how many planets will your life take?! The Nitrogen Cycle One of the most important nutrient cycles found in terrestrial ecosystems Movement of various forms of nitrogen through the hydrosphere, lithosphere and atmosphere The Nitrogen Cycle Composition of Atmosphere Is primarily composed of nitrogen (N2, 78%) The main abiotic source of nitrogen in the cycle The atmosphere is considered a major reservoir of nitrogen Atmospheric Nitrogen Nitrogen, mostly in the form of ammonium and nitrate, reaches the Earth's surface as a result of atmospheric lightning, precipitation and industrial pollution. In the Biosphere Animals consume nitrogen from plants Plants consume nitrogen from the soil Soil gets nitrogen from water or rain that contains nitrogen. In the Biosphere Plants must secure their nitrogen in "fixed" form (incorporated in compounds such as: Nitrate ions (NO3-) Ammonia (NH3) Urea (NH2)2CO Animals secure their nitrogen (and all other) compounds from plants (or animals that have fed on plants). Nitrogen Fixation Three processes are responsible for most of the nitrogen fixation in the biosphere are: atmospheric fixation by lightning biological fixation by certain microbes alone or in a symbiotic relationship with plants industrial fixation Atmospheric Fixation The enormous energy of lightning breaks nitrogen molecules and enables their atoms to combine with oxygen in the air forming nitrogen oxides. These dissolve in rain, forming nitrates, that are carried to the earth. Atmospheric nitrogen fixation probably contributes some 5-8% of the total nitrogen fixed. Biological Fixation The ability to fix nitrogen is found only in certain bacteria. Some live in a symbiotic relationship with plants of the legume family (e.g., soybeans, alfalfa). Ex-Rhizobium bacteria Some establish symbiotic relationships with plants other than legumes (e.g., alders). Legumes and Root Nodules Biological Fixation Some nitrogen-fixing bacteria live free in the soil. Nitrogen-fixing cyanobacteria are essential to maintaining the fertility of semi-aquatic environments like rice paddies. Industrial Fixation Under great pressure, at a temperature of 600°C, and with the use of a catalyst, atmospheric nitrogen and hydrogen (usually derived from natural gas or petroleum) can be combined to form ammonia (NH3). Ammonia can be used directly as fertilizer, but most of its is further processed to urea and ammonium nitrate (NH4NO3). Decay Proteins made by plants enter and pass through food webs just as carbohydrates do. At each trophic level, their metabolism produces organic nitrogen compounds that return to the environment, chiefly in excretions. Decay The final beneficiaries of these materials are microorganisms of decay. They break down the molecules in excretions and dead organisms into ammonia. Nitrification Ammonia can be taken up directly by plants usually through their roots. Most of the ammonia produced by decay is converted into nitrates. This is accomplished in two steps: Bacteria of the genus Nitrosomonas oxidize NH3 to nitrites(NO2-). Bacteria of the genus Nitrobacter oxidize the nitrites to nitrates (NO3-). Nitrification These two groups of autotrophic bacteria are called nitrifying bacteria. Through their activities (which supply them with all their energy needs), nitrogen is made available to the roots of plants. Assimilation Plant roots absorb inorganic ammonia, ammonium ions, and nitrate ions formed by nitrification and nitrogen fixation. Ions are used to make nitrogen containing organic molecules critical to life such as: DNA Proteins Amino Acids Dentrification Denitrification reduces nitrates to nitrogen gas, thus replenishing the atmosphere. Bacteria are the agents. They live deep in soil and in aquatic sediments where conditions are anaerobic. They use nitrates as an alternative to oxygen for their cellular respiration reaction. Human Influence German chemist of WWII, Fritz Haber developed a chemical process in which nitrogen and hydrogen gas combine to form gaseous ammonia. Coupled with irrigation, this input of nitrogen into the soil revolutionized agriculture by increasing crop yields Ways Humans Intervene #1: We emit a large amount of nitrogen into the atmosphere when we burn fuel Ways Humans Intervene #2: We emit heattrapping nitrous oxide gas into the atmosphere through anaerobic bacteria from livestock wastes and commercial inorganic fertilizers Ways Humans Intervene #3: We remove nitrogen from the earth's crust when we mine nitrogen-containing materials for fertilizers Deplete nitrogen from soil by harvesting nitrogen-rich crops Leach water-soluble nitrate ions from soil by irrigation Ways Humans Intervene #4: Remove nitrogen from soil when we burn grasslands and clear forests before planting crops Ways Humans Intervene #5: Add excess nitrogen compounds to aquatic systems in agricultural runoff, sewage, and deposition of nitrogen compounds from the atmosphere Stimulates excess growth of algae and other aquatic plants Breakdown of dead algae by aerobic decomposers deplete water of dissolved oxygen and disrupt aquatic systems and reduce aquatic biodiversity Ways Humans Intervene #6: Add excess nitrogen compounds to terrestrial ecosystems through atmospheric deposition… Atmospheric Deposition The movement of reactive nitrogen compounds, such as nitric acid, nitrogen dioxide, from the atmosphere onto plant leaves and other surfaces Ways Humans Intervene #6, continued: nitrogen becomes available for plant and microbial growth, and can lead to weeds which can better use nitrogen for growth, outgrowing/eliminatin g other plants that can’t use nitrogen as well References http://liftoff.msfc.nasa.gov/academy/space/atmosphere.html http://www.geog.ouc.bc.ca/physgeog/contents/images/lithosphere.gif http://www.oilandgas.org.uk/issues/images/z0002409.gif http://www.webref.org/geology/1/lithosphere.htm http://www.bartleby.com/65/li/lithosph.html http://www.elmhurst.edu/~chm/onlcourse/chm110/outlines/nitrogencycle.html http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/N/NitrogenCycle.html http://web.geology.ufl.edu/Biosphere.html http://www.cas.muohio.edu/~mbi-ws/biogeochemicalcycles/Nitrogen/nitrogen.htm#Ass http://www.marietta.edu/~biol/102/ecosystem.html#TheNitrogenCycle12 Living in the Environment/Eleventh Edition/G Tyler Miller, Jr.
