Presentation on theme: "The Nitrogen Cycle Ch 7 Part 3. Nitrogen Cycle Nitrogen cycle: the routes that nitrogen atoms take through the environment –Nitrogen gas (N 2 ) is the."— Presentation transcript:
Nitrogen Cycle Nitrogen cycle: the routes that nitrogen atoms take through the environment –Nitrogen gas (N 2 ) is the most abundant gas in the atmosphere (78%) Nitrogen gas is inert & cannot be used directly by most organisms. Nitrogen is cycled through animals, soil, & gases. Organisms use nitrogen (in various forms) to make vital organic compounds. –Nitrogen is an important component of biological molecules: DNA, RNA, & proteins. –Nitrogen is a plant nutrient & acts as a limiting factor for plant growth. Too much nitrogen in aquatic systems can cause eutrophication.
Several steps: –Nitrogen fixation nitrogen to ammonia –Nitrification ammonia to nitrates –Assimilation nitrates to proteins, DNA, RNA, etc. –Ammonification decomposition –Denitrification ammonia & nitrates to nitrogen Abbreviations to know: –nitrogen gas (N 2 ) –ammonia (NH 3 ) –ammonium ions (NH 4 + ) –nitrite ions (NO 2 - ) –nitrate ions (NO 3 - )
Nitrogen Fixation Even though N 2 is the most abundant gas in the atmosphere (78%) it must be fixed or converted into a usable form before it can be absorbed and used by multicellular organisms. –Ways of converting N 2 : 1. lightning –causes reaction with O 2 to form NO 3 - 2. Nitrogen fixation: N 2 is combined (fixed) with H 2 to become ammonia, NH 3 –whose water-soluble ions of ammonium, NH 4 +, can be taken up by plants –Done by nitrogen-fixing bacteria
Nitrogen Fixation Specialized bacteria convert N 2 gas to ammonia (NH 3 ) –N 2 + 3H 2 2NH 3 Once converted into ammonia, water-soluble ions in the form of ammonium (NH 4 + ) can be taken up by plants –Done mostly by: Cyanobacteria in soil & water Rhizobium bacteria in nodules on roots of wide variety of plants –(ex. legumes, soybeans, & alfalfa) Requires large amounts of energy
Nitrification Nitrification: bacteria convert ammonium ions, NH 4 + (the water soluble form of ammonia, NH 3, which is usually produced by decay) through a 2- step process into NO 2 - (nitrite) & NO 3 - (nitrate) –Done by 2 groups of specialized aerobic nitrifying bacteria (for use by plants) to: 1. Nitrite ions (NO 2 - ) –toxic to plants »Nitrosomonas »Nitrosococcus 2. Nitrate ions (NO 3 - ) –easily taken up by plants as a nutrient through roots »Nitrobacter
Assimilation Plant roots absorb inorganic ammonia (NH 3 ), ammonium ions (NH 4 + ), & nitrate ions (NO 3 - ). –Formed by nitrification (NH 3 NH 4 + NO 2 - NO 3 - ) & nitrogen fixation (N 2 + 3H 2 2NH 3 ). Ions are used to make N-containing organic molecules such as: –DNA & RNA –Amino Acids –Proteins Animals get their N by eating plants or plant- eating animals NH 3 Assimilation
Ammonification Bacteria in soil & water decompose N-rich organic compounds, wastes, cast-off particles & dead organisms into: –1. simpler N-containing inorganic compounds such as ammonia (NH 3 ) –2. water-soluble salts containing ammonium ions (NH 4 + )
Denitrification Dentrification: other specialized bacteria convert ammonia back into gaseous nitrogen through a multistep process: –Ammonia (NH 3 ) & ammonium ions (NH 4 + ) nitrite ions (NO 2 - ) & nitrate ions (NO 3 - ) nitrogen gas (N 2 ) & nitrous oxide gas (N 2 O) Done by denitrifying bacteria deep in soil & aquatic sediments –Anaerobic conditions… So… use nitrates as an alternative to oxygen for the final electron acceptor in their respiration. Replenish atmosphere & close the nitrogen cycle
Humans Affect the Nitrogen Cycle Haber-Bosch process: synthetic production of fertilizers by combining nitrogen & hydrogen to synthesize ammonia –Dramatically changed the nitrogen cycle since devised in 1950s –huge increases in crop yields (due to fertilization)…cheaply –Humans are fixing more nitrogen than nature does –Reduced biodiversity of plants adapted to low-nitrogen soils Calcium & potassium in soil washed out by fertilizers (depletion) Acidified water & soils Disposing of N-rich municipal sewage Raising cattle in feedlots adjacent to waterways (wastes in runoff) Changed estuaries & coastal ecosystems & fisheries –N in run-off causes bloom in algae & aquatic plants (eutrophication) When photosythesizers die in the water their decomposition by bacteria removes O 2 from the water (hypoxia) & causes death of other organisms. Burning fossil fuels & grasslands & deforestation removes N from the soil & produces NO x (a precursor for acid rain & ozone depletion) Decomposition of wastes & commercial nitrogen fertilizers produces N 2 O (greenhouse gas)
Effects of Human Activities on the Nitrogen Cycle Human activities such as production of fertilizers now fix more nitrogen than all natural sources combined.
Human Inputs of N Into the Environment Fully half of nitrogen entering the environment is of human origin