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THE NITROGEN CYCLE The nitrogen cycle describes the conversion of nitrogen between different chemical forms. The majority of the earth’s atmosphere (about 78%) is composed of atmospheric nitrogen (N2), but it is not in a form that is usable to living things. Complex species interactions allow organisms to convert nitrogen to usable forms and exchange it between themselves. Nitrogen is essential for the formation of amino acids and nucleotides. It is essential for all living things. Described below are five main processes that make up the nitrogen cycle. Nitrogen Fixation: In order for organisms to use atmospheric nitrogen (N2), it must be “fixed” or converted into ammonia (NH3) and ammonium (NH4+). This can happen occasionally through a lightning strike, but the bulk of nitrogen fixation is done by free living or symbiotic bacteria. These bacteria have the nitrogenase enzyme that combines gaseous nitrogen (N2) with hydrogen to produce ammonia (NH3) and ammonium (NH4+). It is then further converted by the bacteria to make their own organic compounds, like proteins and DNA. Some nitrogen fixing bacteria, like Rhizobium, live in the root nodules of legumes where they produce ammonia (NH3) in exchange for sugars. Other nitrogen fixing bacteria live in the soil and produce ammonia (NH3) and ammonium (NH4+). Ammonification: In ammonification, bacteria or fungi convert the organic nitrogen from dead organisms back into ammonium (NH4+). Nitrification can also work on ammonium. It can either be cycled back into plants in a usable form or returned to the atmosphere through denitrification. Bacteria like Bacillus and Proteus are examples of ammonifying bacteria. Nitrification: Nitrification is the conversion of ammonia (NH3) to nitrate (NO3-). It is usually performed by soil living bacteria, such as Nitrobacter. This is important because plants can assimilate nitrate into their tissues, and they rely on bacteria to convert it from ammonia (NH3) and ammonium (NH4+) to a usable form. Assimilation: In assimilation, plants take nitrogen from the soil by absorption through their root hairs in the form of either nitrate (NO3-) or ammonium (NH4+). Most nitrogen obtained by terrestrial animals can be traced back to the eating of plants at some stage of the food chain. De-nitrification: Nitrogen in its nitrate form (NO3-) is converted back into atmospheric nitrogen gas (N2) by bacterial species such as Pseudomonas and Clostridium, usually in anaerobic (without oxygen) conditions. These bacteria use nitrate as an electron acceptor instead of oxygen during respiration. Use the terms listed below to complete the diagram of the nitrogen cycle on page 2. Note that nitrogen fixation is used twice. Terms List Processes: Assimilation by plants, Nitrogen fixation, Ammonification, Nitrification, Nitrogen fixation, De-nitrification Nitrogen-based substances: Nitrates (NO3-), Ammonium (NH4+), 1 Nitrogen (N2) in atmosphere Name _________________________________ THE NITROGEN CYCLE Period _____ Date ______________ Seat ____ Use the terms list at the bottom of page 1 to complete the nitrogen cycle diagram below. Atmospheric nitrogen (N2) De-nitrification Assimilation by plants Nitrogen Fixation Nitrate (NO3-) Ammonification Nitrification Nitrogen Fixation Ammonium (NH4+) 2 Name _________________________________ THE NITROGEN CYCLE Period _____ Date ______________ Seat ____ 1. Explain in detail how nitrogen is cycled through the biosphere with the help of nitrogen fixing bacteria and other organisms; and describe the molecular compounds that the nitrogen is in as it is cycled through the biosphere. Use your textbook page 71 and the picture above. Atmospheric nitrogen (N2) is changed into ammonia (NH3) by nitrogen-fixing bacteria (like Rhizobium) found in the soil and in root nodules of certain plants, like legumes (peanuts, alfalfa). This process is called nitrogen fixation. Some plants can take up ammonia directly and use it to make amino acids (for proteins) and nucleotides (for DNA, RNA, and ATP). Ammonia can also be made by soil bacteria (like Bacillus) that decompose organic matter (from dead plants and animals and animal manure and urine). This process is called ammonification. Other soil bacteria (like Nitrobacter) change the ammonia into nitrates (NO3-) that can also be assimilated by plants. Finally, soil bacteria like Clostridium change nitrates (NO3-) back into nitrogen gas (N2) that is then released back into the atmosphere. 2. How does the way that matter flows through an ecosystem differ from the way that energy flows through an ecosystem? Most energy that drives life comes from the sun, which is outside the biosphere. This energy is converted from one form to another as organisms utilize it. For example, electromagnetic energy (sunlight) is converted by producers into chemical energy (food). This chemical energy is ultimately converted into thermal energy (heat). In time, thermal energy is radiated back to into outer space. For the most part, matter (atoms) stays within the biosphere, being part of different molecules as time goes on. For example, chances are some nitrogen atoms you have breathed in at one time were part of a molecule (like urea) that passed through the urinary tract of a dinosaur. 3. A farmer plants a field of corn and fertilizes it with a low-nitrogen fertilizer. The corn receives plenty of sunlight and water, but does not grow well. What might be preventing the corn from growing? Use your textbook page 73. The corn may not be getting enough nitrogen or some other nutrient (phosphorus, potassium). As a solution, farmers will plant corn in a field that was planted the year before with a legume (alfalfa, peanuts, and soybeans). Legumes are able to replenish the soil with nitrogen that the growing corn will need. In addition, by alternating crops with different nutrient needs, farmers can avoid depleting any one necessary element present in the soil. 3