Background Information: Exploring the Nitrogen Cycle

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1 1 Background Information: Exploring the Nitrogen Cycle Alexandria G. Hounshell Doctoral Student Department of Marine Sciences University of North CarolinaChapel Hill Abstract: This minilesson plan is designed for students who have not had a formal lesson in the nitrogen cycle. If students have already been exposed to the nitrogen cycle, use the background as a quick refresher or skip to the Case Study section of the lesson plan. Learning Objectives: 1. Be able to identify forms of nitrogen and define as either biologically reactive or biologically unreactive. 2. Track nitrogen through an estuarine system and discuss the sources, sinks, and cycling processes within the system. Requirements: Appropriate grade level: High school, 912 th grade Approximate time: 1, 45 minute class period Group size: Any class size, with students arranged in pairs or groups of three. Setting: Indoors Resources: Students: Pen or pencil Educator: Powerpoint capabilities Documents: Worksheet 1B (key provided) Nitrogen Cycle_Figures Exploring the Nitrogen Cycle: 1. Introduce what Nitrogen is: a macronutrient needed for all life from plants to humans. Nitrogen is used in many important biological compounds including DNA, RNA, and proteins. There are many different forms of nitrogen in the environment all with different properties and reactivity s. 2. Go over the different biologically important forms of nitrogen: First, define reactive vs. unreactive and source, sink, and cycling: a) Reactive nitrogen compound that can be used for biomass growth b) Unreactive nitrogen compound that is not biologically available and cannot be used to support biomass growth c) Source a supply of nutrients that can be utilized for biomass growth (i.e., must be available in a reactive form) d) Sink transformation of a reactive form of a nutrient to an unreactive form, so that the nutrient is no longer available for biomass incorporation (unreactive form of nitrogen) e) Cycling when one reactive form is transformed into another reactive form

2 2 Forms of Nitrogen: a. Ammonium (NH 4 ) reactive form of nitrogen that is easily assimilated into biomass (i.e., DNA, RNA, amino acids) b. Nitrate (NO 3 ) reactive and most oxidized form of nitrogen. Can be taken up by many bacteria, internally converted to NH 3, and then incorporated into biomass. c. Nitrite (NO 2 ) intermediate in the process of nitrification. Does not exist in high concentrations in the environment, as it is readily oxidized to NO 3. d. Organic Nitrogen term for nitrogen that has been assimilated into biomass. e. Dinitrogen (N 2) unreactive form of nitrogen gas. Forms 78% of the atmosphere, but cannot be utilized by organisms. 3. Go over the processes of the nitrogen cycle (Worksheet 1B or Nitrogen Cycle_Figures, Slide 13). The numbers (15) correspond to the numbers on the figure. 1. N 2 Fixation: N 2 NH 4 Conversion of the unreactive form of nitrogen (N 2) to biologically (NH 4 ). The process is mediated by bacteria (in the estuarine environment, this includes many species of cyanobacteria) and is one of the only natural process where un can be converted into a reactive form. Because of the triple bond binding the two nitrogen atoms, a lot of energy is needed to fix N 2. If other reactive forms of nitrogen are available in sufficient concentrations, N 2 fixation will be inhibited. The process must be completed in the absence of oxygen (the enzyme, nitrogenase, responsible for N 2 fixation is inhibited by oxygen). Bacteria have learned to overcome this by forming oxygenfree environments within the cell or cell colony. 2. Nitrification: NH 4 NO 3 Conversion of NH 4 into NO 3, where one reactive form of nitrogen is transformed into another reactive form of nitrogen. This is a bacteriamediated process where the bacteria gain energy from the conversion of ammonium into nitrate. The process must be conducted in the presence of oxygen (i.e., in the welloxygenated water column or in the very upper 1mm of the sediments). 3. Denitrification: NO 3 N 2 Process where is converted to un, N 2 gas. The nitrogen is lost from the environment to the atmosphere. The process is mediated by denitrifying bacteria and must be completed anaerobically (i.e., without oxygen). This process typically occurs under the first 1mm of the sediments where oxygen has been depleted to about 0 mg/l. 4. Nitrogen Uptake: NH 4 Organic Nitrogen Nitrogen uptake is the process of converting NH 4 to biomass by plants, phytoplankton, and bacteria into amino acids, DNA, RNA, and proteins. Organisms higher up the trophic food chain consume these primary producers and are able to assimilate the nitrogen. Typically NH 4 is the only form of nitrogen that can be directly taken up for incorporation into biomass, however, a few organisms are able to take up NO 3, internally convert to NH 4, and incorporate into biomass. 5. Nitrogen remineralization: Organic Nitrogen NH 4

3 3 Conversion of detritial material from organic nitrogen to NH 4. The process is mediated by bacteria and converts organic nitrogen back to ammonium which can then be cycled back through the environment and used by phytoplankton, bacteria, and plants for growth. Helpful References (also see The Nitrogen Cycle: Connections to the Neuse River Estuary): Department of Environment and Heritage Protection, Queensland. (2013). Coastal and subcoastal nonfloodplain tree swamp Melaleuca spp. and Eucalyptus spp. Nutrient dynamics, WetlandInfo,. Retrieved November 2, 2014, from

4 4 Worksheet 1B: Exploring the Nitrogen Cycle: Name: Nitrogen Export Nitrogen Loading Nitrogen Burial Image based on: Part I: Fill out Part I with the processes (name and reactions) of the nitrogen cycle discussed. The numbers on the figure (15) correspond with the numbers listed below the figure.

5 5 Part II: Determine if each of the numbered cycles is a source, sink, or cycling of forms. Part I: Reactions of the Nitrogen Cycle Part II: Source, Sink, or Cycling?

6 6 Worksheet 1: Exploring the Nitrogen Cycle: Name: _KEY Nitrogen Export Nitrogen Loading Nitrogen Burial Image based on: Part I: Fill out Part I with the processes (name and reactions) of the nitrogen cycle discussed. The numbers on the figure (15) correspond with the numbers listed below the figure.

7 7 Part II: Determine if each of the numbered cycles is a source, sink, or cycling of forms. Part I: Reactions of the Nitrogen Cycle Part II: Source, Sink, or Cycling? 1. N 2 Fixation: N 2 NH 4 or dinitrogen ammonium Source, un converted to 2. Nitrification: NH 4 NO 3 or ammonium nitrate Cycling, cycled to 3. Denitrification: NO 3 N 2 or nitrate dinitrogen Sink, converted to un 4. Nitrogen Uptake: NH 4 organic nitrogen Cycling, converted to 5. Nitrogen remineralization: Organic nitrogen NH 4 Cycling, converted to