Cornell Science Inquiry Partnerships http://csip.cornell.edu Classroom Aquaponics: Exploring Nitrogen Cycling in a Closed System Teacher’s Guide by Sean Mullen, Cornell University Summary: Nutrient cycling is a fundamental concept in ecology and environmental science. However, many students have trouble visualizing how individual nutrients (e.g., nitrogen) cycle through complex natural ecosystems. To address this issue, this unit outlines an inquiry-based approach to investigating nutrient cycling in a simplified desktop ecosystem involving aquaria and hydroponically grown plants. Subject: Ecology and Environmental Science Audience: 9-12 ecology and general biology classes. However, this unit can be easily modified for younger students. Timer Required: Initial setup takes approximately one 3 hour-lab period. Data collection takes approximately 30 minutes at regular intervals for 3-4 weeks. Background The growth of all organisms depends on the availability of mineral nutrients in the environment. Nitrogen is a particularly important component of the biosphere because it is required in large amounts as an essential component of proteins, nucleic acids and other cellular constituents. Nitrogen is abundant in the earth's atmosphere in the form of N 2 gas. However, in this form nitrogen is unavailable for use by most organisms because there is a triple bond between the two nitrogen atoms that makes the molecule almost inert. Nitrogen can be used for growth if it is first "fixed" (combined) in the form of ammonium (NH 4 ) or nitrate (NO 3 ) ions. Weathering of rocks releases small amounts of these ions but the process occurs so slowly that it has a negligible effect on the availability of fixed nitrogen. Thus, most available nitrogen is derived from other "fixation" pathways. Nitrogen Fixation Nitrogen fixation occurs in two major ways. The first is through high-energy fixation. Large amounts of energy produced by phenomena such as cosmic radiation, meteorite trails, and lightening strikes can drive the combination of nitrogen with the hydrogen and oxygen found in water. However, it is estimated that less than 8.9 kg N/ha is generated annually is this manner. The second, and largest, source of "fixed" nitrogen is through biological conversion. Atmospheric nitrogen can be fixed into biologically available forms, such as ammonia and nitrate, by 1) symbiotic bacteria commonly associated with legumes and with other plants having root nodules, 2) free-living aerobic bacteria, and 3) blue-green algae. Through biochemical pathways, these bacteria break the triple-bonded N 2 molecule into two nitrogen atoms, which then combine with hydrogen to form ammonia (NH 3 ). In this way, biological "fixation" generates approximately 90% of the