https://www.khanacademy.org/science/biology/ecology/biogeochemical-cycles/a/the-water-cycle 2025-08-08T21:39:42.49220884Z The water cycle Learn how water moves through Earth's ecosystems. article https://www.khanacademy.org/science/hs-bio/x230b3ff252126bb6:energy-and-matter-in-biological-systems/x230b3ff252126bb6:biogeochemical-cycles/a/carbon-cycle 2024-05-09T17:42:08.599818576Z The carbon cycle Learn about the carbon cycle and how carbon moves between the biosphere and the geosphere. article https://www.khanacademy.org/science/hs-bio/x230b3ff252126bb6:energy-and-matter-in-biological-systems/x230b3ff252126bb6:biogeochemical-cycles/e/apply-the-carbon-cycle 2025-06-13T16:17:46.628796572Z Apply: the carbon cycle Apply your understanding about the carbon cycle in this set of free, standards-aligned practice questions. Wieteke Holthuijzen exercise https://www.khanacademy.org/science/biology/ecology/biogeochemical-cycles/a/the-nitrogen-cycle 2024-05-29T17:16:29.06533157Z The nitrogen cycle The key role of microbes in nitrogen fixation. How overuse of nitrogen-containing fertilizers can cause algal blooms. article https://www.khanacademy.org/science/hs-bio/x230b3ff252126bb6:energy-and-matter-in-biological-systems/x230b3ff252126bb6:biogeochemical-cycles/e/apply-the-nitrogen-cycle 2024-05-28T21:41:02.467178644Z Apply: the nitrogen cycle Apply your understanding about the nitrogen cycle in this set of free, standards-aligned practice questions. Wieteke Holthuijzen exercise https://www.khanacademy.org/science/bmc-grade-9-science/x345505a97fc4a662:energy-flow-in-an-ecosystem/x345505a97fc4a662:bio-geo-chemical-cycle/v/oxygen-cycle 2026-02-28T16:20:46.436623441Z Oxygen Cycle Learn about the Oxygen Cycle and understand how living (biotic) and non-living (abiotic) processes create and consume oxygen. A clear middle school explanation with examples and summary. Khan Academy video https://cdn.kastatic.org/googleusercontent/ZCdwTudJg6e6n-P2gsaUborP4izvMsGo71pvEVlX9dNYWcLXcP7VHkWpn2grt4TUP1KoJLQP9NswyHBuBLSFTBw Oxygen Cycle Learn about the Oxygen Cycle and understand how living (biotic) and non-living (abiotic) processes create and consume oxygen. A clear middle school explanation with examples and summary. https://cdn.kastatic.org/ka-youtube-converted/ilVmoLQFsMA.mp4/ilVmoLQFsMA.mp4 311 Cycles in Nature