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In Chapter 25, we introduced the carbon cycle as a framework for understanding organisms, ecology, and diversity. The cycles of carbon and other biologically important elements are called biogeochemical cycles because they link organisms and their environment in the chemical cycling of the raw materials needed for life. As noted in Chapter 25, these cycles provide the fundamental ecological circuitry of life, sustaining life in all its diversity over long timescales. The following discussions bring us full circle, showing how the metabolic and functional diversity discussed in earlier chapters of Part 2 link to cycle carbon and nitrogen, structuring biomes on land and in the sea. We also introduce the phosphorus cycle, as phosphorus, like nitrogen, can limit rates of primary production.