Learning Objectives

25.1.1 Summarize the major pools and fluxes associated with global-scale cycles of carbon, nitrogen, phosphorus, and sulfur.

25.1.2 Describe why anthropogenic perturbations to the global carbon cycle are important mediators of environmental change, even though the fluxes are relatively small compared to net primary production and respiration.

25.1.3 Evaluate why changes to individual element cycles at the global scale have implications for the cycling of other elements.

In this section, we will follow the biogeochemical cycling of carbon, nitrogen, phosphorus, and sulfur at the global scale. These particular elements are emphasized both because of their importance to biological activity and because of their roles as pollutants. The cycles are discussed in terms of pools, or reservoirs—the amounts of elements within components of the biosphere—and fluxes, or rates of movement, between pools. For example, terrestrial plants constitute a pool of carbon, while photosynthesis represents a flux—in this case, the movement of carbon from the atmospheric pool to the terrestrial plant pool.