The world's major abiotic reservoirs for nutrients are illustrated in Figure 18.20. The biotas of both terrestrial and aquatic habitats obtain some of their nutrient elements predominantly via the weathering of rock. This is the case, for example, for phosphorus. Carbon and nitrogen, on the other hand, derive mainly from the atmosphere - the first from CO2 and the second from gaseous nitrogen, fixed by microorganisms in the soil and water. Sulfur derives from both atmospheric and lithospheric sources. In the following sections we consider phosphorus, nitrogen, sulfur and carbon in turn, and ask how human activities upset the global biogeochemical cycles of these biologically important elements.
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