Biogeochemical Cycles: Besides energy, all organisms require various nutrients. In addition to water molecules, the elements of primary importance in these nutrients are carbon, nitrogen, oxygen and phosphorus. Nutrients, unlike energy, are recycled through ecosystems in nutrient (biogeochemical) cycles. For each element, the cycle may involve 1) a reservoir, in which the nutrient is present but temporarily unavailable, 2) an exchange pool, which is the primary source of nutrients, and 3) the biotic community, which consists of the organisms through which nutrients pass. Human activities have altered nutrient cycles.