Nutrient Cycles in Ecosystems
Nutrients have been discussed throughout this book in terms of limiting factors for producers, consumers, and decomposers, as electron donors for microbial decomposers, and in a number of other roles. It should be evident, then, that nutrients are inextricably linked to almost all ecological processes. In this chapter, we first look at the transformations, exchanges, and general dynamics of three nutrients: phosphorus, nitrogen, and sulfur. Of course, there are many other important elements that play important roles in ecosystems, such as Si, Fe, Mn, Mo, and many more, some of whose functions have been mentioned in previous chapters. We focus on the dynamics of only three elements (N, P, and S) here, in part for economy of space, and because the interactions and transformations of these three elements do illustrate how nutrients interact with sediments and water, and the great significance of oxidation state and of interactions with organisms. In this chapter, we also examine the stoichiometry by which nutrient cycles are coupled to transformations of organic matter—and the carbon cycle—in ecosystems.