Ecosystems are complex entities composed of diverse organisms expressing a myriad of life histories, body sizes, and metabolic pathways embedded in a dynamic physical and chemical environment. A suite of internal processes create an intricate web of relationships that control the flow of energy and materials within and between ecosystems. Contributing to the complex nature of ecosystems is the breath of spatial and temporal scales over which ecosystems operate. Ecosystems occupy spatial scales from meters to hundreds of kilometers and temporal scales from days to centuries owing to contrasting organismal life spans. For instance, consider boreal forests. There, organism life histories range from a few days (i.e., bacteria in the soil) to centuries (i.e., trees) generating a plethora of timescales over which ecosystem function can be addressed. However, the most characteristic aspect of ecosystem research is that it explicitly incorporates the chemical and physical environment, often requiring multidisci-plinary approaches to characterize patterns and identify controlling mechanisms.

Ecosystem studies are largely organized around two complementary themes, energy flow and elemental cycling. Because ecological stoichiometry examines the nature, control, and implications of elemental balances in ecological processes, it also provides an appropriate framework for linking elemental cycles in nature. Elements required for life, such as carbon (C), nitrogen (N), phosphorus (P), calcium (Ca), and trace metals, are conserved as they move between organisms and their environment. That is, elements cannot be destroyed and are instead used, released back into the environment, reused again by organisms, and so forth, cycle after cycle. Continuous elemental flow between the environment and organisms with specific elemental compositions creates important ecological constraints, the implications ofwhich represent one aspect of ecological stoichiometry.

Ecological stoichiometry offers insight into ecosystem dynamics because it readily applies to the composite nature of ecosystems. Stoichiometric constraints affecting primary producers and higher trophic levels have consequences for the relative cycling rates of elements. This condition has an important implication; if we understand the stoichiometric interactions between organisms, we should be able to predict and understand stoichiometric outcomes at the ecosystem level. However, elemental cycling is not simply the sum of assimilative growth, trophic transfer and mineralization (conversion of organic compounds to inorganic forms). Rather, a suite of other biogeochemical processes also influence the availability and cycling of elements. In this chapter, we discuss the consequences of relative nutrient availability for ecosystem processes and review research that has used a stoichiometric approach to investigate the coupling of nutrient cycles in nature.

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