All things from eternity are of like form and come round in a circle.
-- Marcus Aurelius
The main flow of energy that helps shape conditions on Earth's surface comes from space; and when the energy's work here is done, to space it returns. With respect to energy, then, Earth is an open system. With respect to its chemical endowment, however, Earth is a closed system. That is, the amounts of carbon, hydrogen, oxygen, iron, gold, and other elements in the planet-atmosphere system do not change with time; the chemical arrangement and physical distribution of these elements can and do vary, but essentially nothing enters and nothing leaves the system.1
The elements in this closed system that are essential to life are called nutrients. They can be divided into three categories: the 4 main chemical building blocks of living matter (carbon, oxygen, hydrogen, nitrogen); 7 macronutrients, of which smaller but still significant quantities are required for life; and 13 micronutrients, or trace elements, of which tiny quantities perform essential functions (see Table 3-1).2 The suitability of any terrestrial or aquatic environment for the support of life depends on the availability of nutrients in appropriate forms and quantities. The processes that govern this availability (or lack of it) are known collectively as nutrient cycles, because of the way the individual basic stocks of physical material move cyclically through the living and nonliving parts of the physical world.
These cycles function in support of life not merely by making nutrients continuously available -- in other____________________