The cycling of elements such as carbon and nitrogen is of central importance in ecology, particularly when humans are causing changes to nutrient cycles on a global scale. This book develops a rigorous mathematical framework to illustrate how nutrient cycles operate and interact in plants and soils, forming the foundations of a new ecosystem theory. Powerful predictions can be generated covering a wide range of ecological phenomena related to nutrient cycling from a few basic equations. These predictions are tested extensively against field and laboratory studies of agricultural and forest ecosystems.