The cycling of elements such as carbon and nitrogen is of central importance in ecology, particularly when humans are causing changes to element cycles on a global scale. In this book a rigorous mathematical framework is developed to model how element cycles operate and interact in plants and soils, forming the foundations of a new ecosystem theory. From a few basic equations, powerful predictions can be generated covering a wide range of ecological phenomena related to element cycling. These predictions are tested extensively against field and laboratory studies of agricultural and forest ecosystems. This groundbreaking work will be of interest to graduate students and researchers in theoretical ecology, soil science, forestry and biogeochemistry.