ABSTRACT
The biosphere is defined as that part of the planet that sustains life. It encompasses the lower part of the atmosphere, the hydrosphere (the oceans, lakes, rivers, and streams), and the lithosphere (the earth’s crust) down to a depth of about two kilometers. Biogeochemical cycles are the transport pathways and the chemical and physical interactions of the elements within and among these regions of the biosphere. Atmospheric carbon represents the smallest of the planet’s reserves, containing only 720 billion tons, mostly as carbon dioxide. This quantity is to be compared to the largest of the planet’s reserves, about 27 million billion tons, which reside in the lithosphere and in the deep ocean, mostly as organic sediments or carbonate rock. Despite enormous stores of nitrogen in the lithosphere (about 5 × 1016 tons), only a tiny fraction of the total (about 240 billion tons) that resides in soil takes part in nitrogen’s biogeochemical cycle.
