ABSTRACT
Biogeochemical cycling can be defined as the movement and exchange of both matter and energy
between the four different components of the Earth, namely the atmosphere (the air envelope that
surrounds the Earth), the hydrosphere (includes all the Earth’s water that is found in streams, lakes,
seas, soil, groundwater, and air), the lithosphere (the solid inorganic portion of the Earth, including
the soil, sediments, and rock that form the crust and upper mantle, and extending about 80 km deep)
and the biosphere (all the living organisms, plants and animals). The four spheres are not mutually
exclusive, but overlap and intersect in a quite dynamic way. Soils contain air and exchange gases
with the atmosphere, thus causing the geosphere and atmosphere to overlap; but they also contain
water, so the geosphere and hydrosphere overlap. Dust from the geosphere and water from the
hydrosphere occur in the atmosphere. Organisms are present in water bodies, soils, aquifers, and
the atmosphere, so the biosphere overlaps with the other three spheres. Chemical elements are
cyclically transferred within and among the four spheres, with the total mass of the elements in
all of the spheres being conserved, though chemical transformations can change their form. The
biogeochemical cycle of any element describes pathways that are commensurate with the move-
ment of the biologically available form of that element throughout the biosphere (where the
term biological availability is used to infer the participation of a substance in a “biological” reaction
as opposed to its simple presence in biota). The most efficient cycles are often equated with a high
atmospheric abundance of the element. These cycles ensure a rapid turnover of the element and
have the flexibility to process the element in a number of different forms or phases (i.e., solid,
liquid, gaseous). Except in a few rare but interesting situations (e.g., geothermal/tectonic systems), all biogeochemical cycles are driven directly or indirectly by the radiant energy of the
sun. Energy is absorbed, converted, temporarily stored, and eventually dissipated, essentially in
a one-way process (which is fundamental to all ecosystem function). In contrast to energy flow,
materials undergo cyclic conversions. Through geologic time, biogeochemical cycling processes
have fundamentally altered the conditions on Earth in a unidirectional manner, most crucially by
decomposition of abiotically-formed organic matter on the primitive Earth by early heterotrophic
forms of life, or changing the originally reducing atmosphere to an oxidized one via the evolution of
oxygenic phototrophs. Contemporary biogeochemical cycles, however, tend to be cycling rather
than unidirectional, leading to dynamic equilibria between various forms of cycled materials.
