Environmental Engineering Reference
In-Depth Information
CHAPTER
6
The Carbon Cycle
With a Brief Introduction to Global Biogeochemistry
Jonathan J. Cole
Cary Institute of Ecosystem Studies, Millbrook, NY
WHY STUDY THE CARBON CYCLE?
There are multiple reasons to study the carbon cycle. For one, carbon (C) is the major
building block of all known life and is the second-most important element by mass (after
oxygen) in organisms. Further, C is chemically versatile and can form a diverse array of
organic and inorganic compounds. In the atmosphere, two carbon compounds, carbon
dioxide and methane, are potent greenhouse gases that are on the rise due to human activ-
ity. In many ways the study of climate change is the study of the global C cycle.
In this chapter we will examine the C cycle starting at the global scale at several time
frames and then focus on the C cycle in a few exemplar ecosystems to illustrate certain
aspects. First, let us review the biogeochemistry of C and what a biogeochemical cycle is.
BIOGEOCHEMISTRY OF CARBON
In a biogeochemical cycle, an element can cycle between chemical forms; between major
global reservoirs (e.g., biosphere, lithosphere, and atmosphere); between smaller ecosys-
tems (forest to river in a landscape); or between reservoirs within an ecosystem (soil
to plant, etc.). The chemical forms can cycle among physical states (gas, solid, liquid) or
redox states. No element cycles in isolation. Rather, there are a number of simultaneous
interacting cycles of multiple elements. When we discuss the cycle of an individual element,
we are just choosing to focus on one member of an interacting set of parts. Although focusing
on the biogeochemical cycle of a single element is a useful heuristic tool, we should keep in
mind that this cycle is a small part, or a narrow view, of many coupled biogeochemical
cycles involving multiple elements (
Schlesinger et al. 2011
). As biogeochemical transforma-
tions are partly mediated by oxidation-reduction reactions, before proceeding it may be help-
ful to review the section on redox chemistry (appendix).
