Environmental Engineering Reference
In-Depth Information
CHAPTER
7
T
he Nitrogen Cycl
e
Peter M. Groffman and Emma J. Rosi-Marshall
Cary Institute of Ecosystem Studies, Millbrook, NY
INTRODUCTION
Nitrogen (N) limits primary production over large areas of the earth, especially in tem-
perate forest and saltwater ecosystems, and has a particularly complex and interesting
series of biological transformations in its cycle. Human manipulation of the N cycle is
intense, as large amounts of reactive
1
N are needed for crop production and are produced
as a by-product of fossil fuel combustion. This manipulation leads to an excess of N in the
environment that cascades through ecosystems, leading to problems with air and water
quality and ecosystem integrity (
Galloway and Cowling 2002
;
Galloway et al. 2003;
Driscoll et al. 2003; Vitousek et al. 1997
).
In this chapter, we first present a global perspective on the N cycle and how humans
have altered it. We then discuss the processes and transformations that make up the N
cycle and describe how these processes are expressed and regulated in terrestrial and
aquatic ecosystems.
THE GLOBAL PICTURE
The largest pool of N on Earth is in the atmosphere as dinitrogen (N
2
), a very
stable (not available to most organisms) molecule that accounts for 79% of the atmosphere.
Nitrogen is unique among the elements that are essential for primary production in that it
is not a component of the common rocks that make up Earth's crust. Use of N by primary
producers thus depends on conversion of N
2
into soluble forms.
1
Reactive nitrogen has no standard definition but generally refers to forms of nitrogen capable of
combining with other chemicals in the environment and/or that have biological effects. The vast reservoirs
of stable nitrogen in the atmosphere (N
2
gas) and in soil organic matter are not reactive.
