Environmental Engineering Reference
In-Depth Information
Interestingly, NEP can be either positive or negative. How is this possible? One way
this can occur is if the primary production of an ecosystem is stopped or severely
reduced (the GPP in
Eq. 2.2
0
), but the respiration of stored organic matter continues.
For instance, consider a forest that has just been clear-cut so that there is little or no pri-
mary production but decomposers are still consuming (and respiring) the organic matter
in the forest floor. Another way that negative NEP can occur is if an ecosystem imports
organic carbon, and these imports are respired by heterotrophs along with the carbon
produced within an ecosystem. In both cases the total respiration of the ecosystem
exceeds gross primary production (
R
e
.
B
GPP), thus NEP is negative. Ecosystems with
negative NEP are referred to as heterotrophic ecosystems—these systems respire more
carbon than they produce and the excess respiration either depletes carbon stored in the
system or is subsidized by imports of carbon from outside the ecosystem. In contrast,
ecosystems with positive NEP are autotrophic ecosystems. Ecosystems with negative
NEP are quite common and include many lakes, streams, rivers, and estuaries (
Caraco
and Cole 2004
). These distinctions about relative NEP are important in considering car-
bon sequestration by ecosystems (
Box 2.1
; Chapter 6).
We can also consider NEP in the context of organic carbon accumulation (
dC
org
)inan
ecosystem by considering a mass balance of inputs and losses:
dC
org
5
GPP
I
R
e
2
Ex
Ox
nb
ð
2
4
Þ
1
2
2
:
where the new terms are:
I
imported organic carbon
5
Ex
exported organic carbon
5
Ox
nb
5
nonbiological oxidation of organic carbon (e.g., fire or photo-oxidation)
Since NEP is equal to GPP
R
e
,
Eq. (2.4)
can be written as:
dC
org
I
Ex
Ox
nb
ð
Þ
5
NEP
1
2
:
5
Organic carbon accumulation (
dC
org
) in ecosystems sequestered over long time periods
(centuries to millennia) provides a sink for atmospheric CO
2
and is very important to
those studying global carbon budgets (
Box 2.1
; Chapter 6).
Not all primary production results from aerobic photosynthesis where water is split
and oxygen is produced in the fixation of carbon. Under anoxic conditions, some microor-
ganisms can fix carbon, for example, using hydrogen sulfide (H
2
S) instead of water and
producing sulfur instead of oxygen. Further, some microorganisms, primarily archaea and
bacteria, have chemosynthetic abilities and are also primary producers. There are many
types of chemosynthetic reactions but all oxidize inorganic molecules to produce energy,
which is used to fix CO
2
as organic matter (
Box 2.2
). For example, nitrifying bacteria con-
vert ammonia to nitrite or nitrite to nitrate, and in the process derive energy sufficient to
convert CO
2
to organic matter.
