Environmental Engineering Reference
In-Depth Information
BOX 2.1
NET ECOSYSTEM PRODUCTION
AND CARBON SEQUESTRATION
The fate and especially long-term storage
of primary production in the biosphere can
influence carbon dioxide in the atmosphere.
For example, the current burning of fossil
fuels by humans that is causing atmo-
spheric CO
2
to increase represents the min-
ing of ancient primary production long
stored in the earth. Is it possible to partially
reverse the current course of CO
2
increase
by storing increased amounts of contempo-
rary primary production in long-term reser-
voirs? This type of question drives research
on the carbon cycle and carbon sequestra-
tion in ecosystems. Consideration of carbon
sequestration, however, requires clarity in
terminology and specification about time-
scales over which sequestration occurs. In
some systems NEP is equated to carbon
sequestration; however, this is not necessar-
ily correct. For instance, a portion of NEP
may be exported as organic carbon rather
than sequestered in the ecosystem. In addi-
tion, today's sequestered carbon might
become tomorrow's atmospheric CO
2
.
Consider a regenerating forest that grows
for 50 years accumulating carbon in wood
(90%) and soil organic matter (10%) at a
rate of 100 g C m
2
2
y
2
1
so that 5000 g C
m
2
2
is stored after 50 years. If the forest
burns in year 51 and all the wood is con-
sumed in the fire, the sequestered carbon is
only the 10% stored in the soil (assuming it
did not burn). Consideration of the time-
scale of carbon storage is important given
discussions to develop carbon markets and
global sinks for CO
2
.
These general issues can be clarified by
revisiting the definition of NEP in the con-
text of an ecosystem budget for organic car-
bon (
Lovett et al. 2006
). Remember
Eq. (2.5
)
that
dC
org
5
Ox
nb
.
Clearly, NEP is not simply equivalent to
carbon sequestration (
dC
org
). It is possible
that in some ecosystems
I
,
Ex
, and
Ox
nb
are
low, but most ecosystems lose some carbon
(
Ex
) as dissolved organic carbon and receive
organic inputs (
I
) from the atmosphere and/or
adjacent ecosystems. Further, if an area
burns,
Ox
nb
is high and
dC
org
is strongly
affected but NEP is not. Schemes for car-
bon sequestration must consider NEP and
the fluxes
I
,
Ex
, and
Ox
nb
in the context of
the particular ecosystems where carbon will
be stored. Because some measurement sys-
tems provide an instantaneous estimate of
NEP, the imports, exports, and nonbiologi-
cal oxidation must be accounted for before
organic carbon accumulation can be calcu-
lated. Moreover, the timescale of probable
carbon sequestration should be explicit so
that periodic events like fires and floods can
be incorporated into calculations.
NEP
I
Ex
1
2
2
In most ecosystems that receive significant light energy, chemosynthesis is only a small
proportion of primary production. For example, in three Swedish lakes chemosynthetic pri-
mary production by methane-oxidizing bacteria (which transform methane to acquire energy,
Box 2.2
) was only 0.3% to 7% of total primary production (
Bastviken et al. 2003
). However, in
unlighted ecosystems such as the deep realms of soils, sediments, and caves where
suitable reduced compounds are present (e.g., ammonium, sulfides, methane, and hydrogen),
