Principles of terrestrial ecosystem ecology.pdf

organic and inorganic carbon in groundwater
can be a significant loss from ecosystems,
although it is often poorly quantified.
There can be lateral carbon fluxes (Flateral)
into or out of the ecosystem due to deposition
or erosion.
D
Dt
SOM
= Fpl-soil + Fanim-soil
- ( Rmicrob + Fmicrob-anim
+ F + F + F )
CH4 leach soil-fire
(B6.4)
The pool sizes of plants, animals, and SOM
get larger when inputs exceed outputs and
get smaller when outputs exceed inputs.
Fire, for example, causes an instantaneous
decrease in the pool size of plant and soil
carbon, whereas these pools generally increase
in size during succession after fire (see
Chapter 13).
Ecosystem carbon accumulation depends
primarily on the balance between carbon
inputs through photosynthesis and carbon
losses through respiration and disturbance.
Net ecosystem production is the net annual
carbon accumulation by the ecosystem and
is the sum of the net carbon accumulation in
plants, animals, and the soil plus lateral transfers
of carbon among ecosystems (Flateral)
(Olsen 1963, Bormann et al. 1974, Aber and
Melillo 1991).
( DBplant + DBanim + DSOM)
NEP =
± Flateral
Dt
(B6.5)
NEP is positive when carbon inputs to the
ecosystem exceed carbon losses and is
negative when losses exceed inputs. It is
difficult, however, to measure NEP accurately
from changes in the carbon pools in
plants, animals and SOM, because the
changes in these pools over short time intervals
are small relative to measurement
errors. NEP is therefore often estimated
from changes in those fluxes by which
carbon enters or leaves the ecosystem, ignoring
the fluxes of carbon that occur within the
ecosystem.
Net Ecosystem Production 143
When carbon fluxes are aggregated at the
ecosystem scale, some fluxes cancel out,
because a loss from one component represents
a gain by another component and
therefore does not affect the total quantity
of carbon in the ecosystem. Important fluxes
that cancel out are consumption of plants
and microbes by animals and the carbon
fluxes from plants or animals to soil. Thus
litterfall and herbivory alter ecosystem
carbon budgets primarily by altering the
location of the carbon within the ecosystem,
not by altering directly the carbon inputs to
or losses from the ecosystem. Even large
changes in these fluxes such as occur during
insect outbreaks or hurricanes are simply
internal transfers of carbon within the
ecosystem and need not be represented as
separate terms in an overall budget of
changes in carbon pools.
Some ecosystem fluxes can be aggregated
at the ecosystem scale. Ecosystem respiration
(Recosyst) is the combined respiration of
plants, animals and microbes. It can be
partitioned into plant respiration (Rplant),
also termed autotrophic respiration, and
heterotrophic respiration (Rheterotr)—that is,
the respiration by organisms that
gain their carbon by consuming organic
matter rather than producing it themselves.
Heterotrophs include animals and
microbes.
Rheterotr = Ranimal + Rmicrob
Recosyst = Rplant + Rheterotr
(B6.6)
(B6.7)
It is useful to treat separately those disturbances,
such as fire and harvest, that
directly remove carbon from ecosystems,
because these disturbances are episodic in
nature and frequently involve large fluxes
that occur over a few hours to days.
Fdisturb = Fpl-fire + Fsoil-fire + Fharv (B6.8)
Based on Equations B6.1 to B6.5 we can
describe NEP in terms of carbon fluxes
rather than changes in pool sizes.