Environmental Engineering Reference
In-Depth Information
When storage is zero, then inputs equal outputs by definition, and the ecosystem is in a
steady state. If storage is positive, inputs are accumulating in the system. If storage is neg-
ative, there is a net loss from the system, and outputs exceed inputs.
The calcium budget of a well-studied New Hampshire forested watershed provides an
example of the power of boundaries and budgets (
Likens et al. 1996, 1998
). The boundaries
of watersheds in the Hubbard Brook forest of New Hampshire are easily defined. The for-
est soil is underlain with bedrock and hence the watersheds are nearly watertight. There is
no significant loss of water and solutes through deep ground-water flows (
Likens and
Bormann 1995
). Thus, inputs and outputs in these watersheds can be readily measured.
Calcium enters Hubbard Brook watersheds via atmospheric deposition and weathering
reactions and exits the system in stream water. Within the system there is uptake,
exchange, and cycling of calcium. The movements of calcium can be described as:
P
W
S
B
ð
Þ
1
5
1
9
:
2
where atmospheric inputs are represented by
P
(precipitation); weathering inputs, mean-
ing the mobilization of calcium from rocks via chemical reactions, are represented by
W
;
S
is stream output; and
B
is net storage in biomass.
Equation 9.2
is analogous to the simple
budget of
Eq. 9.1
. Precipitation (
P
) and weathering (
W
) represent new inputs of calcium to
the system. Stream export (
S
) is the primary loss of calcium, while the primary storage of
calcium is in the accumulation of biomass (
B
) in trees. The fluxes
P
,
W
,
S
, and
B
in units of
moles of calcium per watershed area per year were measured over a 30-year period at
Hubbard Brook for specific watersheds. The combined rate of
S
W
(
Figure 9.2
). This apparent imbalance indicates how the constraint of mass balance in eco-
systems can be particularly helpful and lead to important findings.
Where does the calcium come from to account for the excess of outputs over inputs?
The “missing calcium” is mobilized from labile and exchangeable pools in the soil. This is
essentially a new input of calcium that makes up for the shortfall in calcium measured in
the budget of outputs relative to inputs. Precipitation at Hubbard Brook is acidic and this
has caused soil calcium to dissolve to partially neutralize acid inputs (e.g., CaCO
3
dissolu-
tion leading to uptake of hydrogen ions, H
1
, as for example in bicarbonate, HCO
3
, and
the consequent release of calcium). Acid inputs have declined over time because of air pol-
lution controls and consequently the loss of soil calcium has also declined. However, the
long-term net loss of calcium from the soil means that the total stock of calcium in the
watershed has declined. One result of this net loss of calcium is that the system is more
susceptible to acid precipitation. There is less calcium to neutralize acid, because less buff-
ering capacity remains in the soil as calcium is exported at rates far faster than it is replen-
ished (
Likens et al. 1996
). This long-term analysis of calcium would not have been possible
without establishment of watershed (ecosystem) boundaries and measurement of the
major inputs and outputs (i.e., fluxes across boundaries).
Loading
is a term used to describe inputs across a boundary to an ecosystem. In the
Hubbard Brook watershed example, the loading of calcium from outside the system comes
via
P
, the precipitation inputs of calcium to the system. The calcium bound in rocks and
minerals is not available for either chemical reactions or biological uptake. However,
weathering defined as the dissolution of minerals by water releases the calcium and makes
B
far exceeded
P
1
1
