Environmental Engineering Reference
In-Depth Information
Looking at a longer time frame (e.g., 10,000 years ago), we can glean some interesting
information about the C balance at Hubbard Brook. Assuming that the soils and rock
surfaces were free of organic C at the retreat of the glacier some 12,000 to 10,000 years ago,
we can look at the total sequestration of organic C in soils and biomass ( Figure 6.4 ).
All the NEP during this time is either stored in biomass or soil, burned, or was fluvially
exported. This simplifies the problem of estimating rates of C sequestration because all we
need to do is take the total standing stock and divide by elapsed time. We can also
roughly estimate the fluvial export. At the millennial timestep NEP is roughly evenly
divided between storage in the soil and fluvial export ( Figure 6.4 ). The major storage is in
the mineral soil, rather than in the organic horizons or in biomass. On the other hand, this
long-term, direct estimate of C sequestration (about 2.5 g C m 2 2 y 2 1 ) is 45 times lower than
the current estimate that we arrived at by difference. Further, this sequestration estimate is
probably an upper bound since it is likely that the material in the soils was not entirely
C-free when the glacier receded. Over the millennial time step, NEP would be equal to the
sum of long-term C accumulation in soils plus biomass (about 3.5 g C m 2 2 y 2 1 ) plus fluvial
export (about 3 g C m 2 2 y 2 1 ) minus DOC import in rain (1.7 g C m 2 2 y 2 1 ), or about
4.8 g C m 2 2 y 2 1 . This net result at the millennial time step does not imply anything about
short-term variations in the C balance. There are times of rapid storage and rapid loss.
There are intriguing features about C cycling at Hubbard Brook that are not captured
by looking only at the ecosystem-level fluxes. Despite the high rates of photosynthesis in
this forest, the organic C leaving the stream water as DOC has about the same concentra-
tion found in incoming rainwater, thus inputs balance outputs on an annual basis. This is
made even more intriguing by looking at the changes in DOC concentrations as water
moves from rain, to tree canopy, to soil, and out in stream water ( Figure 6.5 ). As this rain
passes through the forest canopy (called throughfall) the concentration increases 10-fold.
In the upper (organic) soil horizons DOC collected in lysimeters (soil water collection
devices) is even higher, three to four times what it was in throughfall. So as water moves
through the forest and soil it increases in DOC concentration by about 30-fold. However,
as this water passes through the mineral horizons, its DOC concentration decreases dra-
matically ( Figure 6.5 ). The concentration as water leaves the lower B horizon and enters
FIGURE 6.5 Changes in the concentration of dis-
solved organic C as it passes through a forest. Values
are mg C/L. Precipitation has low concentrations of
DOC, but as this water moves through the forest can-
opy it acquires DOC from the trees (throughfall) and
even more in the rich organic layers of the forest floor.
DOC concentrations are reduced due to both decompo-
sition and sorption in the mineral layers of the soil
(upper and lower B horizons). As water exits the sys-
tem in stream water it has concentrations much lower
than in throughfall or forest floor soils.
Precipitation - 1.09
Throughfall - 12
Forest floor - 28 to 38
Upper B horizon - 6
(Redrawn from
the data in McDowell and Likens 1988.)
Lower B horizon - 3
Stream water - 3
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