Environmental Engineering Reference
In-Depth Information
Death, however, does not destroy matter but only breaks up the
union of its elements which are then recombined into other
forms
.
11.1
Plants, Ecology, and Biogeochemistry
Democritus of Abdera (460-370 B.C.)
From Browne (1978)
The German biologist Ernst Haeckel (1834-1919), perhaps
best known for his statement, “ontogeny recapitulates phy-
logeny,” and who also was an accomplished artist (as can be
seen in some of his publications), is credited with providing
one of the earliest definitions of ecology (Kormondy 1976)
such that
...
The following sections describe approximations of the
flow of energy and nutrients related to plant survival and,
therefore, application at phytoremediation sites. To simplify
the presentation, the flow of each component essential to
plant life is presented separately. All of these flows occur
simultaneously, however, and water can be considered to be
the common denominator. In fact, the discussion of the
hydrologic cycle in Chap. 2 is itself an introduction to
the biogeochemical cycles discussed here. The rate of
cycling will not be discussed, as the focus here is the path-
way taken. However, there are some interesting details about
the cycling rate of elements that pass through plants. For
example, as would be expected from the fact that between
70% and 80% of the precipitation in a given area is returned
on an annual basis to the atmosphere, eventually all water in
the hydrologic cycle will pass through plants as part of
photosynthesis about one time every 2 million years. In
contrast, the oxygen produced by plants cycles one time
every 2,000 years, and the CO
2
respired by animals and
plants cycles once every 300 years.
Unless plants are grown in the laboratory under hydro-
ponic conditions, most plants used for phytoremediation of
contaminated groundwater are grown in soil. As a result,
plant roots interface with soil particles, soil gas, water, and
microbes. Much of the early work done in the area of the
effects of nutrients and micronutrients on plant growth was
based on observations of plant growth in the absence of a
particular element. These simultaneous interactions are no
better illustrated that in how energy and chemicals move
through the subsurface system, as will be briefly shown
below. In the context of a phytoremediation planting, knowl-
edge of how energy and chemicals move among plants, soil,
and groundwater provides a framework within which the flow
of groundwater contaminants can, therefore, be evaluated.
By ecology we mean the body of knowledge concerning the
economy of nature
ecology is the study of all the complex
interrelations referred to by Darwin as the conditions of the
struggle for survival
.
...
Haeckel's phrase “economy of nature” can be reduced, at
a minimum, to an understanding of the flow of energy and
cycling of nutrients through an ecosystem. Nutrients gener-
ally are discussed in terms of cycles that imply a system at
steady state rather than in terms of a unidirectional flow,
which is more characteristic of the pathway of energy flow.
It probably is more correct to discuss nutrient fate in terms of
flows, because although steady-state conditions of nutrients
can be reached when the loss of a particular nutrient is
balanced by input from another source, nutrients also can
be removed for a length of time by burial through geological
processes and, therefore, are more representative of unidi-
rectional flow. Also, cycles or flows of nutrients all are
driven by solar energy and water as it moves through the
hydrologic cycle. For example, water is the medium in
which life's reactions occur.
Plants must be efficient and tenacious to survive and
reproduce. Even though plants are surrounded by all the
resources they need, these resources often are in dilute
supply. Above ground, CO
2
in the atmosphere is essentially
at concentrations low enough to be considered an impurity,
and below ground most of the essential and micronutrients
to sustain healthy plant growth, such as nitrate and iron,
are either diluted or not readily bioavailable. Also, acquir-
ing some nutrients requires the plant to spend energy, for
some nutrients are characterized by a net negative charge
similar to plants root hairs. Plants need to maintain higher
internal levels of some dilute nutrients, so energy is spent
in bringing these into the cells against a concentration
gradient.
The relation between plants and nutrient cycles or flows
generally is envisioned to occur while the plants are alive
and actively photosynthesizing. However, plants can affect
nutrient cycles even after they die. Except when plants are
removed from a stand for lumber or agriculture or are con-
sumed by fire, the minerals and nutrients that entered the
plant during growth are returned to the soil during decay or
leaf fall. That this cycle has been important was known long
ago, as indicated by
11.1.1 The Flow of Energy and Electrons
The light of the sun, and not the warmth, is the chief reason,
if not the only one, which makes plants yield their
dephlogisticated air (oxygen). A plant not capable of going in
search of its food must find, within the space it occupies, every-
thing which is wanted for itself. The tree spreads through the air
those numberless fans, disposing of themselves to encumber
each other as little as possible in pumping from the surrounding
air all that they can absorb from it, and to present this substance
to the direct rays of the sun, on purpose to receive the benefit
which that great luminary can give it
.
Jan Ingenhousz (1779)
Experiments on Vegetables
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