Environmental Engineering Reference
In-Depth Information
into living biomass (i.e., production). When organisms die, these organically bound forms
are exposed to decomposers, which break down the organic material (i.e., decomposition)
and either incorporate, liberate, or leave behind the elements that were previously part of
other living organisms. Liberated elements may be used by other organisms—for example,
uptake of inorganic nitrogen by plants—or may be bound in inorganic forms in the soil, as
in potassium associated with clay minerals in the soil. Elements in the abiotic pool may be
taken up by organisms or precipitate from solution as secondary minerals. And then there
is weathering, which breaks down the physical and chemical structure of minerals, often
in the presence of organic acids. The soluble products of weathering may be taken up by
organisms or participate in further geochemical reactions.
Individual nutrient cycles are thus, not surprisingly, much more complex than this brief
generalized view and are invariably linked to each other (
Box 5.1
;
Figure 5.2
). The spiral-
ing of nutrients in streams provides an excellent example of how cycling can work in an
BOX 5.1
LINKED ELEMENTAL CYCLES
Both the living and nonliving world are
mixtures of many elements. In the biotic
world, there are over 20 elements essential
for plant growth and reproduction, and
another dozen or so that frequently occur in
organisms (
Schlesinger 1997
). In the mineral
world, oxygen, silicon, aluminum, and iron
are the most abundant elements, but miner-
als may be composed of anywhere from 1
to more than 10 elements, depending on the
size of the sample considered and the num-
ber of impurities in the crystal structure.
So although we are used to thinking about
element cycles separately, this is a conve-
nience, not a common reality, and there
is considerable interest in understanding
the linkages among elemental cycles
(
Schlesinger et al. 2011
and other articles in
Frontiers in Ecology and the Environment
2011, 9(1)).
There are many ways that elemental
cycles can be linked, but two useful catego-
ries of linkage are those that have to do with
(1) structural stoichiometry and (2) the func-
tions of chemical reactivity and material
and energy flow. On the structural side,
stoichiometry describes the ratio of elements
that are present as part of the fundamental
structure of a material or organism, and lin-
kages occur because structures need to be
built with particular ratios of elements.
Alternatively, element cycles can be linked
because materials may move together down
physical gradients or through chemical reac-
tions where the “product” of the reaction is
not itself an organism or mineral of interest.
These two kinds of linkages are inevitably
related to each other since much of the flow
of material and energy goes to building the
structure of organisms in ecological time,
or minerals over geologic time, and these
structures always involve stoichiometry.
Nevertheless, the distinction can be useful in
clarifying our thinking about the reasons for
linkages across element cycles.
In an ecological setting, the most com-
mon linkages in elemental cycles occur
because organisms contain roughly con-
stant ratios of some elements.
Redfield
(1958)
reported that marine phytoplankton
typically contained carbon, nitrogen, and
phosphorus atoms in the ratio 106:16:1. He
