Environmental Engineering Reference
In-Depth Information
Deposition: water with dissolved ions
H
+
H
+
H
+
NO
3
-
SO
4
2-
=
Mg
2+
Na
+
H
+
Mg
2+
H
+
Al
3+
Na
+
Ca
2+
K
+
Na
+
K
+
=
Ca
2+
K
+
H
+
Na
+
Al
3+
Mg
2+
K
+
Na
+
Ca
2+
H
+
-
-
-
-
=
-
-
-
Soil particles
with negative
surface charge
-
=
Ions bound in the soil
may be taken up by
plants or leached
-
-
=
-
-
=
-
-
-
K
+
Mg
2+
NO
3
-
SO
4
2-
Leaching: water with dissolved ions
FIGURE 5.2
Element cycles are linked as materials flow through ecosystems. Water transports elements
such as hydrogen, sulfur, and nitrogen from the atmosphere to soil, where new elements such as calcium,
potassium, sodium, and aluminum are encountered in the soil solution on the soil exchange complex.
Interactions among these elements may result in a different assemblage of elements in soil solution—some of
which may be taken up by plants or lost via leaching—thereby altering the chemical composition of water and
changing what is retained in biotic and abiotic pools in the ecosystem.
ecosystem, and it reminds us that nutrient cycling is not static in space despite our
attempts to picture such cycles in textbooks (
Box 5.2
;
Figures 5.3
and
5.4
).
THE IMPORTANCE OF CHEMICAL
PROPERTIES
Reactions of elements within nutrient cycles and the forms an element will take in vari-
ous pools are controlled largely by their chemical properties—something that can be gen-
eralized in part from an element's position within the periodic table (
Figure 5.5
). Elements
in the first two columns of the periodic table tend to give up electrons easily and so are
commonly cations (ions with a positive charge) when free in ecological systems. They are
also likely to be associated with negatively charged exchange sites in soils. In contrast, ele-
ments in the second-from-right column of the periodic table commonly acquire electrons
and are usually present as anions (ions with a negative charge) in ecological systems.
Elements in the farthest-right column, the noble gases, are highly unreactive, and they
