Environmental Engineering Reference
In-Depth Information
Tracers
As useful as balances are as tools, they tell us about the bulk (or net) movements of
materials through ecosystems, and rarely allow us to distinguish among different path-
ways of material movement within ecosystems. All nitrogen atoms look alike to a mass
balance. Tracers are tools that allow ecosystem scientists to distinguish among particular
pathways of material movement by labeling just some of the atoms or molecules of inter-
est. Ecosystem scientists have used several tracer methods, which have been enormously
powerful in understanding how ecosystems work.
Radioisotopes (
Box 1.3
) were some of the first tracers used in ecosystem science.
Radioisotopes can be detected and quantified at very low concentrations, so they make
excellent tracers, and have had many applications in ecosystem science. In the mid-twentieth
century, ecosystem scientists added small amounts of radioisotopes to ecosystems to trace the
movement of water and the uptake and movement of carbon and limiting nutrients through
ecosystems. Radioisotopes are no longer added to ecosystems as tracers because of associated
health risks, but they continue to be used widely in laboratory studies and measurements
(e.g., to measure microbial production; see Chapter 3). They also are used in “natural abun-
dance” studies where ecosystem scientists use the very low natural abundance of
BOX 1.3
ECOLOGICALTRACERS: ISOTOPES
Most elements exist in several forms that
contain different numbers of neutrons (but
the same number of protons and electrons,
and basically the same chemical properties).
For example, about 99% of the carbon on
Earth is
12
C, which contains six protons, six
electrons, and six neutrons, but about 1% of
the carbon is
13
C, which contains seven neu-
trons. A tiny amount (
isotopes have been used in specialized stud-
ies (see
Figure 1.4
).
The concentration of stable isotopes is
usually expressed in a “del” (
) notation
that compares the abundance of the heavier
isotope to that of the lighter isotope.
Thus, the abundance of
δ
13
C in a sample
is expressed as:
0.0000000001%) of
the carbon is
14
C, which has eight neutrons.
Some isotopes are stable, while others are
radioactive (i.e., they spontaneously decay
into other elements or isotopes). In the case
of carbon,
12
C and
13
C are stable isotopes,
whereas
14
C is a radioisotope that decays
into nitrogen (
14
N) with a half-life of 5730
years. Some isotopes that commonly make
an appearance in ecosystem science include
the radioisotopes
3
H (tritium),
14
C,
32
P, and
35
S, and the stable isotopes
2
H (deuterium),
13
C,
B
12
C
sample
13
C
13
C
!
13
C
ð
m
Þ
5
12
C
standard
2
1
1000
δ
3
The standard in this case is Vienna
Pee Dee Belemnite (a particular kind of fos-
sil). Negative
values indicate that the
heavier isotope is less abundant in the sam-
ple than in the standard, while positive
δ
δ
values indicate that the heavier isotope is
more abundant in the sample than in the
standard.
15
N,
18
O, and
34
S, although many other
