14. Nutrient Transformations
ferred to as potential rates or enzyme activity
measurements.
Isotope Methods
For discussion purposes, I divide isotope methods
into four general categories: (1) tracer techniques,
(2) isotope dilution techniques, (3) isotope models
utilizing numerical solutions, and (4) natural abundance techniques. Of these approaches, isotope dilution techniques and isotope modeling have the
greatest utility for estimating gross nutrient transformation rates in ecosystems; therefore, I emphasize these in my discussion. Additional discussions
of specific isotope techniques can be found in
Chapters 8 and 12 and Knowles and Blackburn
(1993), Schimel (1993), Hart et al. (1994b), Wolf
et al. (1994), and Lajtha and Michener (1994).
Tracer Measurements
In ecosystem studies, the most common use of isotopes is as tracers. A small amount of a radioactive
or stable isotope is added in a particular molecular
form (e.g., 15NHt or 15N03) in order to label a
source pool. Flow of the isotope out of the source
pool into various sink pools is then monitored over
time. Based on the amount of isotope recovered in
a sink pool, one can calculate the total amount of
nutrient that flowed into that pool from the labeled
source pool. For example, the total flow of a nutrient from pool A to pool B in Figure 14.5 is calculated as:
FIGURE 14.5. Hypothetical flow of nutrients during a
tracer experiment.
223
(14.6)
where MAB is total amount of nutrient (added plus
natural isotopes) that flowed from pool A to pool
B during the incubation period (e.g., mg N kg - 1
soil); P B is the size of the sink pool (B) at the end
of the incubation (e.g., mg N kg -1 soil); IB is the
relative amount of isotope, in excess of background, that is found in pool B at end of the incubation (measured as atom % excess if a stable isotope is used or specific activity if a radioactive
isotope is used); and IA is the relative amount of
isotope, in excess of background, that is present in
the source pool, A (units are the same as for I B ).
The total flow of nutrient from A to B calculated
from Equation 14.6 is converted to a rate by dividing MAB by the length of the incubation period.
The assumptions of the tracer method are:
(1) The added isotope acts the same as the naturally
occurring isotope (i.e., no significant discrimination
or fractionation occurs), (2) the source pool is uniformly labeled by the isotope, (3) addition of the
isotope does not stimulate rates, (4) no additional
nutrient moves into the source pool (the source pool
has a constant enrichment), and (5) nutrients are not
lost from the sink pool.
The first assumption, that discrimination or fractionation does not occur, is frequently not valid.
Different isotopes often have different reaction
rates (Handley and Raven 1992). The differences
are usually very small «2%), however, and errors
due to isotope discrimination or fractionation can
usually be ignored when one is working with isotope enrichments that are well above natural
abundance.
The second assumption, of uniform labeling of
the source pool, is often more difficult to achieve
than it seems. In studies of decomposition rates
14
'
C-labeled plant material may be prepared by
growing plants in an atmosphere containing 14C02.
Frequently, plants are exposed to 14C02 for a single
short period of time, as part of a "pulse labeling
technique"; however, uniform distribution of 14C
among structural and soluble plant components
may not occur even when the plant is grown in the
presence of the isotope for several months (Lynch
and Whipps 1990). In other studies measuring rates
of soil nutrient transformations, a syringe maybe
used to inject an isotope solution into soil (e.g.,
ferred to as potential rates or enzyme activity
measurements.
Isotope Methods
For discussion purposes, I divide isotope methods
into four general categories: (1) tracer techniques,
(2) isotope dilution techniques, (3) isotope models
utilizing numerical solutions, and (4) natural abundance techniques. Of these approaches, isotope dilution techniques and isotope modeling have the
greatest utility for estimating gross nutrient transformation rates in ecosystems; therefore, I emphasize these in my discussion. Additional discussions
of specific isotope techniques can be found in
Chapters 8 and 12 and Knowles and Blackburn
(1993), Schimel (1993), Hart et al. (1994b), Wolf
et al. (1994), and Lajtha and Michener (1994).
Tracer Measurements
In ecosystem studies, the most common use of isotopes is as tracers. A small amount of a radioactive
or stable isotope is added in a particular molecular
form (e.g., 15NHt or 15N03) in order to label a
source pool. Flow of the isotope out of the source
pool into various sink pools is then monitored over
time. Based on the amount of isotope recovered in
a sink pool, one can calculate the total amount of
nutrient that flowed into that pool from the labeled
source pool. For example, the total flow of a nutrient from pool A to pool B in Figure 14.5 is calculated as:
FIGURE 14.5. Hypothetical flow of nutrients during a
tracer experiment.
223
(14.6)
where MAB is total amount of nutrient (added plus
natural isotopes) that flowed from pool A to pool
B during the incubation period (e.g., mg N kg - 1
soil); P B is the size of the sink pool (B) at the end
of the incubation (e.g., mg N kg -1 soil); IB is the
relative amount of isotope, in excess of background, that is found in pool B at end of the incubation (measured as atom % excess if a stable isotope is used or specific activity if a radioactive
isotope is used); and IA is the relative amount of
isotope, in excess of background, that is present in
the source pool, A (units are the same as for I B ).
The total flow of nutrient from A to B calculated
from Equation 14.6 is converted to a rate by dividing MAB by the length of the incubation period.
The assumptions of the tracer method are:
(1) The added isotope acts the same as the naturally
occurring isotope (i.e., no significant discrimination
or fractionation occurs), (2) the source pool is uniformly labeled by the isotope, (3) addition of the
isotope does not stimulate rates, (4) no additional
nutrient moves into the source pool (the source pool
has a constant enrichment), and (5) nutrients are not
lost from the sink pool.
The first assumption, that discrimination or fractionation does not occur, is frequently not valid.
Different isotopes often have different reaction
rates (Handley and Raven 1992). The differences
are usually very small «2%), however, and errors
due to isotope discrimination or fractionation can
usually be ignored when one is working with isotope enrichments that are well above natural
abundance.
The second assumption, of uniform labeling of
the source pool, is often more difficult to achieve
than it seems. In studies of decomposition rates
14
'
C-labeled plant material may be prepared by
growing plants in an atmosphere containing 14C02.
Frequently, plants are exposed to 14C02 for a single
short period of time, as part of a "pulse labeling
technique"; however, uniform distribution of 14C
among structural and soluble plant components
may not occur even when the plant is grown in the
presence of the isotope for several months (Lynch
and Whipps 1990). In other studies measuring rates
of soil nutrient transformations, a syringe maybe
used to inject an isotope solution into soil (e.g.,