228
John M. Stark
TABLE 14.3. Sample calculations for a I-day 15NO; isotope dilution experiment performed in intact soil cores (see
Fig. 14.8 for sampling method). Based on procedure described in Hart et al. (1994b) and Davidson et al. (1991).
Core pair I
Core pair II
soil sample:
time harvested (days):
Data
mass of 15NO; - N injected/core
(@ 99.9 atom % 15N) (Jlg N)":
mass of <2 mm oven-dry soil in core
(g o.d. soil):
2 M KCI-extractable NO; - N
(Jlg N g -1 o.d. soil):
mole fraction 15N in KCI-extractable
NO;:
Calculated Values
inner
to
300
320
2.3
0.3820
outer
to
1.6
0.0037 b
inner
tl
300
290
2.0
0.1760
outer
to
2.5
0.0037
mass of 15N added (Jlg N g - 1 o.d. soil):
mass of excess 15N recovered (Jlg N g-I):
extraction efficiency:
300/320 = 0.938
300/290 = 1.034
(2.3 * 0.382) - (1.6 * 0.0037) = 0.873
0.873/0.938 = 0.931
mole fraction 15N excess estimated to be
in the intact core at time - 0:
(1.034 * 0.931 + 2.5 * 0.0037)1(2.5 + 1.034 * 0.931) - 0.0037 = 0.277
time-O
time-1
Calculation of Gross Rates
pool size (Jlg N/g o.d. soil):
mole fraction 15N excess:
2.5 + 1.034 * 0.931 = 3.46
0.277
2.0
0.1760 - 0.0037 = 0.1723
gross nitrification rate (Jlg N g-1 o.d. soil day-I):
(3.46 - 2.0) log(0.277/0.1723)
1.26
1
log(3.46/2.0)
gross nitrate consumption rate
(Jlg N gl o.d. soil day-I):
(2.0 - 3.46)
1.26 -
1
= 2.72
aAll N masses are expressed in terms of the equivalent weight of 14N (i.e., 1 mole of 14N or 15N = 14 g N).
~atural abundance of 15N
For isotope dilution experiments, the isotope is
added as a solution to promote uniform distribution
throughout the soil. Generally, the greater the quantity of solution that is added, the more uniform the
distribution will be. Unfortunately, water addition
may stimulate nutrient transformation rates. Typically, the volume of solution has ranged from about
3 to 6 ml per 100 g of soil. Techniques for adding
isotope through the gas phase have been tested, but
these methods generally result in more than one soil
pool being labeled. Exposure of soil to 15NO gas
results in appearance of a high proportion of the
lsN in soil NO; and NO; pools and smaller proportions in NHt and soil organic N pools (Stark
and Firestone 1995; Rudolph et al. 1996). Exposure
of soils to 15NH3 results in labeling of soil organic
N as well as soil NHt (Burge and Broadbent 1961;
Nommik and Vahtras 1982). Labeling of nontarget
pools complicates the analysis considerably, and in
some cases violates the assumptions for use of the
isotope dilution equations.
The type of extractant used to determine concentrations can effect the rate estimates obtained by
isotope dilution. For example, dilute salt solutions
such as 0.5 M KCI or 0.5 M potassium sulfate
(K 2 S0 4 ) may not extract as much exchangeable
NHt as 2 M KCI does. Therefore, when dilute salt
solutions are used as the extractant during 15NHt
dilution measurements, adsorption of lSNHt onto
exchange sites may be confused with NHt consumption, and desorption of 14NHt from exchange
sites may be confused with mineralization. When
one designs an isotope dilution experiment, it is
critical that an extractant is selected that measures
a well-defined pool, and that all flows affecting that
pool are understood.
Isotope dilution measurements have a number of
advantages relative to tracer measurements. First,
John M. Stark
TABLE 14.3. Sample calculations for a I-day 15NO; isotope dilution experiment performed in intact soil cores (see
Fig. 14.8 for sampling method). Based on procedure described in Hart et al. (1994b) and Davidson et al. (1991).
Core pair I
Core pair II
soil sample:
time harvested (days):
Data
mass of 15NO; - N injected/core
(@ 99.9 atom % 15N) (Jlg N)":
mass of <2 mm oven-dry soil in core
(g o.d. soil):
2 M KCI-extractable NO; - N
(Jlg N g -1 o.d. soil):
mole fraction 15N in KCI-extractable
NO;:
Calculated Values
inner
to
300
320
2.3
0.3820
outer
to
1.6
0.0037 b
inner
tl
300
290
2.0
0.1760
outer
to
2.5
0.0037
mass of 15N added (Jlg N g - 1 o.d. soil):
mass of excess 15N recovered (Jlg N g-I):
extraction efficiency:
300/320 = 0.938
300/290 = 1.034
(2.3 * 0.382) - (1.6 * 0.0037) = 0.873
0.873/0.938 = 0.931
mole fraction 15N excess estimated to be
in the intact core at time - 0:
(1.034 * 0.931 + 2.5 * 0.0037)1(2.5 + 1.034 * 0.931) - 0.0037 = 0.277
time-O
time-1
Calculation of Gross Rates
pool size (Jlg N/g o.d. soil):
mole fraction 15N excess:
2.5 + 1.034 * 0.931 = 3.46
0.277
2.0
0.1760 - 0.0037 = 0.1723
gross nitrification rate (Jlg N g-1 o.d. soil day-I):
(3.46 - 2.0) log(0.277/0.1723)
1.26
1
log(3.46/2.0)
gross nitrate consumption rate
(Jlg N gl o.d. soil day-I):
(2.0 - 3.46)
1.26 -
1
= 2.72
aAll N masses are expressed in terms of the equivalent weight of 14N (i.e., 1 mole of 14N or 15N = 14 g N).
~atural abundance of 15N
For isotope dilution experiments, the isotope is
added as a solution to promote uniform distribution
throughout the soil. Generally, the greater the quantity of solution that is added, the more uniform the
distribution will be. Unfortunately, water addition
may stimulate nutrient transformation rates. Typically, the volume of solution has ranged from about
3 to 6 ml per 100 g of soil. Techniques for adding
isotope through the gas phase have been tested, but
these methods generally result in more than one soil
pool being labeled. Exposure of soil to 15NO gas
results in appearance of a high proportion of the
lsN in soil NO; and NO; pools and smaller proportions in NHt and soil organic N pools (Stark
and Firestone 1995; Rudolph et al. 1996). Exposure
of soils to 15NH3 results in labeling of soil organic
N as well as soil NHt (Burge and Broadbent 1961;
Nommik and Vahtras 1982). Labeling of nontarget
pools complicates the analysis considerably, and in
some cases violates the assumptions for use of the
isotope dilution equations.
The type of extractant used to determine concentrations can effect the rate estimates obtained by
isotope dilution. For example, dilute salt solutions
such as 0.5 M KCI or 0.5 M potassium sulfate
(K 2 S0 4 ) may not extract as much exchangeable
NHt as 2 M KCI does. Therefore, when dilute salt
solutions are used as the extractant during 15NHt
dilution measurements, adsorption of lSNHt onto
exchange sites may be confused with NHt consumption, and desorption of 14NHt from exchange
sites may be confused with mineralization. When
one designs an isotope dilution experiment, it is
critical that an extractant is selected that measures
a well-defined pool, and that all flows affecting that
pool are understood.
Isotope dilution measurements have a number of
advantages relative to tracer measurements. First,