14. Nutrient Transfonnations
lution measurement of gross rates in intact soil
cores presents some additional problems, however
(Davidson et al. 1991; Hart et al. 1994b). The first
problem is obtaining an accurate estimate of the
nutrient concentration and enrichment in the intact
soil core at time O. Subsamples of soil collected
adjacent to the intact core can be extracted to determine initial nutrient concentrations; however, the
greater the fine-scale spatial heterogeneity, the
greater the error associated with these estimates.
This error can be minimized by pounding a largediameter cylinder into the soil in a concentric circle
around the cylinder used for the intact soil core
(Fig. 14.8). The soil between the outer and inner
cylinders is homogenized and extracted immediately. The soil in the inner cylinder is left intact,
injected with isotope solution, incubated, and then
harvested at the end of the incubation period. Theoretically, it should be possible to estimate the initial concentration in the intact inner core by sum,
,
Outer cores
Inner core I
Injected wi 15 NO 3 soln.
Harvested immediately
to determine the
extraction efficiency
of 15N at time-O
Harvested immediately
to determine initial
NO 3 concentrations
Inner core II
Injected Wl 15 NO 3soln.
Incubated intact
Harvested after 1-3 d
FIGURE 14.8. Concentric core design used during
15N03 isotope dilution measurements of gross nitrification and nitrate consumption rates in intact soil cores.
See Table 14.3 for sample calculations.
227
ming the concentration measured in the soil
between the inner and outer cylinders with amount
of isotope added per unit mass of soil. A number
of experiments using 15NHt and 15N01 have
shown that considerably less than 100% of the 15N
is recovered in NHt or NO; pools even when the
soil is extracted within minutes of adding 15N solution (Davidson et al. 1991; Hart et al. 1994b,
Berntson and Aber 2000); thus, for 15N dilution experiments, this method results in overestimation of
the initial concentration in the intact core and overestimation of isotope enrichment. Because recovery
of isotope is often < 100% and varies from soil to
soil, it is necessary to inject the same isotope solution into a replicate soil core, which is then extracted immediately to determine the extraction efficiency (amount of isotope recovered/amount of
isotope injected) (Hart et al. 1994b). The extraction
efficiency is then used to estimate how much isotope would have been measured in the intact core
at time 0, if it had been extracted. Sample calculations showing how isotope dilution measurements can be made with intact cores are presented
in Table 14.3.
As with tracer experiments, the amount of isotope required depends on detection limits for
isotope measurement, the size of the pool to be labeled, and gross rates of production and consumption. For example when 15N is used, >20 Ilg N are
usually required for analysis by diffusion techniques and direct combustion-mass spectrometry
(Barrie et al. 1989, Stark and Hart 1996). If ambient
concentrations in soil are low, then it is generally
wise to add enough N in the isotope solution to
ensure that at least 20 Ilg of 14N + 15N will be
recovered in soil extracts. It is also necessary to add
enough isotope that changes in enrichment during
the incubation can be detected. In this case, detection is not limited by isotope analysis; it is limited
by the ability to accurately estimate time-O concentrations and enrichments. Because time-O concentrations are estimated from soil adjacent to the intact core, errors in estimates may frequently be as
high as ± 20% (1. Stark, unpublished data). For this
reason, sufficient isotope is usually added to create
enrichments of 40 to 60 atom % 15N. During 24to 48-hr incubations of soils with active microbial
populations, enrichments will often decline to 10 to
30 atom % 15N, which is a change that can be easily
detected.
lution measurement of gross rates in intact soil
cores presents some additional problems, however
(Davidson et al. 1991; Hart et al. 1994b). The first
problem is obtaining an accurate estimate of the
nutrient concentration and enrichment in the intact
soil core at time O. Subsamples of soil collected
adjacent to the intact core can be extracted to determine initial nutrient concentrations; however, the
greater the fine-scale spatial heterogeneity, the
greater the error associated with these estimates.
This error can be minimized by pounding a largediameter cylinder into the soil in a concentric circle
around the cylinder used for the intact soil core
(Fig. 14.8). The soil between the outer and inner
cylinders is homogenized and extracted immediately. The soil in the inner cylinder is left intact,
injected with isotope solution, incubated, and then
harvested at the end of the incubation period. Theoretically, it should be possible to estimate the initial concentration in the intact inner core by sum,
,
Outer cores
Inner core I
Injected wi 15 NO 3 soln.
Harvested immediately
to determine the
extraction efficiency
of 15N at time-O
Harvested immediately
to determine initial
NO 3 concentrations
Inner core II
Injected Wl 15 NO 3soln.
Incubated intact
Harvested after 1-3 d
FIGURE 14.8. Concentric core design used during
15N03 isotope dilution measurements of gross nitrification and nitrate consumption rates in intact soil cores.
See Table 14.3 for sample calculations.
227
ming the concentration measured in the soil
between the inner and outer cylinders with amount
of isotope added per unit mass of soil. A number
of experiments using 15NHt and 15N01 have
shown that considerably less than 100% of the 15N
is recovered in NHt or NO; pools even when the
soil is extracted within minutes of adding 15N solution (Davidson et al. 1991; Hart et al. 1994b,
Berntson and Aber 2000); thus, for 15N dilution experiments, this method results in overestimation of
the initial concentration in the intact core and overestimation of isotope enrichment. Because recovery
of isotope is often < 100% and varies from soil to
soil, it is necessary to inject the same isotope solution into a replicate soil core, which is then extracted immediately to determine the extraction efficiency (amount of isotope recovered/amount of
isotope injected) (Hart et al. 1994b). The extraction
efficiency is then used to estimate how much isotope would have been measured in the intact core
at time 0, if it had been extracted. Sample calculations showing how isotope dilution measurements can be made with intact cores are presented
in Table 14.3.
As with tracer experiments, the amount of isotope required depends on detection limits for
isotope measurement, the size of the pool to be labeled, and gross rates of production and consumption. For example when 15N is used, >20 Ilg N are
usually required for analysis by diffusion techniques and direct combustion-mass spectrometry
(Barrie et al. 1989, Stark and Hart 1996). If ambient
concentrations in soil are low, then it is generally
wise to add enough N in the isotope solution to
ensure that at least 20 Ilg of 14N + 15N will be
recovered in soil extracts. It is also necessary to add
enough isotope that changes in enrichment during
the incubation can be detected. In this case, detection is not limited by isotope analysis; it is limited
by the ability to accurately estimate time-O concentrations and enrichments. Because time-O concentrations are estimated from soil adjacent to the intact core, errors in estimates may frequently be as
high as ± 20% (1. Stark, unpublished data). For this
reason, sufficient isotope is usually added to create
enrichments of 40 to 60 atom % 15N. During 24to 48-hr incubations of soils with active microbial
populations, enrichments will often decline to 10 to
30 atom % 15N, which is a change that can be easily
detected.