14. Nutrient Transfonnations
(e.g., C/N ratio) of the substrate available to the soil
microbial community (Hart et al. 1994b).
Non-Isotope Methods
Non-isotope methods for estimating nutrient transformation rates can be divided into the following
categories: (1) net rate measurements combined
with inhibitors, (2) net rate estimates from nutrient
budgets, (3) net rate measurements with "super
sinks," and (4) net rate measurements using substrate analogs as non-isotope tracers. In the vast majority of experiments, estimates of transformation
rates are obtained by combining net rate measurements with some type of inhibitor.
Net Rate Measurements with Inhibitors
As discussed previously, changes in nutrient concentration provide information on gross process
rates only in extremely simple systems in which
either only production or only consumption occurs.
For example, the weathering rate of a primary mineral might be estimated from the changes in the
quantity of mineral present at two points in time
(see Fig. 14.1). The assumptions of this method are
that (1) there are no confounding production or
consumption processes and (2) rates are fast
enough relative to measurement error that a significant change in concentration occurs during the
course of the incubation period. Unfortunately, few
systems are simple enough that changes in concentrations alone provide much information on gross
process rates. Instead, net rate measurements are
almost always combined with process inhibitors of
some type to block confounding processes.
Inhibitors used to block confounding processes
may be either physical or chemical. Physical barriers that prevent nutrient losses by mass flow and
diffusion, or that exclude plant roots and prevent
plant uptake are the most commonly used inhibitors. These include metal or plastic cylinders, solid
or mesh partitions, or soil covers. A variety of
chemical compounds are also frequently used to inhibit specific biogeochemical processes (Table
14.1) (Oremland and Capone 1988).
An experiment reported by Schimel et al. (1984)
provides an elegant example of how gross process
rates can be estimated in a complicated system by
217
measuring net rates in the presence of inhibitors.
Their objective was to measure rates of heterotrophic nitrification during laboratory incubations of
forest soil samples. Heterotrophic nitrification involves the conversion of ammonium (NHt) or organic N to nitrite (NO;) and nitrate (NO;), and in
contrast to autotrophic nitrification (in which
NHt is converted to NO; and NO;), heterotrophic nitrification is unaffected by acetylene. Schimel et al. (1984) used a modification of the nitrification potential assay described by Belser and
Mays (1980) (Fig. 14.2). They incubated soil slurries in flasks, which obviously eliminated leaching
and plant uptake as consumptive processes; they
shook the samples to maintain high oxygen (0 2 )
concentrations, which blocked denitrification; they
added NHt , which inhibited microbial assimilation
of NO; and NO;; and they added acetylene to
block autotrophic nitrification. By blocking all production and consumption processes except the one
process of interest (heterotrophic nitrification) they
were able to use the net rate of NO; and NO;
production to estimate the gross production rate.
Sometimes it is not possible to block all of the
confounding processes. An alternative approach is
to block the process of interest in one soil sample,
and compare net rates in this sample with net rates
in another sample in which the process has not been
blocked. Assuming that all other process rates are
the same in both soil samples, the difference between the two net rates provides an estimate of the
gross rate. For example, Stark and Hart (1997) estimated field rates of gross nitrification in soils from
a variety of forest ecosystems by comparing net
rates of nitrate production in samples either exposed or not exposed to acetylene (C 2 H 2 ). In samples not exposed to C 2 H 2 , net NO; production
equals the difference between gross nitrification
and gross NO; consumption:
net NO; production
= gross nitrification
- gross NO; consumption (14.2)
In samples exposed to C 2 H2, gross nitrification is
blocked:
net NO; productiol\w/c2H2)
= - gross NO; consumption (14.3)
substituting Equation 14.3 into Equation 14.2
gives:
(e.g., C/N ratio) of the substrate available to the soil
microbial community (Hart et al. 1994b).
Non-Isotope Methods
Non-isotope methods for estimating nutrient transformation rates can be divided into the following
categories: (1) net rate measurements combined
with inhibitors, (2) net rate estimates from nutrient
budgets, (3) net rate measurements with "super
sinks," and (4) net rate measurements using substrate analogs as non-isotope tracers. In the vast majority of experiments, estimates of transformation
rates are obtained by combining net rate measurements with some type of inhibitor.
Net Rate Measurements with Inhibitors
As discussed previously, changes in nutrient concentration provide information on gross process
rates only in extremely simple systems in which
either only production or only consumption occurs.
For example, the weathering rate of a primary mineral might be estimated from the changes in the
quantity of mineral present at two points in time
(see Fig. 14.1). The assumptions of this method are
that (1) there are no confounding production or
consumption processes and (2) rates are fast
enough relative to measurement error that a significant change in concentration occurs during the
course of the incubation period. Unfortunately, few
systems are simple enough that changes in concentrations alone provide much information on gross
process rates. Instead, net rate measurements are
almost always combined with process inhibitors of
some type to block confounding processes.
Inhibitors used to block confounding processes
may be either physical or chemical. Physical barriers that prevent nutrient losses by mass flow and
diffusion, or that exclude plant roots and prevent
plant uptake are the most commonly used inhibitors. These include metal or plastic cylinders, solid
or mesh partitions, or soil covers. A variety of
chemical compounds are also frequently used to inhibit specific biogeochemical processes (Table
14.1) (Oremland and Capone 1988).
An experiment reported by Schimel et al. (1984)
provides an elegant example of how gross process
rates can be estimated in a complicated system by
217
measuring net rates in the presence of inhibitors.
Their objective was to measure rates of heterotrophic nitrification during laboratory incubations of
forest soil samples. Heterotrophic nitrification involves the conversion of ammonium (NHt) or organic N to nitrite (NO;) and nitrate (NO;), and in
contrast to autotrophic nitrification (in which
NHt is converted to NO; and NO;), heterotrophic nitrification is unaffected by acetylene. Schimel et al. (1984) used a modification of the nitrification potential assay described by Belser and
Mays (1980) (Fig. 14.2). They incubated soil slurries in flasks, which obviously eliminated leaching
and plant uptake as consumptive processes; they
shook the samples to maintain high oxygen (0 2 )
concentrations, which blocked denitrification; they
added NHt , which inhibited microbial assimilation
of NO; and NO;; and they added acetylene to
block autotrophic nitrification. By blocking all production and consumption processes except the one
process of interest (heterotrophic nitrification) they
were able to use the net rate of NO; and NO;
production to estimate the gross production rate.
Sometimes it is not possible to block all of the
confounding processes. An alternative approach is
to block the process of interest in one soil sample,
and compare net rates in this sample with net rates
in another sample in which the process has not been
blocked. Assuming that all other process rates are
the same in both soil samples, the difference between the two net rates provides an estimate of the
gross rate. For example, Stark and Hart (1997) estimated field rates of gross nitrification in soils from
a variety of forest ecosystems by comparing net
rates of nitrate production in samples either exposed or not exposed to acetylene (C 2 H 2 ). In samples not exposed to C 2 H 2 , net NO; production
equals the difference between gross nitrification
and gross NO; consumption:
net NO; production
= gross nitrification
- gross NO; consumption (14.2)
In samples exposed to C 2 H2, gross nitrification is
blocked:
net NO; productiol\w/c2H2)
= - gross NO; consumption (14.3)
substituting Equation 14.3 into Equation 14.2
gives: