222
bag may be simply buried at a particular depth in
the soil; however, this results in soil disturbance
and precludes repeated measurements at the same
location. Plastic access tubes have been placed at
an angle in the soil, and then the resin bag is lowered down the tube with a string until it contacts
the soil at the end of the tube. A wooden rod can
be used to press the resin bag firmly against the
soil. Attachment of a string to the bag allows it to
be withdrawn and replaced at various time intervals. Other researchers have placed a ring of stiff
rubber tubing inside each resin bag to force the bag
into a disk shape. This ion exchange disk can be
placed horizontally in the soil or attached to the
base of a soil core to measure leaching rates of nutrients (DiStefano and Gholz 1986; Hart and Firestone 1989; Zou et al. 1992). Use of circular ion
exchange disks with known areas allows the flux
rates to be expressed per unit area of soil surface.
In another approach, ion exchange membranes,
rather than IER beads, are buried in soil (Gibson
1986, Subler et al. 1995). A known area of membrane is wrapped around a metal stake, which is
driven into the soil to a predetermined depth. This
"IER spike" can then be removed and replaced with
a fresh one for repeated measurement at the same
location with minimal soil disturbance.
One problem with interpretation of nutrient data
collected using IERs is that fluxes are a function
not only of nutrient production processes and competing consumption processes, but also of nutrient
transport processes. Ions with higher diffusivities
will appear in greater concentrations on IERs than
ions with low diffusivities. Differences in soil water
contents will cause different nutrient accumulation
rates on IERs because of differences in rates of diffusion or leaching. In soils with greater moisture
contents, transport processes will be faster, and a
larger volume of soil will be "sampled" by the
resin. This makes it difficult to express flux rates
on a soil weight or area basis. The use of IER disks
attached to plastic or metal cylinders containing soil
cores, or the use of IER membranes may help solve
this problem.
Another problem with the use of IERs for estimating process rates is that it is usually not known
how efficiently the IERs compete with other processes. If the IERs are a perfect competitor with a
particular consumption process, then the rate of accumulation of nutrient ions on the IERs will proJohn M. Stark
vide a good estimate of the gross rate of that consumption process. If they are only an incomplete
competitor, then data from the IERs will underestimate the gross rate. For example, IER disks sealed
to the base of plastic or metal soil cores are very
efficient at stripping NHt and NO; out of water
that passes through the IER disk, and thus they are
essentially perfect competitors with the leaching
process. On the other hand, IER bags do not absorb
all of the nutrient ions released by mineralization
processes before microorganisms or plant roots
take up the ions. Therefore, the rate of accumulation of nutrient ions on IER bags will underestimate
gross mineralization rates. Because of this, IER
methods are considered to provide an index to mineralization rates and plant uptake rather than an estimate of actual gross rates (Binkley and Hart
1989).
Rate Measurements Using
Substrate Analogs
Enzymes responsible for nutrient transformations
are often capable of transforming other compounds
as well. If these alternate substrates are normally
present in low concentrations, and if relatively simple methods are available for quantification of either the substrate or the product, then this substrate
may be used as a non-isotope tracer for calculating
rates. Examples of compounds that have been used
are: acetylene for estimating rates of N fixation
(Weaver and Danso 1994), sodium p-nitrophenyl
phosphate for estimating rates of P mineralization,
and 2,3,5-triphenyltetrazolium chloride for estimating rates of microbial respiration (dehydrogenase activity) (Tabatabai 1994). While techniques
utilizing substrate analogs as non-isotope tracers
have been used extensively in laboratory studies,
they have been used relatively little for field measurements. One reason for this is that conversion
factors are necessary to calculate nutrient transformation rates based on rates of transformation of
the substrate analog. The conversion factors often
vary with the incubation conditions (e.g., moisture
content, temperature, soil disturbance, etc.), and
control of these incubation conditions may not
be possible under field conditions. A second problem is that addition of the substrate analog is likely
to stimulate rates. Therefore, transformation rates
estimated using substrate analogs are usually re-
bag may be simply buried at a particular depth in
the soil; however, this results in soil disturbance
and precludes repeated measurements at the same
location. Plastic access tubes have been placed at
an angle in the soil, and then the resin bag is lowered down the tube with a string until it contacts
the soil at the end of the tube. A wooden rod can
be used to press the resin bag firmly against the
soil. Attachment of a string to the bag allows it to
be withdrawn and replaced at various time intervals. Other researchers have placed a ring of stiff
rubber tubing inside each resin bag to force the bag
into a disk shape. This ion exchange disk can be
placed horizontally in the soil or attached to the
base of a soil core to measure leaching rates of nutrients (DiStefano and Gholz 1986; Hart and Firestone 1989; Zou et al. 1992). Use of circular ion
exchange disks with known areas allows the flux
rates to be expressed per unit area of soil surface.
In another approach, ion exchange membranes,
rather than IER beads, are buried in soil (Gibson
1986, Subler et al. 1995). A known area of membrane is wrapped around a metal stake, which is
driven into the soil to a predetermined depth. This
"IER spike" can then be removed and replaced with
a fresh one for repeated measurement at the same
location with minimal soil disturbance.
One problem with interpretation of nutrient data
collected using IERs is that fluxes are a function
not only of nutrient production processes and competing consumption processes, but also of nutrient
transport processes. Ions with higher diffusivities
will appear in greater concentrations on IERs than
ions with low diffusivities. Differences in soil water
contents will cause different nutrient accumulation
rates on IERs because of differences in rates of diffusion or leaching. In soils with greater moisture
contents, transport processes will be faster, and a
larger volume of soil will be "sampled" by the
resin. This makes it difficult to express flux rates
on a soil weight or area basis. The use of IER disks
attached to plastic or metal cylinders containing soil
cores, or the use of IER membranes may help solve
this problem.
Another problem with the use of IERs for estimating process rates is that it is usually not known
how efficiently the IERs compete with other processes. If the IERs are a perfect competitor with a
particular consumption process, then the rate of accumulation of nutrient ions on the IERs will proJohn M. Stark
vide a good estimate of the gross rate of that consumption process. If they are only an incomplete
competitor, then data from the IERs will underestimate the gross rate. For example, IER disks sealed
to the base of plastic or metal soil cores are very
efficient at stripping NHt and NO; out of water
that passes through the IER disk, and thus they are
essentially perfect competitors with the leaching
process. On the other hand, IER bags do not absorb
all of the nutrient ions released by mineralization
processes before microorganisms or plant roots
take up the ions. Therefore, the rate of accumulation of nutrient ions on IER bags will underestimate
gross mineralization rates. Because of this, IER
methods are considered to provide an index to mineralization rates and plant uptake rather than an estimate of actual gross rates (Binkley and Hart
1989).
Rate Measurements Using
Substrate Analogs
Enzymes responsible for nutrient transformations
are often capable of transforming other compounds
as well. If these alternate substrates are normally
present in low concentrations, and if relatively simple methods are available for quantification of either the substrate or the product, then this substrate
may be used as a non-isotope tracer for calculating
rates. Examples of compounds that have been used
are: acetylene for estimating rates of N fixation
(Weaver and Danso 1994), sodium p-nitrophenyl
phosphate for estimating rates of P mineralization,
and 2,3,5-triphenyltetrazolium chloride for estimating rates of microbial respiration (dehydrogenase activity) (Tabatabai 1994). While techniques
utilizing substrate analogs as non-isotope tracers
have been used extensively in laboratory studies,
they have been used relatively little for field measurements. One reason for this is that conversion
factors are necessary to calculate nutrient transformation rates based on rates of transformation of
the substrate analog. The conversion factors often
vary with the incubation conditions (e.g., moisture
content, temperature, soil disturbance, etc.), and
control of these incubation conditions may not
be possible under field conditions. A second problem is that addition of the substrate analog is likely
to stimulate rates. Therefore, transformation rates
estimated using substrate analogs are usually re-