14. Nutrient Transformations
Rate Measurements Obtained
from Nutrient Budgets
Nutrient budgets can be used to estimate nutrient
transformation rates if information is available on
a variety of nutrient pools and rates of associated
production and consumption processes. First, the
net change in all associated nutrient pools is calculated, or if the system is at steady state a net
change of zero may be assumed. Then the sum of
all known consumption rates is subtracted from the
sum of all known production rates. Any discrepancy between the net change in nutrient pools and
the balance of production and consumption (i.e., the
left- and right-hand sides of Equation 14.1) is attributed to the unknown process rate. Nutrient budgets have been most frequently used to estimate
ecosystem inputs or outputs of nutrients over relatively long time scales (Bormann et al. 1977; Peterjohn and Schlesinger 1990; Binkley et al. 1992b).
For example, rates of N fixation for entire ecosystems have been estimated by directly measuring inputs, such as wet and dry deposition, and outputs,
such as leaching and denitrification, and then either
measuring the increase in N in the ecosystem or
assuming that steady state exists (for examples see
Chapter 16 and references in Boring et al. 1988).
Budgets can also be used for smaller units than ecosystems. For example, net changes in plant N can
be used to estimate N fixation rates if other plant N
gains and losses can be quantified. A nutrient budget approach could also have been used to estimate
the rate of heterotrophic nitrification (see Fig. 14.2)
if the net change in NOi and NO; were known,
and if instead of blocking the other rates (autotrophic nitrification, plant uptake, microbial assimilation, denitrification, and leaching), it were possible to measure them directly.
One of the most important problems with the use
of budgets is that the rate of interest is estimated
by difference, and thus the error for the rate estimate represents the sum of the errors for all other
rate estimates. Therefore, rate estimates from budgets often have very large uncertainties (Binkley et
al. 1992a).
Net Rate Measurements
with "Super Sinks"
In some cases, it is possible to add a component
that absorbs the nutrient of interest. If the sink
strength of this component is great enough, it will
221
compete with other consumptive processes for the
nutrient, and it will not release the nutrient back
into the system. Accumulation of nutrients in the
added sink can then be used to estimate rates. Examples of such "super sinks" are acid traps, base
traps, and ion exchange resins.
Traps containing a strong acid, such as hydrochloric acid (HCI) or sulfuric acid (H z S0 4 ), have
been used to trap ammonia (NH 3 ) gas that has volatilized from soil or plant surfaces (e.g., Schlesinger and Peterjohn 1991), and traps containing a
strong base, such as potassium hydroxide (KOH)
or lime, have been used to trap carbon dioxide
(CO z ) (Van Cleve et al. 1979). Dishes or vials COntaining the acid or base can be placed inside soil
covers, or a known volume of air can be circulated
through traps (Van Cleve et al. 1979; Klubeck and
Skujins 1981). When used correctly, these traps are
highly efficient and do not release the nutrient back
into the system. Precautions must be taken to ensure that the trapping capacity is sufficient so that
the traps do not approach saturation during the incubation period. Also, the surface area of the absorbing portion of the trap must be high enough
that diffusion into the trap is not restricted. Raich
and Nadelhoffer (1989) observed that during measurement of COz flux from soils, the area of the
alkali trap should be at least 6% of the area occupied by the soil cover.
Ion exchange resins (IERs) are typically used to
examine production of soluble nutrient ions, either
as an index to how much plant-available nutrient is
produced or to measure leaching losses of nutrients
(Sibbeson 1977, Binkley and Hart 1989). Ion exchange resin beads are placed in nylon mesh bags
(made from nylon stockings) and then buried in the
soil for time periods ranging from a few weeks to
several months, depending on nutrient flux rates
and the exchange capacity of the IER (Binkley and
Matson 1983). The resin bags are retrieved, extracted in a strong salt solution (e.g., 2 M KCI), and
the extract is analyzed for nutrient concentrations.
Either cation or anion exchange resins may be used,
depending on whether the nutrient exists as a cation
(e.g., Ca z +, Mgz+), anion (e.g., PO~-, SO~-), or
both (e.g., NUt, NO;). Mixed-bed resins, which
contain a mixture of both cation and anion exchange resins, are most commonly used because
they act as a sink for a variety of nutrient ions.
Various placement methods have been used. The
Rate Measurements Obtained
from Nutrient Budgets
Nutrient budgets can be used to estimate nutrient
transformation rates if information is available on
a variety of nutrient pools and rates of associated
production and consumption processes. First, the
net change in all associated nutrient pools is calculated, or if the system is at steady state a net
change of zero may be assumed. Then the sum of
all known consumption rates is subtracted from the
sum of all known production rates. Any discrepancy between the net change in nutrient pools and
the balance of production and consumption (i.e., the
left- and right-hand sides of Equation 14.1) is attributed to the unknown process rate. Nutrient budgets have been most frequently used to estimate
ecosystem inputs or outputs of nutrients over relatively long time scales (Bormann et al. 1977; Peterjohn and Schlesinger 1990; Binkley et al. 1992b).
For example, rates of N fixation for entire ecosystems have been estimated by directly measuring inputs, such as wet and dry deposition, and outputs,
such as leaching and denitrification, and then either
measuring the increase in N in the ecosystem or
assuming that steady state exists (for examples see
Chapter 16 and references in Boring et al. 1988).
Budgets can also be used for smaller units than ecosystems. For example, net changes in plant N can
be used to estimate N fixation rates if other plant N
gains and losses can be quantified. A nutrient budget approach could also have been used to estimate
the rate of heterotrophic nitrification (see Fig. 14.2)
if the net change in NOi and NO; were known,
and if instead of blocking the other rates (autotrophic nitrification, plant uptake, microbial assimilation, denitrification, and leaching), it were possible to measure them directly.
One of the most important problems with the use
of budgets is that the rate of interest is estimated
by difference, and thus the error for the rate estimate represents the sum of the errors for all other
rate estimates. Therefore, rate estimates from budgets often have very large uncertainties (Binkley et
al. 1992a).
Net Rate Measurements
with "Super Sinks"
In some cases, it is possible to add a component
that absorbs the nutrient of interest. If the sink
strength of this component is great enough, it will
221
compete with other consumptive processes for the
nutrient, and it will not release the nutrient back
into the system. Accumulation of nutrients in the
added sink can then be used to estimate rates. Examples of such "super sinks" are acid traps, base
traps, and ion exchange resins.
Traps containing a strong acid, such as hydrochloric acid (HCI) or sulfuric acid (H z S0 4 ), have
been used to trap ammonia (NH 3 ) gas that has volatilized from soil or plant surfaces (e.g., Schlesinger and Peterjohn 1991), and traps containing a
strong base, such as potassium hydroxide (KOH)
or lime, have been used to trap carbon dioxide
(CO z ) (Van Cleve et al. 1979). Dishes or vials COntaining the acid or base can be placed inside soil
covers, or a known volume of air can be circulated
through traps (Van Cleve et al. 1979; Klubeck and
Skujins 1981). When used correctly, these traps are
highly efficient and do not release the nutrient back
into the system. Precautions must be taken to ensure that the trapping capacity is sufficient so that
the traps do not approach saturation during the incubation period. Also, the surface area of the absorbing portion of the trap must be high enough
that diffusion into the trap is not restricted. Raich
and Nadelhoffer (1989) observed that during measurement of COz flux from soils, the area of the
alkali trap should be at least 6% of the area occupied by the soil cover.
Ion exchange resins (IERs) are typically used to
examine production of soluble nutrient ions, either
as an index to how much plant-available nutrient is
produced or to measure leaching losses of nutrients
(Sibbeson 1977, Binkley and Hart 1989). Ion exchange resin beads are placed in nylon mesh bags
(made from nylon stockings) and then buried in the
soil for time periods ranging from a few weeks to
several months, depending on nutrient flux rates
and the exchange capacity of the IER (Binkley and
Matson 1983). The resin bags are retrieved, extracted in a strong salt solution (e.g., 2 M KCI), and
the extract is analyzed for nutrient concentrations.
Either cation or anion exchange resins may be used,
depending on whether the nutrient exists as a cation
(e.g., Ca z +, Mgz+), anion (e.g., PO~-, SO~-), or
both (e.g., NUt, NO;). Mixed-bed resins, which
contain a mixture of both cation and anion exchange resins, are most commonly used because
they act as a sink for a variety of nutrient ions.
Various placement methods have been used. The