216
John M. Stark
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Illi~'
:::l
"'. $I)
0:=
:::l 0
:::l
Atmosphere
litter
o
'"
;:;:
soil
decomposition
microbes
~~
...... e _ - - - - -
soil
colloids
cr :::l
~()il
solution
weathering
primary
minerals
secondary
minerals
Organic phase
Inorganic phase
Ground water
FIGURE 14.1. Generalized nutrient cycle for terrestrial ecosystems. (Modified from Aber and Melillo [1991].)
It can be seen from Equation 14.1 that the net
production rate equals the gross production rate
only when consumption processes are insignificant.
When nutrient consumption occurs simultaneously
with production, net rates underestimate gross
rates. Measurement of the net change in nutrient
concentration is the most common method for estimating transformation rates; however, when nutrient concentration is all that is measured, it is impossible to know for certain if production and
consumption are co-occurring. For this reason, I
will use the term "net rate" whenever a rate estimate
is based solely on changes in concentration. I will
use the term "gross rate" when isotopes are used to
evaluate whether production and consumption are
occurring simultaneously.
A third term often used to describe rates is the
term "potential rate." This term is used to indicate
that the incubation conditions were not representative of actual field conditions. In some, but not all
cases, the term implies that the rates are maximal.
For example, in the nitrification potential assay of
Belser and Mays (1980), net rates of nitrate
(NO)) production are measured in shaken soil
slurry samples incubated under nearly optimal conditions of moisture, temperature, and substrate
availability. Under these conditions, gross nitrification rates approach maximum values (V max), and
NO) consumption is inhibited so that net rates
equal gross rates. In other assays measuring "potential rates," however, both consumption and production may be stimulated such that net rates are
neither maximal nor indicative of gross rates. Examples are the nitrification potential and mineralization potential assays described by Stanford and
Smith (1972) and Robertson and Vitousek (1981)
in which moist sieved soil samples are incubated in
the laboratory for various time periods (1 to 30
weeks), and net rates of inorganic N production are
measured. In this assay, mixing and sieving tends
to increase the availability of organic matter, which
stimulates microbial activity. Because both production and consumption of inorganic N are stimulated, the potential net mineralization rate is not a
good indicator of the gross mineralization rate. Instead, it is better used as an indicator of the quality
John M. Stark
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Illi~'
:::l
"'. $I)
0:=
:::l 0
:::l
Atmosphere
litter
o
'"
;:;:
soil
decomposition
microbes
~~
...... e _ - - - - -
soil
colloids
cr :::l
~()il
solution
weathering
primary
minerals
secondary
minerals
Organic phase
Inorganic phase
Ground water
FIGURE 14.1. Generalized nutrient cycle for terrestrial ecosystems. (Modified from Aber and Melillo [1991].)
It can be seen from Equation 14.1 that the net
production rate equals the gross production rate
only when consumption processes are insignificant.
When nutrient consumption occurs simultaneously
with production, net rates underestimate gross
rates. Measurement of the net change in nutrient
concentration is the most common method for estimating transformation rates; however, when nutrient concentration is all that is measured, it is impossible to know for certain if production and
consumption are co-occurring. For this reason, I
will use the term "net rate" whenever a rate estimate
is based solely on changes in concentration. I will
use the term "gross rate" when isotopes are used to
evaluate whether production and consumption are
occurring simultaneously.
A third term often used to describe rates is the
term "potential rate." This term is used to indicate
that the incubation conditions were not representative of actual field conditions. In some, but not all
cases, the term implies that the rates are maximal.
For example, in the nitrification potential assay of
Belser and Mays (1980), net rates of nitrate
(NO)) production are measured in shaken soil
slurry samples incubated under nearly optimal conditions of moisture, temperature, and substrate
availability. Under these conditions, gross nitrification rates approach maximum values (V max), and
NO) consumption is inhibited so that net rates
equal gross rates. In other assays measuring "potential rates," however, both consumption and production may be stimulated such that net rates are
neither maximal nor indicative of gross rates. Examples are the nitrification potential and mineralization potential assays described by Stanford and
Smith (1972) and Robertson and Vitousek (1981)
in which moist sieved soil samples are incubated in
the laboratory for various time periods (1 to 30
weeks), and net rates of inorganic N production are
measured. In this assay, mixing and sieving tends
to increase the availability of organic matter, which
stimulates microbial activity. Because both production and consumption of inorganic N are stimulated, the potential net mineralization rate is not a
good indicator of the gross mineralization rate. Instead, it is better used as an indicator of the quality