Principles of terrestrial ecosystem ecology.pdf

188 8. Terrestrial Plant Nutrient Use
P content (mg g -1 )
4
3
2
1
0 0
N-limitation
5
10 15 20
N content (mg g -1 )
P-limitation
co-limitation by N and P
N:P 16
Figure 8.6. Relationship between nitrogen and
phosphorus concentration of leaves in heath plants.
Each datum point represents a site where nutrient
addition experiments show that plant growth is
limited by nitrogen, phosphorus, or both. Plants with
an N : P ratio less than 14 respond primarily to nitrogen,
whereas plants with an N : P ratio greater than
16 respond primarily to phosphorus. (Redrawn with
permission from Journal of Applied Ecology;
Koerselman and Mueleman 1996.)
principle of optimal element ratios is used
in agriculture to determine which nutrients
limit crop growth so nutrient additions can be
matched to plant requirements.
Element ratios are more variable among terrestrial
plants they are among phytoplankton
because of the greater capacity for nutrient
storage. Terrestrial plants have storage organs
(e.g., stems) and organelles (e.g., vacuoles) in
which they store nutrients that are nonlimiting
to growth. In this way, terrestrial plants can take
advantage of short-term pulses of nutrient
supply. Nitrogen and phosphorus, for example,
often show an autumn pulse of availability,
when leaves are shed and leached by rain,
and a spring pulse, after a winter season when
decomposers are more active than plants (see
Chapter 6). Plants absorb these nutrients at
times of abundant supply, altering the ratios of
elements in their tissues. These nutrients are
then drawn out of storage at times when the
demands for growth exceed uptake from the
soil. In arctic tundra, for example, each year’s
production is supported primarily by nutrients
absorbed in previous years, and uptake serves
25
primarily to replenish these stores (Chapin
et al. 1980a). In one field experiment, tundra
plants that were provided with only distilled
water grew just as rapidly as did plants with
free access to soil nutrients, indicating that nutrient
stores were sufficient to support an entire
season’s production (Jonasson and Chapin
1985). Even in the ocean and freshwater ecosystems,
element ratios of algae can be variable
due to storage of nonlimiting nutrients in vacuoles
(see Chapter 10).
Nutrient uptake alters the chemical properties
of the rhizosphere. Nutrient absorption
by plant roots reduces the concentration of
nutrients adjacent to the root. This depletion
of soluble nutrients by root uptake can be
substantial for nutrients that diffuse readily
and create large diffusion shells. The pool sizes
of dissolved nutrients in the soil solution are
therefore a poor indicator of nutrient availability;
dissolved nutrient pools can be small
because of low mineralization rates or rapid
uptake. Plant nutrient uptake is a critical
control over ecosystem retention of mobile
nutrients such as nitrate. Forest clearing or
crop removal, for example, makes soils more
prone to nitrate leaching into groundwater and
streams (Bormann and Likens 1979).
A second major consequence of plant nutrient
absorption is a change in rhizosphere pH.
Whenever a root absorbs an excess of cations,
it secretes hydrogen ions (H + ) into the rhizosphere
to maintain electrical neutrality. This H +
secretion acidifies the rhizosphere. Except
for nitrogen, which can be absorbed either
as a cation (NH4 + ) or an anion (NO3 - ), the ions
absorbed in greatest quantities by plants are
cations (e.g., Ca 2+ ,K + ,Mg 2+ ), with phosphate
and sulfate being the major anions (Table 8.2).
When plants absorb most nitrogen as NH4 + ,
their cation uptake greatly exceeds anion
uptake, and they secrete H + into the rhizosphere
to maintain charge balance, causing
acidification of the rhizosphere. When plants
absorb most nitrogen as NO3 - , their
cation–anion absorption is more nearly balanced,
and roots have less effect on rhizosphere
pH.Ammonium tends to be the dominant form
of inorganic nitrogen in acidic soils, whereas
nitrate makes up a larger proportion of inor-