Principles of terrestrial ecosystem ecology.pdf

Nitrogen uptake (g m -2 yr -1 )
15
10
5
Evergreen forest
Deciduous forest
0
0 500 1000 1500
Production (g m -2 yr -1 )
Figure 8.5. Relationship between nitrogen uptake
of temperate and boreal coniferous and deciduous
forests and NPP. (Redrawn with permission from
Academic Press; Chapin 1993b.)
Changes in root uptake kinetics fine-tune the
capacity of plants to acquire specific nutrients.
Ion-transport proteins are specific for particular
ions. In other words, ammonium, nitrate,
phosphate, potassium, and sulfate are each
transported by a different membrane-bound
protein that is individually regulated (Clarkson
1985). Plants induce the synthesis of additional
transport proteins for those ions that specifically
limit plant growth. Roots of a phosphoruslimited
plant therefore have a high capacity to
absorb phosphate, whereas roots of a nitrogenlimited
plant have a high capacity to absorb
nitrate and ammonium (Table 8.4). Nitrate
reductase, the enzyme that reduces nitrate to
ammonium (the first step before nitratenitrogen
can be incorporated into amino acids
for biosynthesis) is also specifically induced by
presence of nitrate. There are therefore
several adjustments that plants make to
improve resource balance. Plants first alter the
root to shoot ratio to improve the balance
between acquisition of belowground and
aboveground resources. Plants then regulate
the location of root growth to exploit hot spots
of nutrient availability. Finally, plants adjust
their capacity to absorb specific nutrients, which
brings the plant nutrient ratios closer to values
that are optimal for growth.
Nutrient Uptake 187
Nutrient ratios define a stoichiometry of
nutrient cycles in ecosystems. The oceanographer
Redfield (1958) noted that algae with a
ratio of nitrogen to phosphorus (N:P ratio)
greater than 14:1 tend to respond to phosphorus
addition, whereas algae with a lower N:P
ratio are nitrogen limited. This ratio is now
referred to as the Redfield ratio. The important
implication of nutrient ratios is that, if they
are constant, the element that most strongly
constrains production by vegetation defines
the quantities of all elements that are cycled
through vegetation. Marine algae with high
N:P ratios, for example, preferentially absorb
phosphorus. They absorb nitrogen and other
nutrients in proportion (the Redfield ratio)
to the phosphorus that they are able to acquire.
Algae with low N:P ratios preferentially
absorb nitrogen and absorb phosphorus and
other nutrients in proportion to the nitrogen
that they are able to acquire. The most strongly
limiting element therefore determines the
cycling rates of all elements. Experiments in
terrestrial ecosystems suggest that terrestrial
plants also adjust their mineral nutrition to converge
on the same Redfield ratio. Heath plants
with N:P ratios in leaves of less than 14:1
generally respond to experimental additions
of nitrogen, whereas plants with N:P ratios
greater than 16:1 generally respond to added
phosphorus but not to nitrogen (Fig. 8.6). This
Table 8.4. Effect of environmental stresses on rate
of nutrient absorption by barley.
Uptake rate by
stressed plants
Stress Ion absorbed (% of control)
Nitrogen ammonium 209
nitrate 206
phosphate 56
sulfate 56
Phosphorus phosphate 400
nitrate 35
sulfate 70
Sulfur sulfate 895
nitrate 69
phosphate 32
Water phosphate 32
Light nitrate 73
Data from Lee (1982), Lee and Rudge (1987), and Chapin
(1991a).