Principles of terrestrial ecosystem ecology.pdf

180 8. Terrestrial Plant Nutrient Use
tically through the soil by saturated flow whenever
the water content exceeds the soil’s waterholding
capacity. Because nutrient availability
and mineralization rates are generally highest
in the uppermost soils, this vertical flow of
water redistributes nutrients and replenishes
diffusion shells surrounding roots. Both root
growth and vertical soil water movement occur
preferentially in soil cracks, quickly eliminating
diffusion shells around these roots. Saturated
flow is also important in ecosystems where
there is regular horizontal flow of ground water
across an impermeable soil layer. Deep-rooted
species in tundra underlain by permafrost, for
example, have 10-fold greater nutrient uptake
and productivity in areas of rapid subsurface
flow than in areas without lateral groundwater
flow (Chapin et al. 1988). The high productivity
of trees and shrubs in riparian ecosystems
results in part because their roots often extend
to the water table and to groundwater beneath
the stream (the hyporrheic zone), where roots
tap the saturated flow of nutrients through the
rooting zone.
Root Interception
Root interception is not an important mechanism
for directly supplying nutrients to roots.
As roots elongate into new soil, they intercept
available nutrients in this unoccupied soil.
The quantity of available nitrogen, phosphorus,
and potassium per unit soil volume is, however,
always less than the quantity of nutrients
required to construct the root, so root interception
can never be an important mechanism
of nutrient supply to the shoot. Root growth is
critical, not because it intercepts nutrients, but
because it explores new soil volume and creates
new root surface to which nutrients can move
by diffusion and mass flow.
Nutrient Uptake
Nutrient uptake. Who is in charge? Three
factors govern nutrient uptake by vegetation:
nutrient supply rate from the soil, root length,
and root activity. Just as with photosynthesis,
several factors influence nutrient uptake at
the ecosystem scale. Our main conclusions in
this section are as follows: (1) Nutrient supply
rate is the major factor accounting for differences
among ecosystems in nutrient uptake
at steady state. In other words, nutrient supply
by the soil rather than plant traits determines
biome differences in nutrient uptake by
vegetation. (2) Plant traits such as root length
and root activity strongly influence total nutrient
uptake by vegetation in ecosystems in
which biomass is increasing rapidly after disturbance.
(3) Root length is the major factor
governing which plants in an ecosystem are
most successful in competing for a limited
supply of nutrients.
Nutrient Supply
Nutrient uptake by vegetation at steady state is
driven primarily by nutrient supply. There are
unresolved debates about the relative importance
of soil and plant characteristics in determining
stand-level rates of nutrient absorption.
There are several lines of evidence, however,
suggesting that nutrient supply exerts primary
control over nutrient uptake by vegetation at
steady state. The most direct evidence for the
controlling role of nutrient supply in driving
uptake by vegetation is that most ecosystems
respond to nutrient addition with increased
uptake and net primary production (NPP) (Fig.
8.1). This differs strikingly from the controls
over photosynthesis, where ecosystem differences
in carbon uptake are determined
primarily by capacity of vegetation to acquire
carbon (leaf area and photosynthetic capacity),
rather than by the supplies of CO2 or light
(see Chapter 5).
Simulation models support the conclusion
that plant uptake is more sensitive to nutrient
supply and to the volume of soil exploited by
roots than to the kinetics of nutrient uptake,
particularly for immobile ions like phosphate.
At low nutrient supply rates, for example,
variation in factors affecting diffusion (diffusion
coefficient and buffering capacity) and
root length (elongation rate) have a much
greater effect on nutrient uptake than do
kinetics (maximum and minimum capacity for