Principles of terrestrial ecosystem ecology.pdf

182 8. Terrestrial Plant Nutrient Use
Roots grow preferentially in resource hot
spots. Root growth in the soil is not random.
Roots that encounter microsites of high nutrient
availability branch profusely (Hodge et al.
1999), allowing plants to exploit preferentially
zones of high nutrient availability. This explains
why root length is greatest in surface soils (Fig.
7.3), where nutrient inputs and mineralization
are greatest, even though roots tend to be
geotropic (i.e., grow vertically downward).
This exploitation of nutrient hot spots ensures
that plants maximize the nutrient return for a
given investment in roots and reduces the
fine-scale heterogeneity in soil nutrient concentrations.
At a finer scale, root hairs, the
elongate epidermal cells of the root that extend
out into the soil, increase in length (e.g., from
0.1 to 0.8mm) in response to a reduction in the
supply of nitrate or phosphate (Bates and
Lynch 1996). Both of these responses increase
the length and surface area of roots available
for nutrient uptake. Exploitation of hot spots
does not always occur, however (Robinson
1994), and may be more pronounced in fastgrowing
than in slow-growing species (Huante
et al. 1998).
Root length is a better predictor of nutrient
uptake than is root biomass. Root length correlates
closely with nutrient acquisition in
short-term studies of nutrient uptake by plants
from soils. Roots with a high specific root length
(SRL; i.e., root length per unit mass) maximize
their surface area per unit root mass and therefore
the volume of soil that can be explored
by a given investment in root biomass. We
know much less about the morphology and
physiology of roots in soil than of leaves. The
limited available data suggest, however, that
herbaceous plants (especially grasses) often
have a greater SRL than woody plants and that
there is a wide range in SRL among roots in any
ecosystem. Much of the variation in SRL
reflects the multiple functions of belowground
organs. Roots can have a high SRL either
because they have a small diameter or because
they have a low tissue density (mass per unit
volume). Some belowground stems and coarse
roots have large diameters to store carbohydrates
and nutrients or to transport water and
nutrients and play a minor role in nutrient
uptake. There may also be a tradeoff between
SRL and longevity among fine roots, with highdensity
roots being less prone to desiccation
and herbivory than low-density roots. Both the
leaves and roots of slowly growing species often
have high tissue density, low rates of resource
acquisition (carbon and nutrients, respectively)
but greater longevity than do leaves and roots
of more rapidly growing species (Craine et al.
2001).
Mycorrhizae
Mycorrhizae increase the volume of soil
exploited by plants. Mycorrhizae are symbiotic
relationships between plant roots and fungal
hyphae, in which the plant acquires nutrients
from the fungus in return for carbohydrates
that constitute the major carbon source for the
fungus. About 80% of angiosperm plants, all
gymnosperms, and some ferns are mycorrhizal
(Wilcox 1991). These mycorrhizal relationships
are important across a broad range of environmental
and nutritional conditions, including
fertilized crops (Allen 1991, Smith and Read
1997). With respect to nutrient uptake, mycorrhizal
hyphae basically serve as an extension of
the root system into the bulk soil, often providing
1 to 15m of hyphal length per centimeter
of root—that is, an increase in absorbing
length of two to three orders of magnitude.
Because the nutrient transport through hyphae
occurs more rapidly than by diffusion along a
tortuous path through soil water films, mycorrhizae
reduce the diffusion limitation of uptake
by plants. The small diameter of mycorrhizal
hyphae (less than 0.01mm) compared to roots
(generally 0.1 to 1mm) enables plants to exploit
more soil with a given biomass investment in
mycorrhizal hyphae than for the same biomass
invested in roots. Plants typically invest 4 to
20% of their gross primary production (GPP)
in supporting mycorrhizal hyphae (Lambers et
al. 1996). Most of this carbon supports mycorrhizal
respiration rather than fungal biomass,
so a given carbon investment in mycorrhizal
biomass can represent a large carbon cost to
the plant. Mycorrhizae are most important in
supplementing the nutrients that diffuse
slowly through soils, particularly phosphate and