Principles of terrestrial ecosystem ecology.pdf

absorption, indicating the large energetic cost
of nutrient uptake. Elements required in small
quantities are often absorbed simply by mass
flow or diffusion into the root cortical cells
(Table 8.2).
Some plant species tap pools of nutrients that
are unavailable to other plants. Although all
plants require the same suite of nutrients in
similar proportions, nitrogen is available in
several forms (nitrate, ammonium, amino acids,
etc.) that differ in availability among ecosystems.
Many species preferentially absorb
ammonium (and perhaps amino acids) over
nitrate, when all nitrogen forms are equally
available (Table 8.3). Species differ, however, in
their relative preference for these nitrogen
forms, frequently showing a high capacity to
absorb the forms that are most abundant in
the ecosystems to which they are adapted.
Many species that occupy highly organic soils
of tundra and boreal forest ecosystems, for
example, preferentially absorb amino acids
(Chapin et al. 1993, Näsholm et al. 1998), although
even agricultural species use amino acid
nitrogen (Näsholm et al. 2000). An important
community consequence of species differences
in nitrogen preference is that nitrogen represents
several distinct resources for which
species can compete. Species in the same community
often have quite different isotopic signatures
of tissue nitrogen, because they acquire
Nutrient Uptake 185
Table 8.3. Preference ratios for plant absorption of different forms of nitrogen a when all forms are equally
available.
+ -
NH4 :NO3
+
Glycine : NH4 Species preferenceb preferenceb References
Arctic vascular plants 1.1 2.1 ± 0.6 (12) Chapin et al. (1993), Kielland (1994)
Arctic nonvascular plants 5.0 ± 1.5 (2) Kielland (1997)
Boreal trees 19.3 ± 5.8 (4) 1.3 Chapin et al. (1986b), Kronzucker
et al. (1997), Näsholm et al. (1998)
Alpine sedges 3.9 ± 1.3 (12) 1.5 ± 0.4 (11) Raab et al. (1999)
Temperate heath 1.0 Read and Bajwa (1985)
Salt marsh 1.3 Morris (1980)
Mediterranean shrub 1.2 Stock and Lewis (1984)
Barley 1.0 Chapin et al. (1993)
Tomato 0.6 Smart and Bloom (1988)
a Assumes all forms of nitrogen are equally available.
b A preference ratio >1 indicates that the first form of nitrogen is absorbed preferentially over the second. Number of
species studied in parentheses. Research shows that many plants preferentially absorb glycine (a highly mobile amino
acid) over ammonium and preferentially absorb ammonium over nitrate, when all forms are equally available.
nitrogen from different sources—either different
chemical fractions (nitrate, ammonium,
organic nitrogen) or different soil depths
(Nadelhoffer et al. 1996) (Fig. 8.4). Changes in
species composition could therefore alter the
nitrogen pools that are used to support primary
production. In most cases, the species present
are capable of using the prevailing forms of
available nitrogen. If human activities alter
the prevailing form of available nitrogen, for
example through nitrate deposition in coniferous
forests, this novel form of nitrogen may be
used less effectively by the extant vegetation.
Plant functional type
Woody evergreen
Graminoids,
cryptogams
Woody deciduous
Aquatic
Tissue δ 15 -10 -8 -6 -4 -2 0 2 4
N
Figure 8.4. Concentration of 15 N in tissues from different
growth forms of boreal plants. (Figure provided
by K. Kielland; Kielland 1999.)