494
F.1. Woodward and T.M. Smith
a forested early succession to a late successional forest. In addition, if
ericoid species dominate, for example in late succession or on poor soil,
then N supply rate may be even lower. Through the succession, the rate of
phosphorus supply to the plant will be influenced by plant associations with
mycorrhizal mutualists, in addition to the pH of the soil. It has been recently
discovered that ectomycorrhizal and ericoid mycorrhizas but probably not
vesicular-arbuscular mycorrhizas are capable of taking up N from organic N
in the soil (Stribley and Read 1980; Abuzinadah and Read 1986). These
organic sources are not available to the host plant, particularly at low pH
(Abuzinadah and Read 1989).
An important feature of mycorrhizas, particularly ectomycorrhizas, is
their low host specificity, therefore mycorrhizas may connect between the
roots of different species in the same habitat, ensuring a similar capacity to
forage for N (Read 1990). From a modeling viewpoint this is an excellent
simplifying property, as the properties of the mycorrhizal associations of
different species in the same habitat can be considered the same.
The utility of the host-mycorrhizal association for predicting Amax will be
tested by an experimental examination of the effects of mycorrhizal type on
N supply to the plant and its associated Amax. It is hypothesized that the rate
of N supply to the plant will decrease in proportion to the increased reliance
of the plant on N supply from organic sources in the soil. Modeling of
nitrogen supply by mycorrhizas in grasslands (Hunt et al. 1991) supports this
premise. Therefore, through the series from soils with high nutrients, where
plants are predominantly nonmycorrhizal to moderate nutrients (vesiculararbuscular type dominates) to soils with refractory nutrient supply
(ectomycorrhizal and ericoid mycorrhizas) it is predicted that the rate of N
supply to the plant and Amax will both decrease.
23.3.2 Experimental Detail
Fourteen species of herbs and shrubs were grown from seeds on their
natural soils (four sites of acid peat, beech forest soil, limestone grassland,
and old field, in all cases after the removal of surface litter) in the glasshouse
and controlled environmental facility of the Department of Botany, University of Cambridge, over the period from 1984 to 1989. Species were grown
for either 1 year (annual species) or 2 years (perennial species) and on all
combinations of soil type. In each case plants were harvested for growth
analysis and analyzed for gas exchange in the period from May to July and
in the second year for the perennial species. Natural sunlight was used for
the glasshouse experiments, with mean temperatures over the growth period
varying from 18 to 26°C and relative humidities maintained at or above 80%.
Maximum photosynthetic rates and stomatal conductances were measured in
July (ADC LCA 2, portable infrared gas analyzer, or Leybold-Heraus Binos
2 portable analyzer).
F.1. Woodward and T.M. Smith
a forested early succession to a late successional forest. In addition, if
ericoid species dominate, for example in late succession or on poor soil,
then N supply rate may be even lower. Through the succession, the rate of
phosphorus supply to the plant will be influenced by plant associations with
mycorrhizal mutualists, in addition to the pH of the soil. It has been recently
discovered that ectomycorrhizal and ericoid mycorrhizas but probably not
vesicular-arbuscular mycorrhizas are capable of taking up N from organic N
in the soil (Stribley and Read 1980; Abuzinadah and Read 1986). These
organic sources are not available to the host plant, particularly at low pH
(Abuzinadah and Read 1989).
An important feature of mycorrhizas, particularly ectomycorrhizas, is
their low host specificity, therefore mycorrhizas may connect between the
roots of different species in the same habitat, ensuring a similar capacity to
forage for N (Read 1990). From a modeling viewpoint this is an excellent
simplifying property, as the properties of the mycorrhizal associations of
different species in the same habitat can be considered the same.
The utility of the host-mycorrhizal association for predicting Amax will be
tested by an experimental examination of the effects of mycorrhizal type on
N supply to the plant and its associated Amax. It is hypothesized that the rate
of N supply to the plant will decrease in proportion to the increased reliance
of the plant on N supply from organic sources in the soil. Modeling of
nitrogen supply by mycorrhizas in grasslands (Hunt et al. 1991) supports this
premise. Therefore, through the series from soils with high nutrients, where
plants are predominantly nonmycorrhizal to moderate nutrients (vesiculararbuscular type dominates) to soils with refractory nutrient supply
(ectomycorrhizal and ericoid mycorrhizas) it is predicted that the rate of N
supply to the plant and Amax will both decrease.
23.3.2 Experimental Detail
Fourteen species of herbs and shrubs were grown from seeds on their
natural soils (four sites of acid peat, beech forest soil, limestone grassland,
and old field, in all cases after the removal of surface litter) in the glasshouse
and controlled environmental facility of the Department of Botany, University of Cambridge, over the period from 1984 to 1989. Species were grown
for either 1 year (annual species) or 2 years (perennial species) and on all
combinations of soil type. In each case plants were harvested for growth
analysis and analyzed for gas exchange in the period from May to July and
in the second year for the perennial species. Natural sunlight was used for
the glasshouse experiments, with mean temperatures over the growth period
varying from 18 to 26°C and relative humidities maintained at or above 80%.
Maximum photosynthetic rates and stomatal conductances were measured in
July (ADC LCA 2, portable infrared gas analyzer, or Leybold-Heraus Binos
2 portable analyzer).