|Abstract or Summary
- The effect of solution nitrogen (N) and phosphorus
(P) concentrations on biomass production and N₂ fixation
of red alder (Alnus rubra Bong.) seedlings grown in
perlite-filled pots in a climate controlled growth room
were studied. Nodulated seedlings were subjected to 12
different nutrient solution combinations of nitrogen and
phosphorus and one sodium control treatment. Carbohydrate
allocation, nitrogen fixation (using acetylene reduction
(AR) methods) and leaf N and P concentrations were measured
to determine the relative importance of external nitrogen
and phosphorus concentrations on plant growth and nutrient
status, nodule production, and nitrogen fixation activity.
Nodule biomass per plant declined with increasing
solution N concentrations (1, 10, 100, 1000 mg N/l).
Nodule AR rates remained relatively stable over most of
the treatments. This study, however provides some evidence
that nodule and plant AR rates can be enhanced at treatment
combinations of 100 mg N/l and 100 mg N/l over all other
treatment combinations of N and P used in this study.
Leaf P concentrations increased with increasing
solution P concentrations (10, 100, 1000 mg P/l). Nodule
biomass per plant, nitrogenase activity (measured as nodule
AR rates) and plant AR rate were greatest at 100 mg P/l.
Plant biomass production was greatest at solution
concentrations of 100 mg N/l combined with 10 or 100 mg P/l.
All measured variables were depressed at high solution
concentrations of nitrogen (1000 mg N/l) and phosphorus
(1000 mg P/l) probably due to the adverse effects of high
leaf nutrient concentrations.
Plant nitrogen fixation declined with increasing
solution N concentration. The nitrogen content of leaves
per plant, however, was greater at 100 mg N/l than when
N was present at 1 and 10 mg N/l in solution. This suggests
that there was a shift in the relative contribution from
fixed N as a major contributor to the plant nitrogen pool
at solution N concentrations of 1 and 10 mg N/l, to mineral
N at solution concentrations of 100 mg N/l and 1000 mg N/l.
Increased demand for carbohydrate by nitrogen fixation
at low external N concentrations (1 and 10 mg N/l) was
associated with reduced plant growth. First, leaf nitrogen
concentrations appeared to be optimum for plant growth in
all treatments, thus the reduction in plant growth at 1
and 10 mg N/l was not due to a nitrogen deficiency. Second, the decline in nitrogen fixation with increasing
solution N concentrations would result in a reduced demand
for plant carbohydrates by the nodules. At 100 mg N/l
carbohydrate was directed away from the nodules to other
plant parts resulting in an increase in growth. Finally,
it was concluded that the effect of solution N concentrations
on alder nitrogen fixation and growth appeared to
be through a shift in the allocation of carbohydrate to
nodules and other plant parts.
The effect of solution P concentrations on alder
nitrogen fixation appeared to be through a stimulation of
nodule biomass up to solution P concentrations of 100 mg P/l.
Leaf P concentrations ranged from optimum to potentially
toxic for plant growth. Biomass production was highest
when solution P concentrations were at 10 lug P/l and leaf
P concentrations were between 0.2% and 0.4%.
An interaction between N and P indicates that each
may modify the effect the other has on plant growth and
nitrogen fixation. Leaf nutrient concentrations, nodule
biomass, and nodule AR rates were influenced by an interaction
between N and P. An increase in treatment P
concentrations to 100 mg P/l modified the negative effect
of solution nitrogen on nodule biomass so that the rate
of decline of nodule biomass was reduced compared to 10
mg P/l solution treatments. Plant AR activity increased
significantly, when treated with a combination of 100 mg N/l
and 100 mg P/l in solution, as compared to all other treatments.
This increase was a combined result of P enhancement
of nodule biomass and a stimulation of nodule AR activity
by N and P as plant growth increased and more carbohydrate
was produced. It appears that nitrogen fixation may be
enhanced when low amounts of external nitrogen are present
and P availability is not limited.
The negative effect nitrogen concentrations on red
alder nitrogen fixation have been reported in other studies.
However, the importance of phosphorus to nitrogen fixation
has only recently been brought under investigation. This
study indicates that high P availability can modify the
negative effect of external nitrogen concentrations on
nitrogen fixation and increase the potential contribution
of nitrogen to the system by the nitrogen fixing plant.