Effect of solution nitrogen and phosphorus on growth, carbon allocation and nitrogen fixation of red alder seedlings

Abstract

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.