The impact of elevated atmospheric CO₂ and nitrate supply on growth, biomass allocation, nitrogen partitioning and N₂ fixation of Acacia melanoxylon

The interactive effects of nitrate supply and atmospheric CO₂ concentration on growth, N₂ fixation, dry matter and nitrogen partitioning in the leguminous tree Acacia melanoxylon R.Br. were studied. Seedlings were grown hydroponically in controlled-environment cabinets for 5 weeks at seven ¹⁵N-labelled nitrate levels, ranging from 3 to 6400 mmol m^-3. Plants were exposed to ambient (~350 μmol mol^-1) or elevated (~700 μmol mol^-1) atmospheric CO₂ for 6 weeks. Total plant dry mass increased strongly with nitrate supply. The proportion of nitrogen derived from air decreased with increasing nitrate supply. Plants grown under either ambient or elevated CO₂ fixed the same amount of nitrogen per unit nodule dry mass (16.6 mmol N per g nodule dry mass) regardless of the nitrogen treatment. CO₂ concentration had no effect on the relative contribution of N₂ fixation to the nitrogen yield of plants. Plants grown with ≥50 mmol m^-3 N and elevated CO₂ had approximately twice the dry mass of those grown with ambient CO₂ after 42 days. The rates of net CO₂ assimilation under growth conditions were higher per unit leaf area for plants grown under elevated CO₂. Elevated CO₂ also decreased specific foliage area, due to an increase in foliage thickness and density. Dry matter partitioning between plant organs was affected by ontogeny and nitrogen status of the plants, but not by CO₂ concentration. In contrast, plants grown under elevated CO₂ partitioned more of their nitrogen to roots. This could be attributed to reduced nitrogen concentrations in foliage grown under elevated CO₂.