The impact of elevated atmospheric CO2 and nitrate supply on growth, biomass allocation, nitrogen partitioning and N-2 fixation of Acacia melanoxylon

The interactive effects of nitrate supply and atmospheric CO2 concentration on growth, N-2 fixation, dry matter and nitrogen partitioning in the legum inous tree Acacia melanoxylon R. Br. were studied. Seedlings were grown hyd roponically in controlled-environment cabinets for 5 weeks at seven N-15-la belled nitrate levels, ranging from 3 to 6400 mmol m(-3). Plants were expos ed to ambient (similar to 350 mu mol mol(-1)) or elevated (similar to 700 m u mol mol(-1)) atmospheric CO2 for 6 weeks. Total plant dry mass increased strongly with nitrate supply. The proportion of nitrogen derived from air d ecreased 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. C O2 concentration had no effect on the relative contribution of N2 fixation to the nitrogen yield of plants. Plants grown with greater than or equal to 50 mmol m(-3) N and elevated CO2 had approximately twice the dry mass of t hose grown with ambient CO2 after 42 days. The rates of net CO2 assimilatio n under growth conditions were higher per unit leaf area for plants grown u nder elevated CO2. Elevated CO2 also decreased specific foliage area, due t o an increase in foliage thickness and density. Dry matter partitioning bet ween plant organs was affected by ontogeny and nitrogen status of the plant s, but not by CO2 concentration. In contrast, plants grown under elevated C O2 partitioned more of their nitrogen to roots. This could be attributed to reduced nitrogen concentrations in foliage grown under elevated CO2.