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

Abstract

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.