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.
Characterizing CMIP5 model spread in simulated rainfall in the Pacific Intertropical Convergence and South Pacific Convergence Zones
