A study of nodulation and nitrogen fixation of alder on the purplish soils in China

Summary The alder has a perennial nodule cluster. The nodule amount on the roots increases with tree age. The N₂-fixing activity of nodules decreases with nodule age. Purple coloured soils with various soil pHs and CaCO₃ contents are, in the main, the ones which influence nodulation and N₂-fixing. Higher N₂-fixing capacity existed in the neutral and low calcium soils. High calcium soils and acid soils can restrain nodulation and the N₂-fixing rate significantly. On the slope, where calcarous light loams are found, the annual nitrogen fixation capacity of alder and cypress mixed plantations, less than 10 years old, is 16 or 17 kg/ha yr, but in the valley, a pure alder plantation can reach 40 kg/ha yr.