Abstract:The nodulation and nitrogen fixation of Acacia mangium grown on the hill of Heshan Institute of Forestry in Guangdong was studied. The results showed that the diurnal variation of nodule nitrogenase activity for Acacia mangium was related to the energy supply of nitrogen fixation and depended on sunlight and temperature, the range for diurnal variation of nitrogenase activity being 1 to 5 μmol C2H4· g-1 fresh nodules h-1. Seasonal variation of nitrogenase activity was related to the temperature and humidity, it ranged from 3 to 10 μmol C2H4· g-1 fresh nodules h-1 in warm and rainy season from May to October.The growth retardation or death of the nodule and the degradation or loss of nitrogenase activity were affected by dry and low temperature in winter and spring. The nodule biomass for several plots in different years amounted to 104-625 kg·ha-1, and more biomass (over 300 kg·ha-1 ) appeared in early stage of the forest. Biomass decreased with the increase of forest age due to the herb and shrub developing and the root growth downward to the deep soil. Ditching and fertilizing increased the nodule biomass by 57-344kg·ha-1, which is significant for increasing the amount of nitrogen fixation.