Isolation of Nitrogen-Fixing Bacteria and Phosphate-Solubilizing Bacteria from the Rhizosphere of Mangrove Plants and Their Enhancement to the Growth of Kandelia candel Seedlings

Fifty nine isolates of nitrogen-fixing bacteria and phosphate-solubilizing bacteria were isolated based on the characteristics of morphology and growth of the bacteria on the selective growth media from the rhizosphere of mangrove plants including Bruguiera gymnorhiza, B. sexangula, Kandelia candel, Rhizophora stylosa, Sonneratia apetala and S. caseolaris in Zhanjiang and Zhuhai, Guangdong Province and Dongzhai harbor, Hainan Island. Among them 7 isolates of nitrogen-fixing bacteria and 8 isolates of phosphate-solubilizing bacteria were screened as propagule inoculants of K. candel in a series of potted experiments to examine their capability to promote the growth of plants. The results showed that the inoculation with a single isolate of phosphate-solubilizing bacteria He4# resulted in a significant increase in plant height, root mass and total biomass at the rate of 58.4%, 132.1% and 133.2%, respectively. The enhancement of isolate He4# on the plant growth is more significant than that of an alien isolate (Bacillus lichenciformis). The root biomass and total biomass of K. candel inoculated He4# increased by 72.7% and 90.2% than those inoculated B. lichenciformis. All pure inoculations with 7 different isolates of nitrogen-fixing bacteria showed positive response to the plant growth, and the inoculation with isolate Ngqq-R14 resulted in the maximum increase in plant height, stem diameter, stem dry mass, root dry mass and total biomass at the rate of 38.36%, 16.19%, 75.46%, 51.55% and 59.31%, respectively. Dual inoculations with 7 mixtures of one isolate of nitrogen-fixing bacteria and one isolate of phosphate-solubilizing bacteria could result in synergistic effect on the growth of the seedlings of K. candel, and the increase of plant height, stem dry mass, root dry mass and total biomass were 21.1%-69.4%, 70.7%-271.3%, 27.1%-111.3% and 66.9%-123.8% respectively. It can be concluded that using mixed cultures of microorganisms is advantageous over pure cultures, and the inoculation of mangroves with phosphate-solubilizing bacteria and nitrogen-fixing bacteria will promote the growth of mangrove plants.