| Plant growth-promoting rhizobacteria (PGPR) survives in the plant rhizosphere and root surface, with a greater contribution to regulating microorganism dynamics of which directly or indirectly affect plant growth. PGPR also plays a major role in nitrogen fixation, phosphate-solubilizing, generating and releasing plant hormone or antibiotics, mainly including N-fixing, P-releasing and K-releasing bacteria. Eucalyptus seedlings inoculated with the PGPR would temporally be measured from field afforested sites, where were chosen following a method of randomized block experimental design. The impacts of PGPR on rhizosphere microorganisms, soil enzyme activity, soil nutrient concentration are evaluated, as well as inter-relationships between such bacteria and changing of eucalyptus growth and forestry soil fertility. The study would improve our understanding of how to scientifically manage eucalyptus plantations. The main findings have been listed as below:1) The growth indicators (i.e. DBH and tree height), of13-month old,18-month old,25-month old inoculated eucalyptus, were measured through seasonal observations respectively. DBH (ground diameter) and tree height of inoculated individuals are significantly increased than control groups, which can be used to prove that additive PGPR has a significant role in stimulating tree growth and productivity.2) Rhizosphere soil samples, underneath inoculated13-month old and25-month old eucalyptus plantations, were collected respectively. The quantities of different kinds of microorganisms were calculated separately from the soil samples. The research findings indicated that quantities of bacteria, fungus, ctinomyces, N-fixing bacteria, K-releasing bacteria, P-releasing organic bacteria and P-releasing inorganic bacteria were significantly higher than the controls. The increasing number of microbial would directly affect nutrient cycling processes of soils and improve soil fertility as well.3) Soil enzyme activities were also tested from rhizosphere soil samples. The research findings concluded that quantities of soil sucrase, protease, soil polyphenol oxidase and catalase would significantly increase comparing with the controls. Therefore, we confirm that the intervention of inoculated bacteria would play a positive role in promoting soil enzyme activity, which indirectly impact on soil metabolism and would speed up nutrient transforming together with other microbial. The quantity of PGPR can also be seem as an important indicator to evaluate soil fertility.4) Soil total nitrogen, soil alkaline hydrolysis N and available soil phosphorus concentrations have been tested from the same soil samples as testing enzyme ones. Apart from soil alkaline hydrolysis N detected from13-month old and available soil phosphorus from25-month old inoculated individuals, the rest of samples have been proved that there were significant increases after artificial inoculation. However, soil available potassium of the collected samples were lower than the controls, perhaps due to changing patterns of quantities of soil fungi and rapid growing speed of eucalyptus. Soil fertility can directly be evaluated by measuring dynamics of nutrient elements in soils and plants, which can be used to guide a sustainable scheme of fertilizer utilizations.5) By applying principal component analysis, we concluded inoculated eucalyptus plantations would improve tenth of tree growth. A ranked order (N6> NC> N1>P1>40K>14K>9K> CK) can be used to compare quality of soil fertility among those sites. Here, N6proves to be the best case, indicating maintenance of soil fertility is better than the rest. |
