Lasting improvement effects of arbuscular mycorrhizal fungi and Bradyrhizobium japonicum on rhizosphere soil environment in mining subsidence

Abstract: In a mining subsidence experiment in the Shendong mining area in the Shaanxi province of China, agricultural soil was improved by inoculation with arbuscular mycorrhizal fungi and Bradyrhizobium japonicum. This experiment was implemented in mid-May 2008 for the first planting, and in the end of April 2009 for the second time. It was divided into two experiment plots, inoculated and control. After two years of abandoned cropland, in September 2011, 15 random positions were selected under the "S" shape method from each district, and roots and fresh rhizosphere soil samples were collected. The aftereffect of microbial reclamation of the soil was studied systematically, about which there have been no reports until now. The results were 1) the density of external hyphal of the inoculation treatment was 5.58 m/g, 90% higher than the control after abandonment, the difference reaching a significant level, and it was also significantly higher than the two sets of processing before abandonment; 2) the root infection rate of the inoculation treatment was 91%, 52% higher than the control after abandonment, the difference reaching a significant level, and it was significantly higher than the control before abandonment; 3) pH value of the inoculation treatment was significantly higher than that of the control after abandonment; 4) in the inoculation treatment the number of bacteria, microbial total, available phosphorus content, conductivity and acid phosphatase activity were also greater than that of the control after abandonment, the differences not reaching a significant level; 5)compared with before and after abandonment, the soil was in degraded trends; in a comparison between the years of 2009 and 2011, in the control treatment there was a slight increase in hyphal density, the difference not reaching a significant level, while in the inoculation treatment hyphal density was twice than that before abandonment, the difference reaching a significant level; 6）compared with before and after abandonment, the root infection rate, the number of bacteria and acid phosphatase activity were increased in the inoculated treatment, while that of the control was decreased. The results showed that abandoned cropland did not affect the soil mycelium growth and development, and compared with before and after abandonment mycelial density improved greatly; the root infection potential was improved in the inoculated treatment as observed through a microscope; and total soil microbes were increased. These consequences confirmed the advantages of artificial inoculation, affecting the number of rhizosphere microorganisms and the formation of the dominant flora, and increasing microbial activity and species diversity. There was evidence to show that the microbial diversity determined plant biodiversity and ecosystem stability and productivity. Soil acid phosphatase activity was enhanced by inoculation, and the soil available phosphorus content was very scarce; when phosphorus is the main limiting factor for soil microbes and plant growth, microorganisms and plant roots secrete extracellular acidic or alkaline phosphatase enzymes, hydrolyzing organic phosphorus, phosphorus-containing organic compounds into simple inorganic compounds that the plant can absorb. Arbuscular mycorrhizal fungi as a green manure can be effective long-term on soil improvement, without disturbing the soil, but is conducive to the development of the mycelium, having important significance for microbial reclamation of mine subsidence. Arbuscular mycorrhizal fungi and Bradyrhizobium japonicum combined can effectively improve the rhizosphere soil environment, enhance natural succession, and is of great significance to ecological restoration of barren land resulting from mining subsidence.