Effects of species-combined exogenous decomposing micro-organisms on soil microbial community structure and metabolic activity

An incubation experiment was conducted to study the effects of species combination of exogenous decomposing micro-organisms on soil microbial community structure and metabolic activity. The objective of the study was to lay the basis for the optimization of population configuration of decomposing microbial agents. In the study, three microbe species-Paenibacillus pabuli (P), Streptomyces violaceorubidus (S) and Trichoderma aureoviride (T)-were selected. For the experiments, in addition to single P, S and T microbe strains, the microbes were merged to produce two species (PT, PS and ST) and three species (PST) combinations of decomposing microorganisms (forming a total of 7 microbial agents). These microbial agents were then added to red soil sampled from Jiangxi Province in South China. Moreover, a control treatment of red soil added with sterile peat was set to the experimental design. During the incubation period, temporal changes in soil respiration rate and microbial biomass carbon were monitored. Additionally, the changes in total PLFAs content and in the proportion of characteristic microbial population in different treatments after 30 days of incubation were determined. The PLFAs percentages of microbial communities showed the total microbial biomass and composition of soil microbial communities. The results showed that, except for ST and PST, most treatments showed that total microbial biomass increased from 17.2% to 121.6% (P < 0.05). Compared with the control, the proportion of fungus in all the treatments increased by 8.8%-50.6% (P < 0.05). However, the proportion of bacteria in PLFAs remained basically unchanged, increasing from 79.6% to 83.1%. For most of the treatments, except for P and ST, the proportion of actinomyces decreased from 9.4% to 69.8%. Principal component analysis (PCA) of PLFAs data indicated that soil microbial community structure was influenced by different decomposing micro-organisms agents. The change in microbial community structure varied with treatment type, among which single P, S and T microbe strains were smallest and their trio-combination (PST) biggest, compared with the control. The results of soil respiration rate showed the growth of micro-organisms. Treatments of single P and T microbe strains and binary combination of micro-organisms S and T (ST) affected logarithmic growth of soil microbes in the short-term, increasing peak soil respiration rate by 48.7% (P), 53.7% (T) and 78.7% (ST), respectively. Additionally, with increasing number of species of decomposing micro-organisms, it took more time for soil microbes to enter latent phase. From long-term impact of exogenous decomposing micro-organisms on soil fertility, these micro-organisms changed soil microbial metabolic activity, which led to a change in the amount of soil carbon mineralization. The addition of ST combination of microorganisms increased soil microbial metabolic quotient by 28.9%, consequently, the amount of soil carbon mineralization increased by 11.1%. The addition of single S microbe strain decreased soil microbial metabolic quotient by 32.4%, while the amount of soil carbon mineralization only decreased by 7.3%. However, under PS and PST combinations, microbial metabolic activity remained unchanged, while the amount of soil carbon mineralization decreased by 5.8% and 8.7%, separately. There was the need for further study on these treatment combinations. In conclusion, the addition of exogenous decomposing micro-organisms changed soil microbial community structure and growth trajectory. Furthermore, with increasing number of species of decomposing micro-organisms, change in microbial community structure increased. Finally, the study failed to account for any relationship between soil microbial metabolic activity and the number of species of decomposing micro-organisms.