Effects of activated sludge extraction on rhizosphere soil microbial community structure of rice

A root box experiment was conducted to reveal the mechanisms by activated sludge extracts regulating rice growth and improving soil physicochemical properties from the perspective of soil rhizosphere microorganisms. The experiment was designed to investigate the effects of the extracts application with nitrogen reduction on rice soil physicochemical properties and rhizosphere microorganisms by conducting different treatments (T1, blank control; T2, conventional fertilizer control; T3, 30% nitrogen reduction with 0.2 g/kg extract solution; T4, 30% nitrogen reduction with 0.4 g/kg extract solution; T5, full replacement of fertilizer nitrogen with 8 g/kg extract solution equivalent to 75% nitrogen reduction). The results showed that the application of extracts significantly increased the leaf area, biomass and chlorophyll content of rice seedlings. Compared with conventional fertilizer applications, the application of extracts could significantly increase soil pH, reduce soil conductivity, increase soil soluble organic carbon and reduce soil nitrate and ammonium nitrogen content. β-diversity results showed that the effect of extracts application on soil microbial properties was more prominent in rhizosphere and near-rhizosphere soils, with significant differences in bacterial community structure and diversity between the different extracts treated groups, blank group and the control group (P<0.05). Application of extracts significantly increased the relative abundance of microorganisms associated with organic pollutant degradation and carbohydrate metabolism, such as Flavisolibacter, Candidatus_Udaeobacter, and Sphingomonas, which increased by 151% to 541%, 26% to 320% and 55% to 364%, respectively. By FAPROTAX soil function prediction analysis, it was found that the main soil microbial communities enhanced after the application of the extracts were chemoheterotrophy, aerobic chemoheterotrophy, nitrate reduction, nitrogen fixation, nitrification and aromatic compound degradation, which were related to the behaviours of soil carbon cycling, nitrogen cycling and organic pollutant degradation. The results of the study indicated that the application of extracts could improve the structure of soil rhizosphere and near-rhizosphere microbial communities, increase the abundance of related bacteria communities, and regulate carbon and nitrogen cycles of rhizosphere and near-rhizosphere soil, thereby promoting the growth of rice seedlings and improving soil characteristics. Therefore, the results of this study can provide a new pathway for the resource utilization of activated sludge.