【Objective】The safety of Activated sludge extract and the effects of partial substitution of chemical fertilizers on rice seedling growth, soil physicochemical properties and soil bacterial community structure were discussed to provide a theoretical basis for the safety of activated sludge extract and its application as organic fertilizer in agriculture.【Method】Taking rice Nangeng 9108 as the experimental material, a safety assessment experiment of activated sludge extract and an agriculture application experiment with different dosages was established (CK: blank control; CF: conventional fertilization control; LMF: low-amount activated sludge extract with fertilizer; HMF: high-amount activated sludge extract with fertilizer; MF: activated sludge extract applied only). The effects of different treatments on the growth characteristics, chlorophyll content of rice seedlings, soil physicochemical properties, soil bacterial community structure were studied. 【Result】 The content of heavy metal elements in the activated sludge extract could meet the safe application standard of organic fertilizer and soaking seeds with low-concentration activated sludge extract could promote the germination of seeds. The results of the root box experiment showed that compared with the control and conventional fertilization control group, the application of activated sludge extract could promote the growth of rice seedlings, such as plant height, stem width, leaf area, and plant biomass. Compared with conventional fertilization, the combined application of activated sludge extract could significantly increase soil pH by 2.8%-13.8%, reduce soil salinity by 14.3%-66.3%, and increase soil soluble organic carbon content by up to 68.0%. High throughput sequencing analysis of soils with different fertilization treatments showed that the top ten phyla of abundance were distributed in all treatments, and the applied activated sludge extract had a significantly impact on soil bacterial community structure. After applying activated sludge extract, the flora of the phyla related to carbon cycle increased significantly, such as Proteobacteria
, etc.; Firmicutes
, which are common in stressful environments, were significantly reduced.【Conclusion】The application of activated sludge extract could promote the germination of rice seeds, and the agricultural safety of the activated sludge extract can be guaranteed. At the same time, the application of a high amount of activated sludge extract could significantly improve the growth characteristics of rice seedlings, improve soil physical and chemical properties, and optimize the structure of soil bacterial, which is comparable to the efficiency of organic fertilizer application. In practice, the treatment of reducing nitrogen fertilizer application rate by 30% nitrogen by high amount of activated sludge extract could be preferred.