Safety Of Activated Sludge Extract And It’s Effects On Rice Seedlinggrowth And Soil Properties

In response to the problems of huge sludge production,difficult treatment and disposal,severe shortage of resource utilization,and agricultural non-point source pollution caused by large amounts of pesticides and fertilizers used in rice cultivation in urban sewage treatment plants in China,this study evaluates the agricultural safety of activated sludge extraction liquid,a sludge resource product,through safety tests,The effect and mechanism of activated sludge extract on promoting the growth of rice seedlings and roots were studied through hydroponic experiments.The effect and mechanism of activated sludge extract on promoting rice growth and improving soil were studied through root box experiments.The effect of activated sludge extract combined with reduced fertilizer application on rice growth traits,yield,soil properties,and reduction of nitrogen and phosphorus residues in rice fields was studied through field experiments.The main research findings are as follows:1.The safety test results showed that the values of Hg,As,Cd,Pb and Zn in the extracts were only 0.01,0.53,0.05,3.07 and 1.44 mg/L,which were much lower than the corresponding standard limits of 5,10,10,50 and 50 mg/L in the Water-soluble fertilizers containing amino-acids.Extracts from 0.33% to 10% had significant promotion effects on rice seed germination and root elongation,with 3.3% extracts having the most significant promotion effect.The results of the earthworm toxicity test showed that the actual production application concentration of the extract was2.13 mg/kg,which was much lower than the LC50 of 14 d for the Eisenia foetida >5000 mg/kg.2.The results of hydroponic experiments showed that 0.04～0.11% extracts could significantly promote the growth of rice roots,improve the root vigour and oxidase activity of rice seedlings,enhance the glutathione content and significantly reduce the malondialdehyde content of rice seedlings,with 0.11% extracts having the most significant effect.3.The results of the root box test showed that,compared with conventional fertilizer application,the application of extract significantly promoted the growth of rice seedlings,with the best result of 30% N reduction with high amount of extraction solution,which could increase the number of tillers by 20%,plant height by 8.60%,above-ground biomass by 40.54%,below-ground biomass by 48.00%,leaf area by19.25% and chlorophyll content by 28.57%.The application of extract also increased the pH and dissolved organic carbon content of the soil and reduced the electrical conductivity of the soil.The most significant effect was seen in the full extract with phosphorus and potassium fertiliser treatment.The results of the bacterial community structure analysis at the soil microbialgenus level showed that the relative abundance of Flavisolibacter,Candidatus＿Udaeobacter and Sphingomonas spp.associated with carbon metabolism and plantgrowth-promoting rhizobacteria in the soil increased by138～320%,38～321% and 88～123% respectively after the application of the extract solution.The results of the functional prediction analysis showed that the expression of functions such as chemoheterotrophy,aerobic＿chemoheterotrophy and aromatic＿compound＿degradation were significantly enhanced in the soil.4.The results of the field trial showed that there was no significant difference in yield between the extracts and fertiliser reduction compared to the control field.The extracts,in combination with fertiliser reduction,significantly reduced the total N and P content of the soil at all depths during maturity,increasing the abundance of soil phyla such as Proteobacteria,Nitrospirae and Verrucomicrobia,where the Proteobacteria is the plant probiotic,the Nitrospirae is the main microorganism responsible for the nitrosation reaction.and the Verrucomicrobia is positively correlated with the expression of C-cycle related genes.Their increased abundance helps to optimize the soil microbial community structure of rice fields.In summary,the application of a certain concentration of activated sludge extract can promote rice root growth,improve rice enzyme activity and root vitality,improve soil physical and chemical properties,optimise soil microbial community structure,ensure yield while reducing chemical fertiliser application,thereby reducing soil nitrogen and phosphorus content at maturity and,to a certain extent,reducing the effect of farmland surface pollution at source.