Remediation of polluted sediments is important to reduce algal bloom in eutrophic lakes. Biochar with high surface area and great adsorption capacity has been used in pollution control and carbon sequestration. Nevertheless, biochar's effects on CH4 emissions and nutrients in algae-contaminated sediment are unclear. In this study, changes in CH4 emissions, nutrients and microbial communities were investigated in water column containing algae-contaminate sediments covered with 0 (CK) or 1-6 cm of biochar. The results showed that biochar covering improved water quality, decreased nutrients in sediment, and mitigated CH4 emissions. Compared to CK, biochar covering reduced by about 11.48 %, 24.79 %, 46.43 % and 66.37 % of mean CH4 fluxes in 1CM, 2CM, 4CM and 6CM treatments, respectively. Furthermore, the average concentrations of NH4+-N, TN and COD in overlying water were 45.86 %-83.10 %, 2.60 %-38.75 % and 22.00 %-51.77 % lower in biochar treatments than in CK, respectively. Considering the experimental effect and economic cost, 4 cm thickness biochar is an ideal choice. PLS-PM analysis showed that biochar changed physical and chemical parameters of water, and thus affecting CH4 fluxes (p < 0.001). Microbial diversity increased in water and sediment, and phylum Bacteroidota, Actinobacteriota, and Chloroflexi abundance increased but Cyanobacteria abundance reduced in biochar treatment groups compared to CK. Through co-occurrence network, it was found that bacterial interactions were more intricate in sediment than overlying water, and the hub species with low abundance might contribute significantly to maintain microbial community stability. It should be noted that biochar increased the ratio of methanotrophs/methanogens in surface sediments. Functional prediction suggested that biochar promoted C, N and P cycling in sediment and water, and accelerated nutrients mineralization. This study highlights biochar covering can improve water quality, shape microbial community and reduce CH4 emissions in the short term, and provides useful information for eutrophic lake sediment remediation and management.
