Manganese-modified reed biochar decreased nutrients and methane release from algae debris-contaminated sediments

Biochar is one of the ways for carbon storage, pollution control and biosolid reuse. Aquatic plant reeds are widely used in nutrient removal in wetlands and have huge biomass. Nonetheless, little is known regarding the effects of reed-based biochar on sediments. In this study, reed-based biochar (BC) modified with or without Mn-oxidizers (MBC) was prepared to investigate their impacts on nutrient removal, methane (CH₄) emission fluxes, and CH₄ concentration and microbial community in sediments for 20 days. We found that BC and MBC significantly reduced CH₄ emission fluxes by 56.84 ± 10.47% and 69.95 ± 0.76% (p < 0.05) compared to control (CK), respectively. In addition, BC and MBC had a higher efficiency of nutrient removal, and the removal rate increased by 4.4% for NH₄⁺-N and 10.13% for TN in BC and by 3.21%, 8.43% and 18.29% for NH₄⁺-N, TN and TP in MBC, respectively. Proteobacteria, Chloroflexi, Bacteroidota, Firmicutes, Desulfobacterota and Acidobacteriota were the predominant phyla in sediments and might contribute to nutrient removal. Network analysis revealed that biochar addition promoted interspecific competition in sediments, which could be more beneficial for enhancing the stability of microbial community structures. The decreased mcrA (CH₄ bioproduction) abundance but increased pomA (CH₄ oxidation) abundance was detected in BC and MBC compared to CK, explaining biochar-reduced CH₄ emissions. This study highlights that reed straw-based biochar can be used in the in-situ remediation of polluted sediments and provides a choice for carbon storage and pollution control for managers.