Biochar derived from hydrolysis of sewage sludge influences soil properties and heavy metals distributed in the soil

Sewage sludge contains a large number of nutrients and dangerous substances, when sludge was processed into sludge hydrochar that was added to the soil, which not only solve the problem of sludge disposal, but also amend the soil and fix pollutants in the soil. However, it was lack of report on the effect of the sludge hydrochar on soil compositions and soil microorganism community structures until now. In the present study, the hydrothermal carbonization method is used to prepare hydrochar from sewage sludge at temperatures of 180 ℃ and 240 ℃ at durations of 6 h and 15 h in this paper. The effects of the prepared sludge hydrochar on soil-derived dissolved organic matter (DOM), the content of total dissolved nitrogen (TDN) and NO₃^--N in soil, and the community structure of soil bacteria and fungi were evaluated. Furthermore, the change rules in heavy metal speciation in soils treated with sludge hydrochar were investigated. With the increase in the preparation temperature and dosage of sludge hydrochar, the main components of DOM changed from soluble microbial byproducts to fulvic acid-like and humic acid-like fractions through UV and fluorescence characterization. The sludge hydrochar prepared at low temperature could significantly increase the contents of TDN and NO₃^--N in the soil. Affected by sludge hydrochar, the dominant phylum of the bacterial community changed from Proteobacteria to Actinobacteria, and the dominant phylum in the fungal community did not change, but its relative abundance increased. Finally, the sludge hydrochar obtained when the carbonization time was 15 h was more beneficial to reduce the total amount and available content of heavy metals in the soil. The study provides a basis for sludge hydrochar application for the soil amendment.