Effects of hydrochar derived from hydrothermal treatment of sludge and lignocellulose mixtures on soil properties, nitrogen transformation, and greenhouse gases emissions

In this study, hydrochar samples derived from hydrothermal treatment (HTT) of sludge and sludge–biomass mixtures were applied to a sandy soil and their effects on soil properties, soil nutrients, greenhouse gas (GHG) emissions, and soluble heavy metals were investigated. The application of untreated sludge and hydrochar derived from HTT of sludge at 180 °C led to the highest soluble nitrate, CO2 and N2O emissions, followed by the application of hydrochar samples derived from HTT of sludge–biomass mixtures at 180 °C. Although the application of hydrochar samples derived from HTT of sludge alone and sludge–biomass mixtures at 240 °C in sandy soil led to the lowest emissions of CO2 and N2O, it resulted in lower levels of soil electrical conductivity (EC), cation exchange capacity (CEC) and soluble phosphorus. The application of hydrochar samples derived from HTT at 240 °C led to the production of CH4 and lower nitrate–N contents than hydrochar samples derived from HTT at 180 °C. These results indicated that the soils containing hydrochar samples from HTT at 240 °C were anaerobic, which might inhibit the growth of plants. The application of hydrochar samples derived from HTT of sludge–biomass at 180 °C led to significantly improved contents of soil soluble phosphorus (2.56 and 2.84 g kg−1 soil) and soil nitrate–N (160.2 and 263.2 mg kg−1 soil) at the end of 60 days of incubation. However, these contents were lower than the contents of soluble phosphorus (3.71 and 4.45 g kg−1 soil) and nitrate–N (528.3 and 583.2 mg kg−1 soil) with the application of untreated sludge and sludge derived from HTT of sludge alone at 180 °C. Although more studies are needed to understand the mechanisms and effects on different soils, this study provides useful insights into the application of hydrochar derived from sludge–biomass mixture in soil.