The Sewage Sludge Biochar at Low Pyrolysis Temperature Had Better Improvement in Urban Soil and Turf Grass

In recent years, continuous efforts have been made to understand the impact of biochar on arable soil fertility. Little is known about whether the biochar derived from municipal sewage sludge has positive impacts on urban soil. In this study, we pyrolyzed spray-dried municipal sewage sludge at 200 °C, 300 °C, 500 °C, and 700 °C for 2 h in a muffle furnace and then amended it into an urban soil to grow turf grass in pots. The outcomes demonstrated that biochar incorporation caused remarkable increases in soil organic C, black C, total N, available P, and K by 3–8, 7–25, 2–9, 10–19, and 1.4–2 times, respectively. The dry matter of turf grass increased by 43–147%, probably due to the nutritional improvement after biochar addition. The turf grass grown in biochar-added soil had 4–70% lower heavy metals than that in the control, although the soils had much higher total heavy metals, which might imply that biochar amendment reduced the bioavailability of heavy metals. Considering the cost of biochar production and its impacts on both urban soil and grass, it would be alternative to convert the spray-dried municipal sewage sludge into biochar at 200 °C for 2 h and then used as an urban soil amendment.