Influence of Sludge-Based Biochar on the Soil Physicochemical Properties and the Growth of Brassica chinensis L

Poor soil has become a serious global problem, threatening soil health and food security. Although returning sludge to soil in the form of coal char is an ideal method for soil improvement, the improvement of soil physical and chemical properties by different co-pyrolysis materials is not clear. We selected three co-pyrolysis species and sludge to prepare biochar, and studied their effects on soil improvement. The influences of different sludge-based biochar FeCl3 (BC1), Na2SiO3 (BC2), and Ca(H2PO4)(2) (BC3) on soil pH, cation exchange capacity (CEC), ammonium nitrogen, effective phosphorus, organic carbon, and the growth of Brassica chinensis L through a pot experiment with three parallel were investigated. The results showed that all sludge-based biochar were beneficial to the growth of Brassica chinensis L and did not exceed the heavy metal standard in the soil (GB 1618-2018, Chinese Standard). In addition, the above-ground parts of Brassica chinensis L met the requirements of food safety (GB 2762-2017, Chinese Standard). BC3 was the most effective in increasing soil ammonium nitrogen, effective phosphorus, and organic carbon by 29.81%, 229.41%, and 25.12%, respectively. BC3 also increased plant yield and chlorophyll content by 54.5% and 27.9%, respectively. In addition, BC3 inhibited the migration of heavy metals to the above-ground and reduced the overall contamination level of heavy metals from 0.8467 to 0.3277. The human health risk assessment showed that the target hazard quotient (THQ) and total target hazard quotient (TTHQ) values were less than 1, indicating that the health risks associated with the consumption of Brassica chinensis L (heavy metal intake) were negligible.