Effects of soil layer replacement combined with organic fertilizer application on soil quality and maize yield in heavy metal-contaminated farmland

High geological background, coupled with mining activities, cause heavy metal enrichment in karst topsoil. A field experiment was carried out with two soil replacement depths (T0: 0–20 and 20–40 cm; T1: 0–30 and 30–60 cm), combined with three application rates of biochar, cow manure, and liquor brewing sludge-derived organic fertilizer. Soil physical property index (SPI), soil chemical property index (SCI), soil microbial property index (SMI) and soil quality index (SQI) were adopted to evaluate soil improvement, heavy metal reduction, and maize yield. The results showed that T1 reduced Cd, Pb, and Zn by 45.8%–71.1%, complying with GB 15618 2018 limits. The high-concentration liquor brewing sludge-derived organic fertilizers (CYH) achieved the highest SQI value. Partial least-squares path modeling verified SQI as the primary yield determinant. Overall, T1 + CYH is an effective strategy for heavy metal mitigation, soil quality improvement, and maize yield promotion. These findings provide scientific support for the safe utilization of heavy metal-contaminated karst farmland.