Combination of low-accumulation kumquat cultivars and amendments to reduce Cd and Pb accumulation in kumquat grown in contaminated soil

Kumquat, the main economic fruit of Guangxi, China was selected as the research object of this study. Pot and field experiments were conducted to systematically study the safe utilization of Cd- and Pb-contaminated orchard soil, which is a typical karst zone surrounding a historical Pb–Zn mining area in Yangshuo County, Guilin City, Guangxi Province. The pot experiment compared the Cd and Pb accumulation ability of different kumquat cultivars. The results showed that Cuipi (CP) had a lower Cd and Pb accumulation ability. Compared with Yingmu (YM), the Cd and Pb contents of CP leaves were reduced by 39.71% and 32.83%, respectively. In the field experiment (one year), CP with low accumulation was combined with 0.5% amendments including rice husk biochar (RB), pig manure (PB), soil conditioner (SC), and organic fertilizer (OF) in real Cd- and Pb-contaminated kumquat orchard soils. The results showed that all amendments reduced non-carcinogenic health risks and improved agricultural sustainability. In terms of health risk reduction, the amendments significantly reduced the accumulation of Cd and Pb in fruit of CP. In particular, RB decreased Cd and Pb by 76.90% and 25.59%, respectively, achieving the highest health risk reduction (37.16%). Moreover, RB is more advantageous owing to its low cost. To determine agricultural sustainability, the soil quality and kumquat production were evaluated. After adding amendments, the available Cd and Pb contents in soil decreased by 42.19–60.94% and 41.64–57.79%, respectively. SC and OF improved soil quality more effectively, while RB increased the fruit biomass and the basic quality of CP. Comprehensive analysis, RB has more advantages. The mechanism of immobilization by the different amendments was further analyzed. RB and OF primarily immobilized the heavy metals by adsorption, and that of SC and PB mainly due to adsorption and precipitation. This study provides a theoretical and practical basis for the remediation of heavy metal-contaminated orchard soil.