## 1 Introduction  
Metal pollution in soils is a serious environmental problem with potentially harmful consequences for agriculture and human health. Modern agricultural practices and industrial activities have resulted in Cd, Cu, Pb, and Zn accumulation in soils [1]. Metals occur in soil naturally and their concentrations can differ due to the soil type. [2]. Research has shown that the accumulation of Cu, Cd, Zn, Hg, and Pb, and especially Cd in soils were enhanced by geological background and carbonate rock weathering [3, 4]. The long-term use of phosphate fertilizers that contain these metals and the application of contaminated sewage sludge can further enrich agricultural soils with metals [5]. Because these trace elements can be found at relatively high concentrations in crops, their presence in plants is of concern for humans [6]. Hence, there is a need to either remove them from the environment or to avoid their uptake by crop plants that are consumed by humans. Tobacco has the potential to accumulate Cd and Pb from contaminated soil [7]. Excessive Cd taken up by tobacco inhibits the absorption of N, P, and K, reducing the biomass of shoot and root [8], while Pb mainly influences seed germination and photosynthesis [9]. In contrast, Cu and Zn, which are essential mineral elements for plant growth, are crucial for maintaining chloroplast structure, enzyme activity, tobacco aroma and growth [10]. Research has shown that soil with 2.0 to 4.8 mg/kg Zn benefits tobacco growth, making it possible to increase production and quality, and accelerate the uptake of nutrients [11]. However, tobacco can be poisoned by high concentrations (>9.8 mg/kg) of Zn in the soil [12; 13]. It is known that heavy metals in tobacco get into the human body from the smoke that gets through the lungs of people who smoke. Indeed, tobacco is considered a major source of toxic metals in human blood and various organs [14]. Unfortunately, heavy metals in tobacco have not aroused widespread public awareness, and administration related to the content of heavy metals in tobacco is still lacking. Total metal concentrations alone cannot evaluate the metal's mobility, bioavailability, and environmental impact sufficiently [15]. The Community Bureau of Reference (BCR) sequential extraction method is widely used to determine the binding forms and the mobilities of metals in soils. Furthermore, diethylene triamine pentaacetate acid (DTPA) extracted metals have been reported to have better correlations between soil metal content and crop tissue content [16; 17]. In general, Pb, Cd, and Zn are preferentially taken up by tobacco due to their potential bioavailability [18; 19]. In comparison to the total metal concentrations, the available or exchangeable and reducible fractions are highly correlated [20]. Some studies have shown that the available, exchangeable, and reducible fractions mainly determine the mobility and bioavailability [21] or phytotoxicity [22] of, for example, Cd, Cu, Pb, and Zn in soils. Passivation methods, by transfer metals to stable phases, have been widely used in the remediation of metal-contaminated soils [23]. Various methods and techniques have been used for passivation of Cd, Pb, Cu, and Zn, but the application of humic acid (HA) is one of the most promising [24; 25]. HA has a high complexation capacity for these metals as it contains carboxylic and phenolic groups as complexation sites, which can reduce the bioavailability and mobility of metals [26; 27; 28; 29]. Therefore, quantifying the available, exchangeable, and reducible fractions after HA application is necessary to assess metal mobility. The application of humic acid to stabilize (i.e., immobilize) metal ions, especially cations, in soil has been widely used [21; 24; 25]. HA amendments have been proven to be effective to improve soil functions, and especially to enhance the ability of anti-acidification and fertility [24; 30; 31], allowing for the establishment of a plant cover.