2.1. Pot Trials

Soils used in pot experiments were obtained from the upper layer (0–20 cm) of a slope in an area unpolluted with Tl in the suburbs of Guiyang City (106°42′ E, 26°34′ N), Guizhou Province, Southwest China. The physicochemical properties of the pot soils are summarized in Table 1. The initial soil collected for the experiment was classified as weak acidic (pH values 6.27) and characterized by low CEC (mean at 21.5 cmol/kg) and high SOM (mean at 70.3 g/kg). The exchangeable Ca concentration in the initial soil was 660 mg/kg, averaging 6.70% of the total Ca. The total Tl concentration in the initial soil was 0.85 mg/kg, which was within the background value of Chinese soils (0.29–1.17 mg/kg Tl) [48].

Physicochemical properties of the pot soil (mean ± sd, n = 3).

¹ SOM: soil organic matter. ² CEC: cation exchange capacity.

The pot soil was mainly composed of quartz and smectite, and the detailed mineral compositions are summarized in Table 2. Stones, plant roots, and other sizable debris were removed from the soil by passing through a 2 mm stainless-steel sieve, following which the soil was air-dried and combined.

Mineral composition of the initial pot soil.

Two oxidation states, namely monovalent Tl(I) and trivalent Tl(III), exist in Tl and differ with respect to their toxicity and chemical reactivity [49]. Although Tl(III) may be stabilized by hydrolysis and colloid formation or sorption to Fe(III)-colloids, Tl(I) tends to dominate over Tl(III), due to the high redox potential of Tl(III)/Tl(I) couple (Eh = 1.28 V) [50,51]. Thus, Tl was artificially combined with each soil sample as Tl⁺ (12 mg/kg, TlCl) and Tl³⁺ (8 mg/kg, Tl(NO₃)₃·3H₂O). A control treatment without Tl spike was carried through the experiment. Following three saturation cycles (15 d per cycle) with deionized water (aiding the Tl fraction distribution to obtain a relative balance) and 2.5 kg air-drying, soil samples were added to plastic pots (25 cm height, 20 cm diameter). Prior to seeding, 10 g of soil was collected from each pot for Tl fractionation and pH assessment. Green cabbage seeds were obtained from the Vegetable Research Institute of Guizhou Agricultural Academy, China. Once the seeds had been superficially sterilized in 0.5% sodium hypochlorite (NaOCl) and thoroughly rinsed with deionized water, they were placed onto a feeding block contained in a climate chamber (RXZ-300C-type). After approximately 5 d, plants exhibiting similar growth were transferred into each pot (one plant in one pot). One week later, the soil was supplemented five times with Ca²⁺ (Ca(NO₃)₂·4H₂O) to obtain Ca concentrations of 1.0, 1.5, 2.0, 2.5, and 3.0 g/kg. The treatments were performed in triplicate. The plants were cultivated for 12 weeks in a plant growth chamber (temperature of 25–30 °C, humidity of 40–60%). Tap water was added to the pots to ensure that the moisture level was just less than field water capacity to prevent leaching from the pots. Each pot was fertilized approximately once a week with 0.1 L 50% Hoagland’s solution [52] lacking Ca(NO₃)₂·4H₂O and trace element solution for each pot.

Following the three-month growth phase, the green cabbage samples were gathered and divided into root, stem, and leaf samples, which were then rinsed with deionized water to eliminate Tl contamination from dust or soil particles on the plants. The samples were then air-dried at 80 °C for 72 h. Plant tissue dry weight was then calculated. The plant samples were then pulverized using a micro plant grinding apparatus (FZ102, TAISITE, Tianjin, China) and passed through a 60-mesh sieve. Selected rhizosphere soil samples were also obtained during plant tissue collection. A shaking-off method [53] was applied to obtain the rhizosphere soil samples. All of the soil samples were air-dried, powdered in a ceramic disc mill, and passed through a 100-mesh sieve.