Effects of mercapto-palygorskite application on cadmium accumulation of soil aggregates at different depths in Cd-contaminated alkaline farmland

Mercapto-palygorskite (MP) is a novel immobilization material for cadmium (Cd) pollution, but the immobilization mechanism on alkaline Cd contaminated soil is not completely clear. In this paper, field experiment was carried out to study the effect of MP on the transfer of Cd in aggregates at different depth, the contribution of soil aggregates to the reduction of Cd in bulk soil and the immobilization mechanism of MP. The results showed that MP had no significant influence on the total Cd content, soil aggregates distribution, pH value, CEC value and enzyme activities no matter at any depth. At the depth of 0–20 cm, MP significantly reduced the DTPA-Cd in bulk soil by 60.7%, and increased the GWD and R₀.₂₅ value. Similarly, the content of DTPA-Cd in the soil aggregates was deceased by 40.2–63.6%, the OM, DOC, available Fe, Mn and S in soil aggregates were significantly increased by 15.0–19.1%, 19.2–41.7%, 24.7–41.2% and 12.5–35.1% respectively. The Cd fraction of aggregates, especially exchangeable Cd (EXE-Cd) and bound to Fe/Mn oxide Cd (OX-Cd), was reduced by 5.4–28.1% and increased by 22.3–50.4%. In addition, MP had different effects on the GSF value of soil aggregates, but there was a downward trend for AFX value at 0–20 cm soil depth. MP almost had no significant influence on the above indexes at the depth of 20–40 cm and 40–60 cm, but except the Cd fraction, the GSF and AFX value in individual aggregates. Small aggregates (<1 mm) and large aggregates (>1 mm) contributed 59.1% and 22% to the reduction of Cd in bulk soil. Partial Least Structural Equation Model (PL-SEM) revealed that S promoted the production of available Fe, Mn, OM and DOC, while the content of DOC inhibited the formation of EXE-Cd and the available Fe and Mn boosted the production of OX-Cd.