Effects of Wood Vinegar Application and Planting Density on Cadmium Uptake and Accumulation in a Maize–Wheat Rotation System Under Straw Removal Management

Cadmium (Cd) contamination in agricultural soils threatens food safety and human health. To address this issue, wood vinegar (WV) and high-density planting were combined in our field study. The results revealed that the combined application significantly increased maize (Zea mays) grain and straw biomass while enhancing Cd accumulation in aboveground tissues. Notably, grain Cd concentrations remained relatively low, ranging from 0.032 to 0.047 mg·kg−1, whereas the Cd enrichment coefficients in stems and leaves rose by 70.03% and 67.42%, respectively. Additionally, WV application altered soil Cd speciation and reduced the exchangeable Cd fraction. The ZT2 treatment reduced the exchangeable Cd fraction by 24.56%, while the ZT3 treatment reduced total soil Cd by 6.91%, indicating its potential to enhance Cd removal. In the subsequent wheat (Triticum aestivum) season, maize straw removal combined with high-density planting considerably decreased Cd concentrations across wheat tissues, with maximum reductions of 25.59% in grain and 45.63% in straw. However, wheat grain Cd concentrations remained above the applicable food safety limit. These management practices were also associated with shifts in the soil microbial community, including changes in the relative abundance of Actinobacteria and Acidobacteria. Partial least squares path modeling (PLS-PM) indicated that Cd accumulation in maize straw showed the strongest negative association with Cd concentrations in wheat grain. Overall, integrating WV application with high-density planting represents a promising agronomic strategy for enhancing Cd phytoextraction by maize and mitigating Cd accumulation in subsequent wheat.