Unraveling water–biochar synergy under cadmium stress: Coupled enhancement of photosynthetic physiology and bacterial community structure in maize seedlings

Cadmium (Cd) is a highly toxic element that impairs plant growth and disrupts soil metabolism. Biochar is widely used to remediate Cd-contaminated soil; however, its immobilization performance may be altered by aging, particularly under different water regimes. In this study, a pot experiment was conducted to investigate the combined effects of biochar (0, 1.5%, and 3.0% w/w) and water management (50%, 60%, and 70% field capacity) on maize physiological responses, rhizosphere soil enzymatic activity, and bacterial communities in Cd-contaminated soil. Compared with deficit irrigation, full irrigation (70% θf) mitigated Cd-induced photosynthetic impairment more effectively. Antioxidant parameters responded differently to water and biochar: POD activity, SOD activity, and MDA content increased with increasing soil moisture, whereas CAT activity and GSH content decreased. The addition of biochar reduced POD and CAT activity, whereas the responses of SOD and GSH were V shaped. Soil moisture and biochar significantly influenced microbial community richness and diversity, and the M-B2 treatment (60% θf + 3% biochar) optimized bacterial diversity and promoted the enrichment of Sphingomonas (21.18%); these effects were linked to higher antioxidant enzyme activity. This pot-scale study provides reliable experimental evidence for the synergistic effects of water and biochar on maize seedlings under Cd stress and suggests a mechanistic framework that warrants further validation under field conditions.