Synergistic effects of Trichoderma harzianum and polyaspartic acid enhance cadmium phytoremediation via core microbiota and network stability in Brassica juncea

Trichoderma harzianum ( T. harzianum ) and Polyaspartic acid (PASP) have been shown to enhance phytoremediation of Cd-contaminated soils. However, their effects on soil bacterial and fungal community structure and network stabilityremain unexplored. This study systematically investigated microbial community dynamics in response to phytoremediation, employing high-throughput 16S rRNA and ITS sequencing coupled with co-occurrence network analysis. The results showed that the combined treatment (PASP + T. harzianum , referred to as AT) increased the Cd removal efficiency in soil from 21.71 % to 38.27 %. Soil CEC, OM and Cd bioavailability were significantly increased. PASP contributed to increasing the node constancy and community composition stability, thereby increasing network stability (natural connectivity, robustness and positive cohesion) in AT treatment. During AT treatment, Trichoderma emerged as the core fungi, while Pseudomonas and Arenimonas became the core bacterial taxa. The ecological strategy transformation and the synergistic effect between T. harzianum (r-strategy) and PASP (K-strategy) increased the stability of microbial community. This then promoted functional enrichment in the function of cis-vaccenate biosynthesis and TCA cycle II (plant and fungi). The expression of the ZIP gene family and TC.HME transporters was significantly upregulated in AT treatment. Fungal network structure and stability directly influenced bacterial network stability and function, which via soil enzyme activity, promoted plant Cd accumulation. Overall, this study emphasises the crucial role of microbial network restructuring and functional transitions in ensuring the long-term effectiveness of phytoremediation. Identifying core microbial taxa and their responsive ecological strategies provides the theoretical basis and practical guidance needed to develop targeted remediation strategies in complex contaminated soils. • T. harzianum and PASP enhanced the growth and Cd accumulation of B. juncea. • Combined amendments improved community stability and positive cohesion. • Bacterial community stability and function were regulated by fungal interactions. • Core species promoted community cooperation and ecological strategy alteration. • Fungal functions and soil enzyme activity displayed significant potential to facilitate plant Cd accumulation.