Biochar, an eco-friendly soil amendment, holds promise for remediating contaminated soils, yet its impacts on coastal wetland soils under combined microplastic (MP) and heavy metal (HM) pollution remain underexplored. This study examined the efficacy of 2% Spartina alterniflora-derived biochar (BC) in rehabilitating soils co-contaminated with cadmium (Cd) and two MPs—polyethylene (PE) and polylactic acid (PLA)—at 0.2% and 2% (w/w). The results indicated that biochar significantly elevated soil pH (8.35–8.43) and restored electrical conductivity (EC) to near-control levels, while enhancing organic matter content (up to 130% in PLA-contaminated soils), nutrient availability (e.g., phosphorus, potassium), and enzyme activity. Biochar reduced bioavailable Cd by 14–15% through adsorption and ion exchange. Although bacterial richness and diversity slightly declined, biochar reshaped microbial communities, enriching taxa linked to pollutant degradation (e.g., Proteobacteria, Bacteroidota) and upregulated functional genes associated with carbon, nitrogen, and sulfur cycling. Additionally, biochar boosted Suaeda salsa (S. salsa) biomass (e.g., 0.72 g/plant in A1B) and height (e.g., 14.07 cm in E1B) while reducing Cd accumulation (29.45% in shoots) and translocation. Remediation efficiency was most pronounced in soils with 0.2% PLA. These findings bridge critical knowledge gaps in biochar’s role in complexly polluted coastal wetlands and validate its potential for sustainable soil restoration.

Microplastics are persistent contaminants in coastal wetlands, yet the mechanisms of their microbial transformation remain poorly understood. This study examined the interactions between a wetland sediment-derived microbial consortium and polyethylene terephthalate (PET) fibers over a 60-day incubation. After 60 days, the consortium caused a PET weight loss of 13.7 ± 0.9%, whereas the abiotic control showed a less than 2% loss. The water contact angle decreased from 77.5 ± 1.2° to 75.8 ± 0.4°, suggesting enhanced surface hydrophilicity. Multi-scale surface analyses (SEM, WCA, and FTIR) confirmed progressive microbial colonization, increased surface roughness, and enhanced hydrophilicity through microbially mediated modification. High-throughput 16S rRNA sequencing unveiled a distinct community succession; PET exerted selective pressure that reduced alpha-diversity while enriching specific functional taxa such as Acinetobacter and Pseudomonas. Moreover, isolation and co-culture assays confirmed the importance of synergistic microbial interactions in PET transformation, with co-culture of four representative isolates causing 9.2 ± 0.1% PET weight loss, compared with only 1.7–3.2% in monocultures. These findings underscore the intrinsic natural attenuation potential of wetland ecosystems and provide a critical scientific basis for developing nature-based management strategies. By identifying key functional taxa and PET-associated transformation pathways, this work supports the establishment of early-warning mechanisms to safeguard the ecological integrity and soil health of coastal World Natural Heritage sites like the Tiaozini Wetland.