Anthropogenic Carbon Particles Trigger Oxidative Reprogramming of Mangrove Physiology and Disrupt Rhizosphere Microbial Networks

seedling and rhizosphere responses to subsurface-applied polystyrene nanoplastic (PS NP) or biochar (BC) were evaluated through integrated physiological, microscopic, and multiomics analyses. PS NPs followed a soil-root-stem-leaf gradient (63.4, 24.33, 4.72, 3.66 μg/g), indicating restricted upward transport, whereas BC showed limited internalization. Compared to controls, PS NP reduced root volume by 6.68%, caused chloroplast disintegration, and increased leaf reactive oxygen species by 1.14-fold, decreasing ΦPSII by 11%. BC reduced ΦPSII by 20.7% and induced severe root-tip disintegration; urease activity declined by 83%. Multiomics revealed PS NP upregulated arachidonic acid metabolism and aromatic amino acid pathways by 2.56- and 2.71-fold and reduced microbe-gene-metabolite connectivity by 28.3%, while BC reduced unsaturated fatty acid abundance by 28% and microbial network connectivity by 23.9%. Both treatments altered core C-N cycling genes, with PS NP promoting carbon degradation and BC inducing compensatory carbon fixation. Collectively, both PS NP and BC disrupt the soil-microbe-plant interface, with BC's negative effects exacerbated in oxygen-limited mangrove sediments.