Cadmium-oxytetracycline co-contamination disturbs soil ecology and triggers plant self-defense mechanisms

Co-contamination of agricultural soils with cadmium (Cd) and oxytetracycline (OTC) from organic fertilizers and wastewater irrigation threatens food safety. A pot experiment was conducted to examine the responses of soil and pak choi ( Brassica rapa subsp. chinensis ) to combined Cd (2/5 mg kg −1 ) and OTC (1/10/30 mg kg −1 ) exposure. Soil available Cd was primarily governed by the Cd addition level (F = 886.70, p  < 0.001), but its response to OTC was concentration-dependent (F = 53.39, p  < 0.001): available Cd increased by 2.89–3.11-fold with rising OTC under the Cd2 background but decreased under the Cd5 background, whereas the Cd bioconcentration factor was consistently lower in Cd5- than Cd2-treated soils ( p  < 0.001), largely independent of OTC. The combined pollution reshaped bacterial communities and drove co-enrichment of antibiotic resistance genes (e.g., sul 1, sul 2, tet G, tet C) and the mobile genetic element int I1, whose abundance under Cd5 was up to 3.13-fold higher than under Cd2 in rhizosphere soils. The O10 treatments (Cd2O10 and Cd5O10) exhibited the highest comprehensive toxicity, with soil enzymatic IBR values reaching 3.79 and leaf IBR values reaching 7.9 × 10 −4 , alongside fewer negative microbial co-occurrence correlations and reduced network modularity. Under these high-toxicity conditions, the stress-tolerant genera Cupriavidus (Cd2O10) and Nocardiopsis (Cd5O10) were selectively enriched in root endophytes, and plant antioxidant defense responses (SOD, POD, GSH) were activated. Structural equation modeling indicated that Cd contamination directly and indirectly governed the edible safety of pak choi (λ = 0.75, p  < 0.001). These findings reveal non-linear, dose-dependent risk patterns under Cd–OTC co-contamination and suggest microbial-assisted mitigation strategies for co-contaminated agricultural soils, providing insights for pollution control without compromising productivity.