[Biochar Remediation of Plastic-cadmium Co-contaminated Soil and Its Effect on Amaranthus mangostanus L. Growth]

A pot experiment was conducted to investigate the effects of biochar on soil properties， cadmium （Cd） bioavailability， and amaranth （Amaranthus mangostanus L.） growth in soils co-contaminated with polypropylene （PP） plastics and Cd， across varying PP dosages （0.1% and 1%） and particle sizes. The results indicated that in low-dose （0.1%） plastic-Cd co-contaminated soil， biochar significantly increased soil cation exchange capacity， ammonium nitrogen， and available phosphorus by 4.14%-22.75%， 92.82%-130.57%， and 2.86%-18.91%， respectively. In contrast， in high-dose （1%） plastic-Cd co-contaminated soil， greater increases in soil pH and dissolved organic carbon content were observed. Biochar reduced the available Cd content， amaranth Cd content， and the Cd enrichment factor of amaranth in low-dose plastic-Cd co-contaminated soil by 10.29%-23.35%， 4.40%-39.06%， and 10.92%-34.45%， respectively， primarily through physical adsorption and chemical passivation. However， in the high-dose plastic-Cd co-contaminated soil， biochar unexpectedly promoted Cd uptake by amaranth， with greater Cd accumulation observed under smaller plastic particle sizes. Furthermore， biochar significantly increased amaranth biomass and antioxidant enzyme activity， exhibiting more pronounced effects under conditions of low-dose and large-size plastic contamination. Redundancy analysis and partial least squares path modeling revealed that biochar alleviated co-contamination stress by adsorbing and immobilizing Cd， improving soil physicochemical properties and nutrient availability， suppressing the plastic-heavy metal synergistic effect， and activating the plant antioxidant defence system. In conclusion， rice husk biochar significantly improved soil physicochemical properties， reduced Cd bioavailability， and promoted plant growth in low-dose plastic-Cd co-contaminated soil. For high-dose plastic-Cd contamination， a combined strategy of biochar with phytoremediation is required to enhance Cd immobilization.