Biochar-earthworm-plant interactions regulate soil organic carbon stabilization and Cd immobilization in vegetable...

A dual strategy for carbon (C) sequestration and Cd remediation was developed by leveraging the synergistic effects of biochar and earthworms on soil organic C transformation and Cd bioavailability. However, the combined capacity of these amendments to simultaneously enhance soil organic C storage and reduce crop Cd uptake, particularly through the regulation of dynamic organic C pools, remains insufficiently understood. A field experiment was conducted to evaluate the individual and combined effects of biochar (600 g plot⁻¹) and earthworms (Amynthas aspergillum; 50 worms plot⁻¹) on soil organic C dynamics and Cd transfer in corn (Zea mays) and vegetables (Brassica campestris). To account for soil heterogeneity, samples were collected from non-planted, bulk, and rhizosphere soil zones. Microbial biomass was highest in corn fields treated with biochar alone, whereas earthworm activity significantly altered microbial community structure, particularly the Gram-positive-to-Gram-negative (G⁺/G^-) and fungal-to-bacterial (F/B) ratios. Distinct soil organic C stabilization patterns were observed between crop systems: vegetable fields exhibited increased mineral-associated organic C (MAOC) and dissolved organic C (DOC), while corn fields promoted particulate organic C (POC) and higher POC/MAOC ratios. Redundancy analysis (RDA) revealed that POC was positively correlated with amorphous Fe and the activities of catalase, urease, and fluorescein diacetate (FDA) hydrolase, whereas MAOC was significantly associated with invertase activity and microbial community ratios (G⁺/G^- and F/B). Partial least squares (PLS) analysis identified POC as the dominant organic C pool, primarily regulated by crop type and mediated by soil chemistry, microbial structure, and Fe oxide forms. Biochar application immobilized Cd by increasing POC content, the POC/MAOC ratio, exchangeable Ca, free Fe, and urease activity, while also promoting more stable Cd fractions, including oxidizable and residual forms. Consequently, biochar reduced Cd uptake by 8% in vegetables and 6% in corn. In contrast, earthworm activity enhanced MAOC, DOC, soil electrical conductivity (EC), and microbial community ratios (G⁺/G^- and F/B), but also mobilized soil Cd, increasing total Cd accumulation by 15% in vegetables and 8% in corn under the combined treatment system. Random forest analysis identified crop biomass and the POC/MAOC ratio as the primary drivers of Cd uptake. Pearson correlation analysis further demonstrated that POC negatively correlated with Cd uptake, whereas MAOC positively influenced crop Cd accumulation. Overall, this study provides important insights into how the co-application of earthworms and biochar influences soil organic C storage and Cd dynamics in agricultural systems. While the combined strategy may improve soil C sequestration, caution is warranted because it may simultaneously compromise Cd stabilization, thereby increasing Cd bioavailability and accumulation in crops.