Mg/Fe-layered double hydroxide-loaded biochar enhances nitrogen retention and reduces geochemically reactive metal concentrations in paddy soil: Bacterial community responses

Intensive rice cultivation faces the dual challenges of low nitrogen use efficiency and heavy metal pollution risk. In this study, a previously developed magnesium–iron layered double hydroxide-loaded biochar (Mg/Fe-LDH@BC) composite was evaluated in paddy soil to determine its effects on nitrogen retention, heavy metal activity, crop growth and bacterial community organization. With the addition of the 1% Mg/Fe-LDH@BC, NH 4 + −N retention increased by 41.7% while NO 3 - −N accumulation decreased by 9.8%. Meanwhile, NH 3 volatilization decreased by 24.2%, and N 2 O emissions were reduced by over 50%. These changes were accompanied by increased rice biomass and grain dry mass, together with decreased concentrations of multiple geochemically reactive metals. Microbial community analysis showed that this material reshaped the topology of the bacterial co-occurrence network. The 1% Mg/Fe-LDH@BC material formed the most complex network (1446 nodes and 11,063 edges) and shifted key species from slow-growing groups (such as Syntrophus ) to metabolically active groups associated with carbon, iron, and sulfur cycles (e.g., Streptomyces , Pelobacter , and Thiobacillus ). This network exhibited both high connectivity and a highly modular structure, indicating a more interconnected and compartmentalized pattern of microbial associations. Furthermore, the porous structure of BC may create micro-anaerobic niches that support the survival of anaerobic taxa (e.g., Anaerolinea ). Although the LDH can directly immobilize heavy metals through physicochemical mechanisms, sulfur cycle-related bacteria (e.g., Desulfatitalea ) were identified as key species in the network, suggesting that microorganisms may contribute to in situ heavy metal immobilization through sulfide production.