Co-application of biochar and light rare earth elements: An “immobilization-activation” strategy for enhanced Amorphophallus konjac productivity

Amorphophallus konjac productivity is constrained by soil degradation and suboptimal nutrient availability. While biochar and light rare earth elements (LREE) can act as effective soil amendments, the mechanistic underlying their co-application in tuber-crop systems are not fully understood. This study investigated the effects of biochar (1–3%) and LREE-based composite (1–3%) applied in a 3 × 3 factorial experiment, with an additional untreated control (CK), on A. konjac growth, konjac glucomannan (KGM) content, and rhizosphere chemical and microbial characteristics. Soil chemical profiling, metagenomics analysis, and physiological assessments were used to analyze rhizosphere responses. Co-application, particularly 3% biochar with 2% LREE (D32), significantly increased corm yield by 21.6% and KGM content by 26.2% compared with the CK. Significant biochar × LREE interactions influenced soil pH and inorganic nitrogen speciation, contributing to NH + 4 -N stabilization. Metagenomic analysis indicated rhizosphere microbial communities shifted, including the enrichment of specific taxa (e.g., Rhizophagus and Sphingobium ), the decreased relative abundance of Fusarium , and the concurrent upregulation of carbon and nitrogen cycling genes. Furthermore, biochar application maintained corm LREE accumulation far below safety limits. These results indicate that biochar-LREE co-application can simultaneously enhance growth, KGM quality, and maintain LREE safety in A. konjac , supporting its potential as a soil amendment strategy under degraded soil conditions.