Magnesium enhances soil carbon sequestration by modulating carbon-cycling microorganisms under reduced fertilisation conditions

• Reduced fertilisation led to a decline in soil organic carbon (SOC) in tea plantation soils. • Magnesium fertilisation reversed SOC loss by improving soil pH, nutrient availability, and organic inputs. • Magnesium enhanced SOC stabilisation by modulating carbon-cycling genes (AAs, CBMs, PLs) and microbial communities. • Exchangeable magnesium and the enrichment of carbon-cycling functional genes were the key drivers of SOC accumulation. Soil organic carbon (SOC) depletion remains a central challenge for tea plantations under intensive fertilisation. We evaluated three fertilisation strategies including farmer fertilisation (FF), optimised fertilisation (OF), and optimised fertilisation with magnesium (OF+Mg)—to assess their effects on soil properties, microbial carbon-cycling functions, and SOC dynamics. Compared with FF, OF modestly improved nutrient supply but did not raise soil pH or halt SOC decline, indicating continued imbalance between nutrient inputs and organic-matter retention. In contrast, OF+Mg increased soil pH, enriched exchangeable Mg and Ca, lowered ammonium concentrations, and reversed SOC losses, yielding a 19.5 % SOC increase relative to OF. Metagenomic profiling showed a shift from glycoside hydrolases toward auxiliary activity enzymes, carbohydrate-binding modules, and polysaccharide lyases, consistent with enhanced carbon stabilisation. Redundancy analysis and structural equation modelling identified exchangeable Mg and carbon-cycling enzyme groups (AAs, PLs) as key drivers of SOC sequestration. Overall, while OF optimised nutrient delivery, only OF+Mg strengthened soil buffering capacity, increased microbial functional potential, and promoted SOC accumulation, providing a practicable pathway toward carbon-neutral soil management in acidified tea plantations.