Differential Effects of Microbial Carbon Chain Elongation Metabolism Induced by Organic Inputs in Soil: Carbohydrates, Lipids, Organic Acids, and Alcohols

The soil carbon pool is critical for global climate regulation, and microbial carbon chain elongation (MCE) plays an indispensable role in the anaerobic soil organic carbon (SOC) transformation. However, how different exogenous carbon sources regulate MCE metabolism and the interaction between MCE and the priming effect (PE) of SOC turnover remain unclear. This study conducted anaerobic soil microcosm experiments with five representative individual exogenous carbon sources (glucose, glycerol, lactic acid, ethanol, and acetic acid) and a carbon-free control. Results showed distinct MCE performance enhancement by carbon sources: ethanol exhibited the highest MCE efficiency (4.05 mM caproic acid) and induced the strongest negative PE (−46.26%); glucose initiated MCE rapidly but caused positive PE (+27.48%); glycerol and lactic acid enabled moderate MCE, while acetic acid failed due to insufficient electron donors. Microbial community analysis revealed exogenous carbon reduced alpha diversity with Clostridium as the core MCE-functional genus. Metagenomics indicated that the reverse β-oxidation pathway dominated MCE, and there was a significant negative correlation between MCE activity and PE, which was mediated by the in situ fixation of CO2 to generate acetyl-CoA for MCE. This study clarifies the microbial carbon metabolisms of MCE regulated by carbon sources, providing a theoretical basis for optimizing soil carbon sequestration strategies.