Soil developments along glacier forelands, closely related to microbial activities, are generally considered a carbon sink; however, those of continental glacier forelands remain unclear. This study investigated carbon characteristics along a chronosequence of the Laohugou Glacier No. 12 foreland on the Tibetan Plateau, analyzing soil bacterial community structure and microbial carbon-related functional potentials. Unexpected carbon loss was found, with soil organic carbon decreasing significantly after 50 years of glacial retreat. Structural equation modeling verified the significant positive impacts of bacterial community changes on carbon loss. Lower carbon fixation efficiency was supported by fewer autotrophic bacteria and carbon fixation genes related to the reductive tricarboxylic acid cycle. Lower carbon availability and higher carbon requirements were identified by an increasing bacterial copy number and a shift in dominant bacterial communities. Findings show that continental glacier foreland carbon loss is significantly affected by bacterial community changes, highlighting the need to avoid overestimating the carbon sink characteristics of glacier forelands in climate models.
