Unexpected high carbon losses in a continental glacier foreland on the Tibetan Plateau

Closely related with microbial activities, soil developments along the glacier forelands are generally considered a carbon sink; however, those of continental glacier forelands remain unclear. Continental glaciers are characterized by dry conditions and low temperature that limit microbial growth. We investigated the carbon characteristics along a chronosequence of the Laohugou Glacier No. 12 foreland, a typical continental glacier on the Tibetan Plateau, by analyzing soil bacterial community structure and microbial carbon-related functional potentials. We found an unexpected carbon loss in which soil organic carbon decreased from 22.21 g kg^-1 to 10.77 g kg^-1 after receding 50 years. Structural equation modeling verified the important positive impacts from bacterial community. Lower carbon fixation efficiency along the chronosequence was supported by less autotrophic bacteria and carbon fixation genes relating to the reductive tricarboxylic acid cycle. Lower carbon availability and higher carbon requirements were identified by an increasing bacterial copy number and a shift of the dominant bacterial community from Proteobacteria and Bacteroidetes (r-strategists) to Actinobacteria and Acidobacteria (K-strategists). Our findings show that the carbon loss of continental glacier foreland was significantly affected by the changes of bacterial community, and can help to avoid overestimating the carbon sink characteristics of glacier forelands in climate models.