Global warming has accelerated glacier retreat, exposing soils in glacier forelands. These forelands are characterized by harsh environments with low temperatures and poor nutrient contents, particularly carbon, a crucial element in soil development. While studies in various regions have shown carbon accumulation in glacier forelands, regional variations in carbon resources, climatic, and edaphic factors significantly influence this process. The foreland of Hailuogou Glacier (a temperate glacier on the Tibetan Plateau) shows significant SOC accumulation, contrasting with the smaller increase observed in the Urumqi Glacier No. 1 (a sub-continental glacier). Continental glaciers, covering a significant area in China, are characterized by low temperatures and dry conditions, making them sensitive to climate change. Laohugou Glacier No. 12, a representative continental glacier, experienced significant area reduction. Understanding the carbon characteristics of continental glacier forelands is crucial to improve our understanding of glacier retreat and its feedback effects on global warming. Biological activities, primarily driven by microorganisms (especially bacteria), are major nutrient sources in glacier forelands. The balance between microbial metabolic respiration and carbon fixation greatly influences carbon storage. As exposure time increases, microbial metabolism strengthens, impacting carbon storage. These shifts are closely related to changes in microbial community composition, including the relative proportions of autotrophs and heterotrophs and the abundance of functional genes. This study aimed to investigate the dynamics of carbon mediated by bacteria along a soil chronosequence in the continental glacier forelands of Laohugou Glacier No. 12 to improve our understanding of carbon accumulation in these environments.

Previous research has demonstrated variable patterns of carbon accumulation in glacier forelands, depending on factors such as glacier type, climate, and initial soil conditions. Studies in the Italian Alps, Norway, Iceland, the Antarctic Peninsula, and the High Arctic have documented increases in soil organic carbon (SOC) in glacier forelands. However, the rate of accumulation varies significantly across different regions. The accumulation rate is generally greatest in the initial stages of soil development. Temperate glaciers, like Hailuogou Glacier, exhibit higher rates of SOC accumulation compared to sub-continental glaciers, such as the Urumqi Glacier No. 1. These differences highlight the importance of considering regional variability and glacier type when assessing carbon sequestration potential. The role of microbial communities in driving soil carbon dynamics is well-established. Microorganisms, especially bacteria, act as primary colonizers in glacier forelands, influencing carbon accumulation through their metabolic activities. Studies have highlighted the importance of both carbon fixation and degradation processes in determining the net carbon balance. However, detailed investigations focusing on the microbial community structure and functional gene abundance in continental glacier forelands remain limited. This research gap motivated the current study on the Laohugou Glacier No. 12 foreland.

This study used a chronosequence approach to examine carbon dynamics in the foreland of Laohugou Glacier No. 12, the largest continental glacier on the Tibetan Plateau. Five sampling sites were selected based on the glacier's retreat rate, representing 0, 10, 15, 31, and 50 years of deglaciation. Soil samples were collected in triplicate from each site. Soil physicochemical properties (pH, water content, SOC, TN, TP, DOC, ammonium, and nitrate) were analyzed using standard methods. Microbial community analysis was conducted using high-throughput Illumina sequencing of the V3-V4 region of the 16S rRNA gene. Quantitative microbial element cycling (QMEC) was employed to quantify 32 functional genes related to carbon cycling using the 16S rRNA gene as a reference. Statistical analyses included one-way ANOVA, Fisher's LSD, Pearson correlation, principal coordinate analysis (PCOA), PERMANOVA, and structural equation modeling (SEM). SEM was used to assess the impacts of retreat time, bacterial community (represented by copy number and the ratio of carbon fixation to carbon degradation genes), and soil pH on SOC changes. Alpha diversity indices (observed richness, Chao1, Shannon, and Pielou's evenness) were calculated to assess microbial diversity. Beta diversity was evaluated using Bray-Curtis distance and PERMANOVA.

The study revealed an unexpected pattern of carbon loss in the Laohugou Glacier No. 12 foreland. Despite the generally accepted view that glacier forelands act as carbon sinks, SOC content decreased from 22.21 g kg⁻¹ to 10.77 g kg⁻¹ over 50 years of deglaciation. Soil pH increased slightly over time. While total nitrogen (TN) content increased significantly, total phosphorus (TP) remained relatively stable. The C:N and C:P ratios were highest at 10 years post-deglaciation and decreased significantly afterward. The bacterial copy number increased significantly from 1.07 × 10⁵ copies g⁻¹ soil to 4.95 × 10⁶ copies g⁻¹ soil over 50 years, peaking at 31 years post-deglaciation. The abundance of almost all carbon-related functional genes also increased, peaking at 31 years post-deglaciation, except for several genes. The ratio of carbon fixation to carbon degradation genes (FD ratio) peaked at 10 years post-deglaciation and then decreased. The dominant bacterial phyla shifted from Proteobacteria and Bacteroidetes (r-strategists) to Actinobacteria and Acidobacteria (K-strategists) over time. Proteobacteria abundance decreased significantly, while Actinobacteria abundance increased significantly. Structural equation modeling (SEM) showed that the FD ratio had a strong direct positive effect on SOC, while bacterial abundance had a weaker direct positive effect. Retreat time and soil pH had negative total effects on SOC, with retreat time having a strong direct negative effect.

The observed carbon loss in the Laohugou Glacier No. 12 foreland contrasts with findings from other glacier forelands, highlighting the unique characteristics of continental glacier ecosystems. The increasing bacterial copy number suggests a higher demand for carbon, leading to increased respiration and potentially exceeding carbon fixation. The shift in bacterial community composition from r-strategists to K-strategists reflects the decreasing availability of carbon resources. The decreased efficiency of carbon fixation, evidenced by the reduction in genes related to the reductive tricarboxylic acid (rTCA) cycle and the decrease in autotrophic bacteria, contributes to the carbon loss. The simplified carbon degradation gene groups and the shift in bacterial life strategy further support the hypothesis of lower carbon availability. The SEM results clearly demonstrate the interplay between bacterial community dynamics and SOC changes, emphasizing the importance of microbial processes in shaping carbon cycling in this environment. This study underscores the need to consider the specific characteristics of continental glacier forelands when modeling carbon dynamics and projecting future carbon sequestration potential.

This study reveals unexpected carbon loss in a continental glacier foreland, challenging the common assumption of glacier forelands as carbon sinks. The findings emphasize the critical role of microbial community structure and function in determining carbon cycling in these environments. The observed decrease in carbon fixation efficiency and the shift toward K-strategist bacterial communities suggest that carbon limitation and increased respiration are major contributors to carbon loss. Future research should focus on further exploring the interactions between environmental factors, microbial communities, and carbon cycling in continental glacier forelands under changing climate conditions. This is essential for improving predictions of carbon storage in these ecosystems and for understanding their feedback to global climate change.

This study focused on a single continental glacier foreland on the Tibetan Plateau, limiting the generalizability of the findings to other regions and glacier types. The sampling was conducted at a single point in time, preventing assessment of temporal variability in carbon dynamics. The study primarily focused on bacterial communities, and future studies should incorporate analyses of other microbial groups (e.g., fungi) and their roles in carbon cycling. Further research should investigate the specific mechanisms underlying the observed shifts in bacterial community composition and their influence on carbon fixation and degradation processes.