Siltation of check dams alters microbial communities and thus limits organic carbon mineralization

Check dam interception in the Loess Plateau leads to a buildup of silt-based deep soil reserves. These deep soils tend to be high in organic carbon (C), and could dramatically impact our estimates of terrestrial C storage. Despite a growing interest in the terrestrial C cycle, little work has focused on deep soil organic carbon (SOC) pools. In this study, we observed how soil depth and consequential oxygen concentration may influence the way in which microorganisms act on the SOC mineralization process. We found that the change from aerobic to anaerobic environment led to a decrease in cumulative mineralization from 1.37 mg to 1.06 mg and a significant decrease in mineralization rate, which had a limiting effect on SOC mineralization. The reduction of oxygen concentration (approximating depth) also led to changes in the dominant population of bacteria/fungi. Microorganisms with higher C metabolism capacity in aerobic conditions had higher utilization of C sources than that in anaerobic conditions, and vice versa. The limiting factors affecting the amount of SOC mineralization in aerobic soil were carbohydrate, Rokubacteria and Mortierellomycota, and that in anaerobic soil were Amino acid, Mortierellomycota and Ascomycota. Microorganisms with high C metabolism in anaerobic conditions have a weaker metabolism in aerobic conditions, and vice versa. We further quantified the direct and interactive contribution of each factor to SOC mineralization and found that when the soil was aerobic, the contribution of single factor to SOC mineralization was significantly lower (0.32) than the interactions (0.65). Conversely, when the soil was anaerobic, the contribution of single factor to SOC mineralization was much higher (0.76) than the interactions (0.20). When the soil is in anaerobic condition for a long time, the contribution of single factor will gradually increase and the interactions among factors will gradually weaken, further limiting SOC mineralization. In this study, the mechanism of SOC mineralization based on microbial activity in anaerobic and aerobic environment of check dam was reported for the first time, providing theoretical support for understanding the contribution of check dam construction to SOC pool on the Loess Plateau.