Moisture-mineral interactions drive bacterial and organic matter turnover in glacier-sourced riparian sediments undergoing pedogenesis

Glacial recession is occurring at unprecedented rates resulting in increased sediment accumulations in some riverine ecosystems providing new mineral surfaces for soil formation. Soils and sediments have an enormous potential to retain carbon (C), predominantly due to sorption to mineral surfaces. However, C persistence may be sensitive to climate-change induced temperature and moisture variations. We coupled ultrahigh resolution organic matter composition classification with bacterial characterization and respiration measurements to test the combined pedogenic effects of temperature (4 vs 20 °C) and moisture (50 vs 100% water-filled pore space) on C turnover in sediments maintained under different mineralogical conditions (illite-amended vs non-amended). Here we show that the inhibition of CO₂ emissions from the combined effect of increased moisture content and illite was reflected in the turnover of key molecular signatures, such as the nominal oxidation state of C, often irrespective of temperature. However, shifts in bacterial communities from a coupled moisture-mineral interaction, were temperature-dependent. Our results highlight the importance of moisture in driving mineral-organic interactions and suggest that C in clay-rich, water-saturated sediments is both thermodynamically unfavorable and mineral-protected from microbial consumption.