Permanent land use and microbial contributions shape soil carbon stabilization in lake floodplains

The study was conducted in the floodplain of Lake Saint-Pierre (LSP), Québec, Canada (46,12°N, 72,49°W, alt. 6.2 m a.s.l). We examined how SOC storage and stabilization vary in the floodplain of Lake Saint-Pierre, along an anthropogenic disturbance gradient of six land uses: conventional and improved croplands, temporary and permanent meadows, marshes, and forested swamps. In all 6 land uses, we quantified SOC stocks, mineral-associated organic matter - C (MAOM-C), particulate organic matter - C (POM-C), soil δ13C and sugars. Land use effects on SOC storage and stabilization were most pronounced in the topsoil (0-10 cm), with forested swamps showing the highest vegetation biomass, SOC, and MAOM-C. According to sugar analyses, microbe-derived inputs were more abundant in MAOM, which is more stable than POM, whereas plant-derived C dominated the latter. Soil C pools increased with decreasing disturbance, with a tipping point occurring in the transition from temporary to permanent meadows. Our results highlight the importance of conserving permanent and low-disturbance land uses to promote SOC accumulation in floodplain ecosystems and emphasize the central role of microbial metabolism in SOC stabilization processes.