Lake floodplains as sinks for stable soil organic carbon: is the carbon plant- or microbe-derived, and why does permanent land use matter?

Floodplain ecosystems play a key role in soil organic carbon (SOC) storage, as they integrate inputs from both vegetation and sediments. Promoting land uses with low anthropogenic disturbances helps maintain the function of these ecosystems as soil carbon (C) sinks. However, the tipping points along a disturbance gradient where land use transitions generate the largest SOC losses or gains remain unclear, though they are key for effective land use management and climate change mitigation. Because flood events can negatively impact labile C, determining the main origin of stable SOC, whether from plants or soil microorganisms, is also important to identify the optimal combination of land use, vegetation, and soil type for SOC stabilization in floodplains. We examined how SOC storage and stabilization vary in the floodplain of Lake Saint-Pierre, Quebec, Canada 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 {delta}13C 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. Microbe-derived inputs were more abundant in MAOM, whereas plant-derived C dominated POM. SOC gains 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 persistence in floodplain ecosystems and emphasize the central role of microbial metabolism in stabilization processes.