The roles of parent material, climate, and geomorphology in soil organic carbon response to short-term climate change in moist boreal forests

Boreal forests store significant soil organic carbon (SOC) where increasing temperature and extreme precipitation are expected with climate change. Yet, the impact on subsoil SOC is unclear. Dissolved organic matter (DOM) facilitates Al weathering to precipitate Al organo-metal complexes (AlOMC) that stabilize mineral horizon SOC. An enhanced DOM input is expected with climate change, however, a high water-flux associated with extreme precipitation may limit AlOMC precipitation. This thesis investigates the roles of parent material composition, climate and geomorphology in boreal SOC dynamics using climate and hillslope transects to enhance: (1) subsoil SOC predictions and (2) its response to short-term climate change. A SOC content predictive model was developed using soils from four climate regions with varying parent material. Unsurprisingly, AlOMC content was the main predictor, with greater SOC in regions with high Al availability. Further interactions with depth-dependent factors, C saturation of AlOMC (C:AlOMC) and their proportion of SOC, suggested infiltration depth is key within a region. As such, greater SOC is supported on gentle slopes via deeper infiltration. Controls on SOC response to extreme precipitation were evaluated experimentally with increasing soil moisture to emulate events occurring on dry summer soils to wet late autumn soils. Antecedent soil moisture and C:AlOMC controlled SOC response under high water flux regardless of parent material or climate. I present a simple predictive model demonstrating shallow SOC loss but deeper storage potential, with greater potential on gentle slopes. Further, enhanced loss from dry soils indicates late summer SOC is most vulnerable to loss (~1‰). Therefore, event timing and infiltration depth are key for SOC response to extreme precipitation. Weathering profiles (mass transfer coefficients), AlOMC and SOC distributions suggest that Al availability sets the water flux threshold for AlOMC precipitation. Extreme events surpass this threshold, limiting new SOC storage. Enhanced SOC storage is expected with an increasing DOM flux with climate change in Al-rich regions supporting higher AlOMC precipitation thresholds, while Al-poor regions may experience SOC loss. These results find regional parent material overrides climate controls on boreal SOC, and this will inform carbon feedbacks to improve Earth Systems models.

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