This study aimed to evaluate the potential of two carbon (C)-rich carrier materials derived from agricultural residues, spent mushroom substrate (SMS) and composted wheat straw (CWS) for enhancing soil carbon stabilization in an Andisol, with a view towards their future application in smart fertilizer design. We investigated how their contrasting nitrogen contents and application rates affect soil organic carbon dynamics and contribute to sustainable soil management and climate change mitigation. Soil incubations were conducted over 365 days, during which C mineralization, enzymatic activities, and C and N sequestration were assessed. SMS addition at both low and high rates (0.5% and 1% soil C increase, respectively) enhanced soil organic carbon (SOC) stabilization, increasing mean residence times (40.6 and 48.8 years) and half-life times (28.1 and 33.8 years) of the stable C pool compared to unamended soil (35.9 and 24.9 years). High-rate CWS application (1% soil C increase) promoted native SOC decomposition, increasing C losses (5.8%) and reducing C sequestration potential (96%). However, low-rate CWS application (0.5% soil C increase) showed promise, increasing mean residence time (46.8 years) and half-life time (32.4 years) of the stable C pool. Spearman correlations revealed positive associations between electrical conductivity, total N, humification indices, and C stabilization parameters, highlighting the importance of nutrient availability and humification potential for C stabilization. Incorporating C-rich carrier materials with balanced nutrient content, such as SMS, can enhance soil C stabilization and support climate-smart agriculture goals. Low-rate CWS application also shows potential as an alternative C-rich carrier material. However, careful consideration of application rates and material properties is crucial to avoid adverse effects on native SOC mineralization.