Terrestrial soil organic carbon (SOC) dynamics play an important but uncertain role in the global carbon (C) cycle. Current modeling efforts to quantify SOC dynamics in response to global environmental changes do not accurately represent the size, distribution and flux of C from the soil. Here, we modified the Daily Century (DAYCENT) biogeochemical model by parameterizing conceptual SOC pools with C fraction data, followed by historical and future simulations of SOC dynamics. Results showed that simulations using modified DAYCENT (DCmod) led to better initialization of SOC stocks and distribution compared to default DAYCENT (DCdef) at long-term research sites. Regional simulation using DCmod demonstrated higher SOC stocks for both croplands (34.86 vs 26.17 MgC ha-1) and grasslands (54.05 vs 40.82 MgC ha-1) compared to DCdef for the contemporary period (2001-2005 average), which better matched observationally constrained data-driven maps of current SOC distributions. Projection of SOC dynamics to land cover change (IPCC AR4 A2 scenario) under IPCC AR5 RCP8.5 climate scenario showed absolute SOC loss of 8.44 and 10.43 MgC ha-1 for grasslands and croplands, respectively, using DCmod whereas, SOC losses were 6.55 and 7.85 MgC ha-1 for grasslands and croplands, respectively, using DCdef. The projected SOC loss using DCmod was 33% and 29% higher for croplands and grasslands compared to DCdef. Our modeling study demonstrates that initializing SOC pools with C fraction data led to more accurate representation of SOC stocks and individual carbon pool, resulting in larger absolute and relative SOC losses due to agricultural intensification in the warming climate.