Microbial processes underpin soil organic carbon (SOC) stabilization, yet how afforestation regulates these mechanisms across deep soil profiles remains poorly understood. Here, we combined metagenomic and microbial biomarker (amino sugars) analyses along a 0-900 cm soil profile on the Chinese Loess Plateau to investigate how cropland afforestation with Robinia pseudoacacia reshapes microbial strategies and metabolic functions, and their links to microbial necromass carbon (MNC) and carbon cycling potential (CCP). Afforestation significantly increased SOC, total nitrogen, and extracellular enzyme activities in surface soils (0-20 cm), resulting in a fourfold increase in MNC, although no significant changes occurred in deep soils. Moreover, afforestation significantly altered microbial life-history strategies, shifting communities from a predominantly growth-yield (Y) strategy toward coordinated resource-acquisition and stress-tolerance (A-S) strategies. These shifts were associated with pronounced changes in deep soil resource availability, particularly soil moisture depletion and nutrient redistribution following afforestation. Importantly, variations in microbial life-history strategies were closely linked to MNC and CCP across the soil profile. Notably, thiosulfate oxidation (soxACDXYZ) emerged as a key pathway potentially linking microbial energy metabolism to SOC stabilization. Taken together, our results demonstrate that afforestation-induced reorganization of microbial functional strategies across the 0-900 cm soil depth is strongly associated with changes in MNC and CCP, highlighting the important role of subsurface microbial processes in impacting carbon dynamics of afforested ecosystems.