The long-term impacts of intensive agricultural reclamation on the turnover and stabilization of soil organic carbon (SOC) in desert ecosystems remain poorly understood, particularly throughout deep soil profiles. Using a paired-site approach in northwest China, we investigated how the conversion of sandy land to cropland following 15 years of reclamation shaped the vertical distribution (0-200 cm) and composition of SOC fractions, focusing on particulate (POC) and mineral-associated organic carbon (MAOC). Our results show that over 60% of total SOC stock is stored below 60 cm depth, with deep-soil carbon increasing significantly from 18.4 t ha-1 in natural land to 27.2 t ha-1 following reclamation, underscoring the critical role of subsoil carbon sequestration in arid regions under land-use change. Reclamation fundamentally shifted SOC composition from POC dominance to MAOC dominance, with subsoil MAOC increasing by up to 133.9%, indicating enhanced stability. A strong correlation between microbial necromass carbon (MNC) and MAOC in the topsoil suggests a predominantly microbial-mediated pathway for MAOC formation in surface layers. MAOC correlated negatively with aliphatic-C and positively with polysaccharide-C, collectively pointing to microbial transformation and subsequent mineral stabilization as key processes in MAOC formation. Key factors including total nitrogen, available nitrogen, clay content, and soil moisture were identified as primary predictors of MAOC accumulation, with depth-dependent influences. These findings demonstrate that long-term reclamation markedly promotes MAOC accumulation and carbon sequestration capacity in deep soil, while clarifying associated biological and physicochemical stabilization mechanisms. These insights into SOC persistence under land-use change are crucial for developing sustainable soil management and climate-adaptive agriculture in drylands.