Converting perennial alfalfa (Medicago sativa L.) to annual crops for soil moisture recovery risks substantial soil organic carbon (SOC) loss. While it is often presumed that alfalfa-converted croplands require less fertilization due to their pre-conversion fertility legacies, the critical role of nutrient management in SOC dynamics during alfalfa conversion has been largely overlooked. We conducted a 15-year split-plot field experiment on China's Loess Plateau to examine how nitrogen (N; 138 kg ha(-)(1) yr(-)(1)) and phosphorus (P; 45 kg ha(-)(1) yr(-)(1)) fertilization affect SOC dynamics in alfalfa-converted versus conventional cropland. Results revealed that alfalfa-converted soils were functionally P-limited but N-sufficient, with available P primarily driving crop biomass accumulation, while available N showed negligible influence. Without fertilization, these soils exhibited twice the SOC decline rate of conventional cropland (-0.098 vs. -0.047 g kg(-)(1) yr(-)(1), p < 0.05). Fertilization stabilized SOC at pre-conversion levels in alfalfa-converted cropland through four synergistic mechanisms: (1) P-driven productivity gains increased crop biomass by 51 % and root biomass by 64 %, enhancing carbon inputs; (2) elevated heavy fraction organic carbon (HFOC; +13 %) linked to a 4.9 % pH decrease that suppressed polyphenol oxidase activity (-9 %) while boosting beta-glucosidase activity (+407 %); (3) increased microbial necromass C contribution to SOC (+44 %) driven by higher soil total N (+12 %) and microbial biomass C (+29 %); and (4) reduced specific SOC mineralization (-18 %) associated with elevated available N (+209 %). Notably, fertilization enhanced SOC retention efficiency by 108 % in alfalfa-converted versus conventional systems, leveraging inherent SOC and N legacies to favor HFOC and fungal necromass stabilization. Our findings clarify distinct nutrient roles: P sustains carbon inputs, while N enhances stabilization and reduces mineralization. Balanced fertilization is thus essential for SOC conservation in alfalfa-converted croplands, warranting future research to establish optimal thresholds that maximize retention while minimizing environmental trade-offs.