Soil organic carbon (SOC) stabilization in degraded arid ecosystems depends on understanding shifts between plant- and microbial-derived C pathways during restoration. Using a 50-year chronosequence of open-canopy Robinia pseudoacacia plantations in China's Mu Us Sandy Land, we quantified plant-derived C (using lignin phenols) and microbial-derived C (using amino sugars) contribution to SOC, along with particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) pools. Microbial-derived C increased with stand age, peaking at 30 years (65-70% SOC). This shift is mechanistically explained by (i) enhanced root exudation and enzyme activity fueling the "microbial carbon pump," (ii) preferential stabilization of fungal residues (58-61% SOC) through organo-mineral complexation with Fe/Al oxides as MAOC, and (iii) phosphorus limitation after 30Y, causing a subsequent decline. Plant-derived C declined sharply despite increasing biomass, reflecting accelerated lignin biotransformation rather than chemical preservation. Random forest analysis identified TDN, total phosphorus, pH, and CBH as key predictors of microbial-derived C, while beta-1,4-glucosidase activity, MBN, and AGB predicted plant-derived C. We conclude that microbial necromass, particularly fungal, rather than plant inputs, drives SOC stabilization in afforested sandy soils, with the most significant shift occurring at the 30-year-old forest stand, challenging the paradigm that plant litter recalcitrance governs long-term carbon persistence in restored arid ecosystems.