Microbial carbon use efficiency (CUE) regulates the allocation of assimilated carbon between microbial biomass production and respiration during grassland restoration; however, its aggregate-scale drivers remain unclear. We investigated how phosphorus limitation affects microbial CUE across different aggregate fractions during a 45-year restoration chronosequence on the Loess Plateau of China. Soil samples were separated into large macroaggregates (LMA, >2 mm), small macroaggregates (SMA, 0.25-2 mm), and microaggregates (MI, <0.25 mm). Enzyme stoichiometry vector analysis indicated consistent shifts from carbon to phosphorus limitation across all aggregate fractions as restoration progressed. Despite these convergent metabolic constraints, CUE exhibited aggregate-specific dynamics: LMA showed continuous decline (24 % reduction), SMA displayed a U-shaped response (initially decreasd significantly by 36 %, followed by a significant recovery), while MI maintained relatively stable CUE throughout the chronosequence. Structural equation modeling demonstrated that microbial biomass C:N ratio was the primary positive driver of CUE in large macroaggregates, whereas the dissolved organic carbon: available nitrogen ratio negatively influenced CUE in both small macro- and microaggregates. Our results demonstrate that soil aggregate architecture fundamentally mediates stoichiometric imbalances and microbial metabolism during ecosystem restoration. The adverse correlation between phosphorus limitation and CUE in LMA contrasts with adaptive responses in SMA, underscoring the necessity of considering microhabitat-specific constraints when predicting soil carbon dynamics, which has implications for optimizing restoration practices to enhance soil carbon sequestration.