In reconstructed grasslands, soil organic matter (SOM) is the largest CO2 and reactive N sink but SOM gains after reconstruction rarely achieve precultivation levels. Through a chronosequence of reconstructed grasslands 1 to 21 yr after establishment, we explored which physicochemical mechanisms protect accumulated soil organic C (SOC) and N from mineralization. After 21 yr, total SOC and soil N concentrations increased by 32 and 23%. The SOC concentration was within 5% of a new equilibrium but was 64% of a never‐cultivated remnant. Chemically stabilized C and N pools on free silt and clay surfaces increased with time. Coarse particulate organic matter C increased with time but accounted for <12% of SOC. Microaggregate‐stabilized SOM did not change. The positive linear relationship between total SOC and free silt and clay C indicates that 21 yr after establishment, reconstructions have unsatisfied capacity for further SOM storage, despite proximity to a new SOC equilibrium. The accumulated C and N associated with free silt and clay suggest that ammonium oxalateextractable Fe (AmOx‐Fe) and polyvalent cation concentrations could be correlated with total C and N stocks. These promote SOM stabilization and were possibly affected by human activity before reconstruction. However, AmOx‐Fe and polyvalent cation concentrations were not associated with total SOC or soil N and could not explain the slowing SOM accumulation. Regardless of time since reconstruction, AmOx‐Fe was highly concentrated on microaggregate surfaces compared with other fractions and was positively associated with microaggregate C and N, suggesting a link between Fe and microaggregate stabilization.