Natural-abundance δ13C reveals a mismatch between macroaggregate carbon accumulation and finer-fraction carbon incorporation after vegetation restoration on the Loess Plateau

Vegetation restoration commonly increases soil organic carbon (SOC) and macroaggregate abundance. However, it remains unclear whether macroaggregate-centered SOC recovery is coupled with greater incorporation of restoration-derived C into finer aggregate fractions or remains largely confined to macroaggregates. Here, we compared active cropland with two woody vegetation types restored for 24 years, forestland and shrubland, in the hilly-gully region of the Loess Plateau. We quantified plant inputs, soil properties, aggregate structure, SOC pools, δ 13 C signatures, new/old C allocation, and Δ δ 13 C -based isotopic distances at 0–10 and 10–20 cm. Vegetation restoration enriched macroaggregates and increased bulk-soil SOC storage by 75.5–79.3% in the 0–10 cm layer and by 24.3–24.6% in the 10–20 cm layer. Macroaggregates dominated restored SOC storage, contributing 67.3–71.8% in topsoil and 64.0–64.7% in subsoil, compared with 58.4% and 58.8% in cropland, respectively. Newly derived C accounted for 52.9–74.8% of restored topsoil SOC and was preferentially retained in macroaggregates, whereas finer fractions showed proportionally greater old-C enrichment. In the 0–10 cm layer, macroaggregate-silt + clay isotopic distance was significantly greater in both restored vegetation types than in cropland, whereas macroaggregate-microaggregate and microaggregate-silt + clay distances did not differ significantly among vegetation types. The 10–20 cm pattern was less consistent, with a significant macroaggregate-silt + clay difference detected only between cropland and shrubland. Thus, after 24 years of restoration, newly accumulated C remained concentrated mainly in macroaggregates, without a corresponding strengthening of its association with finer fractions. These findings distinguish where restoration-derived C is stored from how strongly it is associated with finer, potentially more persistent fractions.