Soil aggregates reveal tree species and land-use legacy effects on early carbon storage pathways during reforestation

Reforestation is a leading natural climate solution, but bulk soil organic carbon (C) often responds slowly, obscuring early belowground change. We tested whether soil aggregates reveal early structural reorganization that affects C retention pathways at a three-year-old experimental reforestation planting established on former pasture in Oregon, USA. We sampled soils at 0-20 and 20-40 cm beneath incense cedar (Calocedrus decurrens; arbuscular mycorrhizal [AM]), black cottonwood (Populus trichocarpa; AM and ectomycorrhizal [EcM]), ponderosa pine (Pinus ponderosa; EcM), and treeless controls. We measured bulk soil C concentration and C:N, aggregate size distribution and mean weight diameter (MWD), fraction-associated C, and, in a subset of surface aggregate fractions, natural-abundance {delta}13C to evaluate soil C pools, physical structure, C distribution, and C processing. After three years, reforestation did not produce significant differences in bulk soil C between planted trees and treeless controls. In surface soil, MWD averaged 36% higher under incense cedar and 58% higher under black cottonwood than under controls, whereas ponderosa pine remained similar to controls. The clearest treatment differences in C distribution occurred in macroaggregates. Incense cedar and black cottonwood had higher large (>2000 m) macroaggregate-associated C than controls, while black cottonwood combined a greater proportion of large macroaggregates with lower C concentrations, indicating that structural development and C accumulation were partly decoupled. Species patterns were broadly consistent with stronger early aggregate responses under AM-compatible species in former-pasture legacy conditions; by contrast, the EcM-associated ponderosa pine remained closer to treeless controls across multiple aggregate measures. Treatment effects weakened with depth and were limited in microaggregates (250-53 m) and silt-and-clay (<53 m) fractions. Smaller aggregate fractions were progressively enriched in {delta}13C, consistent with greater C processing in protected fractions, but treatment-level isotope differences had not yet emerged. Soil aggregates showed species-associated belowground reorganization, revealing how tree identity and former-pasture legacy may shape early soil C organization before consistent differences emerged in bulk soil C. Aggregate measures can complement bulk soil C measurements in restoration monitoring as early indicators of belowground soil C trajectories shaped by tree species identity and land-use legacy.