Stand age, fire and clearcutting affect soil organic carbon and aggregation of mineral soils in boreal forests

Soil organic carbon (SOC) in mineral soil accounts for a large portion of total ecosystem carbon (C) in boreal forests. We evaluated the effects of stand age and disturbance origin on SOC, soil aggregate stability, and aggregate-associated SOC in the boreal forests of Ontario, Canada. Mineral soils at 0–15 cm depth were sampled in 27 stands of six post-fire age classes (2- to 203-year-old) and three post-clearcut age classes (2- to 29-year-old), each with three replications. In post-fire stands, the SOC pool increased from 2- and 10-year-old to 29-, 85- and 140-year-old, and then decreased in 203-year-old stands. Aggregate-associated SOC showed a similar trend. Abundance of water stable aggregates (>0.25 mm in diameter) was the highest in 2-year-old stands. Compared with the same-aged post-fire stands, the SOC pool and aggregate-associated SOC were higher, and aggregate stability was lower in 2- and 10-year-old post-clearcut stands. But the differences in SOC pool, aggregate-associated SOC, and aggregate stability between the two stand origins diminished or became less dramatic in 29-year-old stands. Our results indicate that aggregate stability is more dependent on thermal modification of SOC by fire than on aggregate-associated SOC. Our results also show higher SOC pool and aggregate-associated SOC but lower aggregate stability in post-clearcut than post-fire stands shortly after disturbance; however, differences between the two stand origins diminish when stands become older.