Afforestation-induced large macroaggregate formation promotes soil organic carbon accumulation in degraded karst area

Cropland afforestation-induced soil organic carbon (SOC) accumulation is one of the major strategy for mitigating atmospheric CO2 and climate change, which plays an important role in the terrestrial ecosystem carbon cycle and sustainable land use management strategies. However, soil aggregate formation and its interaction with SOC are still unclear, and the changes in new and old organic carbon (OC) within aggregates after afforestation, especially in a fragile karst ecosystem, are rarely studied. Soil samples from cropland (reference) and two afforestation types (natural forest and managed forest) at 0–20 cm depth were collected. We measured aggregate amounts, OC concentrations, and stocks in bulk soil and aggregates and quantified new and old OC according to the natural abundance of δ13C in bulk soil and aggregates. Afforestation remarkably increased the proportion of large macroaggregate (LMA) and decreased the proportions of small macroaggregate (SMA), microaggregate (MI), and silt + clay (SC) fraction compared with cropland in 0–20 cm depth. Afforestation remarkably increased OC stock in LMA but decreased OC stocks in SMA, MI, and SC. The SOC sequestration in total soil with afforestation depends on the increase in OC in LMA, which was due to the increased LMA amounts rather than the increased OC concentration in LMA. The afforestation-induced aggregation of LMA resulted in an increased physical protection of SOC, which led to substantial SOC accumulation from these aggregates. Moreover, the new and old OC stocks in LMA and SMA dominated in total new and old OC stocks, and the accumulation rates of new OC stocks tended to increase as the aggregate size increased. These results indicated that SOC accumulation was related to the rapid increase in new OC and the slow loss of old OC in bulk soil and aggregates. Natural forest could be a better afforestation strategy, as it facilitates LMA formation better than managed forest and promotes SOC sequestration.