The formation of large macroaggregates induces soil organic carbon sequestration in short-term cropland restoration in a typical karst area

Abstract

Cropland restoration induces litter and root inputs and promotes the development of biological soil crusts (BSCs), which may promote aggregate formation and soil organic carbon (SOC) sequestration. However, litter, roots and BSCs have not been simultaneously considered when assessing soil aggregate and aggregate-associated SOC fraction responses to cropland restoration in subtropical areas. Here, we measured particulate organic carbon (POC) and mineral-associated organic carbon (MOC) in bulk soils and soil aggregates after 15 years of cropland restoration. Soil samples of cropland (CL) and four cropland restoration types (plantation forest [PF], forage grassland [FG], mixed plantation of forest and forage grassland [FF], and abandoned natural grassland [NG]) from depths of 0–30 cm were collected. Cropland restoration significantly increased SOC and POC in bulk soil at the 0–5 cm depth. However, only in FG did SOC significantly increase at depths of 5–15 cm, and POC significantly increased at depths of 5–30 cm. The large macroaggregate (5–10 mm and 2–5 mm) proportions increased significantly at the 0–15 cm depth after cropland restoration, and FG, FF and NG also increased the 5–10 mm aggregate proportions at the 15–30 cm depth. The SOC sequestration in bulk soil with cropland restoration was attributed to increases in the aggregate-associated organic carbon (AAOC) pool in large macroaggregates, which was mainly attributed to the increased aggregate amount rather than the increased AAOC concentration in large macroaggregates. Our results also indicated that an increase in aggregate-associated particulate organic carbon (AAPOC) led to an increase in AAOC. Variation partitioning indicated that the formation of large macroaggregates was controlled by the litter-moss-root interactive effect in this karst area. FG could be a better short-term cropland restoration strategy, increasing large macroaggregates in deeper soil layers better than the other vegetation types and promoting soil carbon sequestration in deeper soil layers.