Variations in soil organic carbon storage and stability with vegetation restoration stages on the Loess Plateau of China

Soil organic carbon (SOC) plays the most significant role in climate change mitigation and ecosystem productivity. However, dynamic changes in the storage and stability of SOC during vegetation restoration and succession, especially the physical protection role of soil aggregates, are not well understood. This study, therefore, explored the variations in SOC storage and stability along with vegetation restoration by combining the physical protection of soil aggregates with the chemical structural stability of organic carbon (OC). OC fractions and carbon functional groups of bulk soil and soil aggregates were investigated using soil samples collected from abandoned farmland, grassland, miscellaneous shrubland, pioneer arbor forest, and coniferous forest plots representing typical vegetation restoration consequence on the Loess Plateau of China. Vegetation restoration enhanced clumps of clay particles with visible pores and cementing substances. The contents of OC fractions (total, inert, and active carbons) in bulk soil and aggregates significantly increased during vegetation restoration process. Total and active OC in macroaggregates (>0.25 mm) were significantly higher than those in other aggregate fractions, while inert OC had no significant difference among different aggregate fractions. The elevated absorption intensity, i.e., absolute content of OC functional groups, especially stable OC functional groups, enhanced SOC chemical stability during vegetation restoration. From the abandoned farmland to miscellaneous shrubland, the relative content of active OC functional groups significantly increased and was primarily distributed in macroaggregates (84.94–86.32%), while silt–clay aggregates (<0.053 mm) had higher relative content of stable functional groups (21.84–26.53%). A significant increase was observed in the relative content of the stable functional group from miscellaneous shrubland to pioneer arbor forest, primarily in silt–clay aggregates (∼27.15%). In the coniferous forest, stable functional groups mainly distributed in macroaggregates also showed the most pronounced increase (∼36.15%). We concluded that vegetation restoration improves the soil structure and storage and stability of SOC due to the increased accumulation of OC in macroaggregates and enhanced chemical stability of OC in silt–clay aggregates from the abandoned farmland to the pioneer arbor forest. However, SOC stability in the coniferous forest is significantly influenced by macroaggregates in the soil.