Responses of soil labile organic carbon and water-stable aggregates to reforestation in southern subtropical China

Aims Reforestation can enhance soil carbon (C) stability and promote soil C accumulation. Experimental results are, however, highly variable, and the efficacy of reforestation in enhancing soil C stability is still in debate. Consequently, it remains unclear how the different soil C pools respond to reforestation in forest ecosystems. Methods The response of different soil C fractions to reforestation was examined in five subtropical forests, including the plantations of Eucalyptus urophylla (EU), Acacia crassicarpa (AC), Castanopsis hystrix (CH) and 10 species mixed (MX), and a naturally recovered shrubland (NS). Soil labile C fractions (readily oxidized organic C by KMnO4: ROC; dissolved organic C: DOC), distribution of aggregate-size classes and aggregate-associated C from different soil layers (0-10, 10-20, 20-40 and 40-60 cm) were evaluated. Important Findings We found that reforestation and forest type did not affect ROC concentration, yet the highest DOC concentration was detected in NS at four soil layers. Aggregate C concentration was the highest in all aggregate-size classes of CH at 0-10 cm depth. In addition, forest type did not alter the proportion of soil water-stable aggregates at four soil layers. However, soil depths significantly affected the distribution of soil aggregates with >0.25 mm aggregates dominating in the topsoils (0-20 cm), but 0.053-2 mm aggregates being dominant in the deep soils (20-60 cm). These results indicate that reforestation and forest type affected soil DOC (0-60 cm) and aggregate C (0-10 cm). Furthermore, soil DOC and aggregate C were more susceptive to reforestation than ROC. The findings suggest that plantations reduce soil DOC concentration, highlighting that C leaching loss may decrease compared with natural recovery. Moreover, C. hystrix plantation may enhance soil C stability by physical protection in topsoil. This study provides valuable information on tree species selection for reforestation concerning soil C sequestration in southern subtropical China.