Wetland restoration after agricultural abandonment enhances soil organic carbon efficiently by stimulating plant- rather than microbial-derived carbon accumulation in Northeast China

Agricultural abandonment is a common measure for wetland restoration, which has been suggested as a policy goal with multiple environmental benefits, including improving regional microclimate and promoting soil organic carbon (SOC) accumulation. However, the processes of SOC accumulation during wetland restoration after agricultural abandonment are not clear. Here, we investigated variations in SOC pools and sources in the wetlands restored for one, three, and four years from the paddy field in Northeast China by measuring the δ13C values of SOC, amino sugars, and lignin phenols. Wetland restoration for three and four years increased the contents of SOC and new C by 22.6–37.2 and 27.9–43.0 g C kg−1, respectively, while did not affect old C content. This indicated that increased SOC was mainly due to new C accumulation during wetland restoration after agricultural abandonment. Wetland restoration for three and four years increased microbial-derived C content by 59.2–102 %, but did not change its proportion in SOC; in contrast, wetland restoration for three and four years increased plant-derived C content by 213–301 %, and its proportion in SOC by 106–122 %. Among lignin phenols in plant-derived C, wetland restoration for three and four years increased the contents of cinnamyl-, syringyl-, and vanillyl-monomers and the ratio of cinnamyl-/vanillyl-monomer, indicating that wetland restoration after agricultural abandonment was favorable to plant-derived C, especially easily decomposable component accumulation. Plant biomass and soil Fe2+/Fe3+ ratio were identified as key factors influencing SOC content. The increasing plant inputs and retarding lignin degradation through suppressing peroxidase activity in lower oxidizing conditions promoted the accumulation of plant-derived C in SOC during wetland restoration after agricultural abandonment. Our result suggested that enhanced SOC during wetland restoration after agricultural abandonment was primarily due to efficient accumulation of plant-derived C rather than due to microbial-derived C accumulation in Northeast China.