Soil Organic Carbon and Nitrogen Variations with Vegetation Succession in Passively Restored Freshwater Wetlands

Passive restoration after agricultural abandonment has been widely practiced to improve soil quality and recover the ecological functions of degraded wetlands. However, studies concerning the relationships between soil and vegetation during natural succession are still lacking. In this study, the variations of soil organic carbon (SOC) and total nitrogen (TN), as well as their relationships with soil and vegetation properties were evaluated in passively restored freshwater wetlands with a chronosequence (2, 4, 8, 13, 16, and 20 years) on the Sanjiang Plain, China. An adjacent natural wetland was chosen as a reference system. Results indicated that soil and vegetation in restored wetlands changed substantially overtime, and gradually came to resemble natural wetlands. SOC and TN contents in the 10-30 cm soil layers required less time to achieve a natural level than those in the 0-10 cm soil layers. They were significantly correlated with soil water content and conductivity, especially in the 0-10 cm layer. Moreover, SOC and TN storages were synergistically improved, and highly dependent on plant diversity, height, coverage, and biomass. These results suggest that passive restoration is an efficient measure for forming wetland plant communities after agricultural abandonment, and ultimately enhances SOC and TN accumulation in restored wetlands.