Top-meter soil organic carbon stocks and sources in restored mangrove forests of different ages

Knowledge of the changes in the soil organic carbon (OC) stock and its sources with forest age is crucial to understanding the carbon processes and carbon sequestration benefits associated with mangrove restoration. The present study compared the soil OC contents and stocks among three restored Kandelia obovata mangrove forests with different ages (12, 24 and 48 years) in South China. The contributions of mangrove and allochthonous suspended particulate organic matter (SPOM) in tidal water to soil OC were also estimated. The results showed increases in soil OC content and density with forest age, and the OC stocks in the top meter of soil were 13.0 +/- 0.5 kg C m(-2), 15.1 +/- 0.7 kg C m(-2) and 17.0 +/- 0.9 kg C m(-2) at the 12-year site, 24-year site and 48-year site, respectively. We also found a higher soil labile OC content at the 48-year site. The higher soil OC content was accompanied by more C-13-depleted OC in the older forests, and the significantly negative relationship between soil OC contents and delta C-13 values indicated that the increase in soil OC content relied on the input of C-13-depleted mangrove organic matter. Isotope mixing calculations showed that the soil OC was dominated by SPOM at the 12-year site but was dominated by both mangrove-derived OC and SPOM at the 48-year site. Moreover, the findings also suggested that the effect of mangrove restoration on the soil OC content and stock was more substantial in the upper soil layers.