Soil carbon stocks and accumulation in young mangrove forests

Mangrove reforestation and afforestation programs have been initiated in many countries recently to compensate for historical losses. At the same time, awareness of the high carbon (C) sink potential of mangrove forests is growing, and C sequestration is beginning to be considered among forestation goals. To assess whether and at what rate C accumulates in the soil of young mangrove forests following afforestation, we conducted a field study at an afforestation project in southeast China, including repeated measures taken over six years at two young forests (consisting of Kandelia obovata and Sonneratia apetala, aged 0-6 years old), and also a chronosequence of forests aged 0 (mudflat), 6 (both species), 20 (S. apetala), and 70 (K. obovata) years old. In the repeated measures, surface (0-10 cm) soil C concentration (%C of dry soil mass) increased significantly over six years, from 1.14% to 1.52% (K. obovata) and 1.23% to 1.68% (S. apetala). The rates of increase did not differ significantly between the two species, despite much greater biomass of S. apetala. In the chronosequence, soil C also increased with age across sites, but only the 70-year-old forest was statistically different, suggesting that localized environmental differences may obscure age-related patterns in soil C. At all sites, soil C concentration for 1-m soil depth (0.62%-2.43%) was low compared to published global averages, yet the estimated soil C accumulation rate (155 g C m−2 y−1) was comparable to published averages for mature forests. We supported this field study with a literature review of similar studies containing soil C concentration data from young mangrove forests: data compiled from 15 studies, comprising 31 sites, showed consistent, positive changes in soil C concentration with forest age, even in the youngest (<5 years old) forests, supporting our field observation that soil C increases over time following mangrove afforestation.