Abstract: Reclamation aquaculture and invasions by alien species have caused severe degradation of coastal tidal-flat wetlands in China. Mangrove restoration is an important pathway for improving the ecological functions of these tidal flats. At the northern limit of mangrove afforestation in China (Wenzhou, Zhejiang Province), the effects of mangrove planting on the sequestration capacity and stability of soil organic carbon (SOC) remain unclear. In this study, we analyzed the contents and fractions of soil organic carbon, as well as their association characteristics with iron and aluminum oxides across these different land-use types in Yueqing Bay of Wenzhou as the research area. There were four representative land-use types, including bare mudflat representing the pristine state, Kandelia obovata forest representing mangrove restoration, Spartina alterniflora marsh representing alien plant invasion, and aquaculture ponds representing intensive anthropogenic use. The results showed that: 1) Soils under K. obovata and S. alterniflora had higher contents of SOC, labile organic carbon, and dissolved organic carbon, which ranged from 10.76 to 10.85, 1.122 to 1.125, and 0.311 to 0.312 g·kg^-1, respectively, being significantly higher than those in the bare mudflat and aquaculture ponds. 2) Compared with the other soil types, the K. obovata soil exhibited the highest contents of free iron oxide and free aluminum oxide, reaching (12.11±0.74) and (3.91±0.31) g·kg^-1, respectively. Moreover, the proportions of Fe(Ⅲ) and Al-O in the K. obovata soil reached 82.3% and 93.6%, respectively, which were significantly higher than those in S. alterniflora (66.7% and 57.3%), bare mudflat (65.7% and 81.2%), and aquaculture ponds (65.7% and 84.8%). 3) The molar ratios of carbon to iron (C/Fe) in all soils were below 1, indicating that organic carbon was predominantly retained through association with iron oxides by adsorption. The content of iron-bound organic carbon (Fe-OC) was highest in the K. obovata soil (5.82±1.03 g·kg^-1), which was significantly higher than that in S. alterniflora (2.98±0.43 g·kg^-1), aquaculture ponds (2.95±0.16 g·kg^-1), and bare mudflat (2.21±0.31 g·kg^-1). Our results demonstrated that K. obovata mangroves could enhance the sequestration and stability of soil organic carbon in coastal wetlands by promoting the transformation of iron and aluminum oxides into crystalline forms and forming high-valence mineral-organic complex interfaces.

Key words: coastal blue carbon, mangroves, soil organic carbon, iron oxide, mineral protection