Effects of different land use types on soil organic carbon and its stability in coastal wetlands of Wenzhou.

The results indicated that: 1) The soils under K. obovata and S. alterniflora had the highest contents of SOC, labile organic carbon (LOC), and dissolved organic carbon (DOC), which ranged from 8.85 to 10.85 g·kg-1, 0.876 to 1.125 g·kg-1, and 0.203 to 0.312 g·kg-1, respectively; these values were 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 (Fed) and free aluminum oxide (Ald), reaching 12.11 ± 0.74 g·kg-1 and 3.91 ± 0.31 g·kg-1, respectively.

Moreover, the proportions of Fe(III) and Al–O in the K. obovata soil reached 82.2% 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, at 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). This study demonstrates that K. obovata mangroves can markedly 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.