Distribution characteristics and sources of organic carbon in wetlands of the Yellow River Basin

Wetlands in the Yellow River Basin constitute an important component of China’s wetland resources and play a critical role in conserving the diversity of wetland ecosystems. To elucidate the characteristics of carbon cycling and carbon pools in wetlands of the Yellow River Basin, this study selected four typical wetland sites across the basin: Tianhe Bay Wetland in Ningxia (upper reaches), Sanmenxia Reservoir Wetland and Mengjin Yellow River Wetland in Henan (middle reaches), and the Yellow River Delta Wetland (lower reaches). The organic carbon contents and coupled δ¹³C-TOC signatures of wetland vegetation and soils were determined to clarify the spatial distribution and source apportionment of organic carbon across the Yellow River Basin. The results showed that the mean vegetation organic carbon contents in the upper, middle, and lower reaches were 30.21%, 34.76% and 38.13%, respectively. For aboveground vegetation tissues, the corresponding mean organic carbon contents were 30.23%, 41.21% and 35.68%, while those of belowground roots were 30.20%, 28.30% and 40.57%, respectively. Vegetation organic carbon ranked in the order: lower reaches>middle reaches>upper reaches (p>0.05). In the 0-25 cm soil layer, the mean soil organic carbon contents across the upper, middle, and lower reaches were 0.34%, 0.29% and 0.53%, respectively, following the order of lower reaches>upper reaches>middle reaches (p>0.05). In the upper reaches, the vertical variation of soil δ¹³C-TOC values across different plant communities ranged from −27.80‰ to −25.31‰, falling within the typical δ¹³C range of C₃ plant-derived organic matter. Specifically, the soil δ¹³C-TOC signatures of the Phragmites australis community were closer to those of its aboveground stems and leaves, whereas soils under the Catabrosa aquatica and Salix matsudana communities exhibited δ¹³C-TOC signatures closer to those of their respective belowground roots. In the middle reaches, the vertical soil δ¹³C-TOC values across different communities varied from −28.02‰ to −21.72‰, the soil δ¹³C-TOC of partial plant communities exceed the canonical δ¹³C interval for pure C₃ plant-derived organic matter and reflecting mixed contributions from terrestrial C₃ and C₄ higher plants with dominant C₃ signals. The soil δ¹³C-TOC signatures of the Typha orientalis, S. matsudana, and P. australis communities at the Sanmenxia Wetland and the T. orientalis community at the Mengjin Wetland were all consistent with the δ¹³C-TOC of their respective belowground roots. By contrast, soils from the P. australis community at the Mengjin Wetland matched the isotopic signatures of aboveground stems and leaves of P. australis. In the lower reaches, the vertical soil δ¹³C-TOC values ranged from −28.08‰ to −21.01‰, encompassing the isotopic domains of both terrigenous C₃ plant-derived and marine autochthonous organic matter, with a predominance of C₃ plant sources. The soil δ¹³C-TOC signatures of the P. australis, mixed S. matsudana-P. australis, and P. australis in tidal flat were all closer to those of the aboveground stems and leaves of the dominant species P. australis.