Dissolved organic carbon content and characteristics in relation to carbon dioxide partial pressure across Poyang Lake wetlands and adjacent aquatic systems in the Changjiang basin

Dissolved organic carbon (DOC) plays diverse roles in carbon biogeochemical cycles. Here, we explored the link between DOC and pCO₂ using high-performance size-exclusion chromatography (HPSEC) with UV₂₅₄ detection and excitation emission matrix (EEM) fluorescence spectroscopy to determine the molecular weight distribution (MW) and the spectral characteristics of DOC, respectively. The relationship between DOC and pCO₂ was investigated in the Poyang Lake wetlands and their adjacent aquatic systems. The results indicated significant spatial variation in the DOC concentrations, MW distributions, and pCO₂. The DOC concentration was higher in the wetlands than in the rivers and lakes. pCO₂ was high in wetlands in which the dominant vegetation was Phragmites australis, whereas it was low in wetlands in which Carex tristachya was the dominant species. DOC was divided into five fractions according to MW, as follows: super-low MW (SLMW, <1 kDa); low MW (LMW, 1–2.5 kDa); intermediate MW (IMW, 2.5–3.5 kDa); high MW (HMW, 3.5–6 kDa); and super-high MW (SMW, > 40 kDa). Rivers contained high proportions of HMW and extremely low amounts of SLMW, whereas wetlands had relatively high proportions of SLMW. The proportion of SMW (SMW_p) was particularly high in wetlands. We found that pCO₂ significantly positively correlated with the proportion of IMW, and significantly negatively correlated with SMW_p. These data improve our understanding of the MW of bioavailable DOC and its conversion to CO₂. The present results demonstrate that both the content and characteristics of DOC significantly affect pCO₂. pCO₂ and DOC must be studied further to help understanding the role of the wetland on the regional CO₂ budget. - Highlights: • The molecular weight of DOC showed spatial variations across the lake systems. • pCO₂ was related to the proportion of intermediate-molecular-weight DOC. • pCO₂ was negatively related to the proportion of super high molecular weight of DOC. - The molecular weight of DOC showed spatial variations across the Poyang Lake aquatic systems. pCO₂ was related to the proportion of intermediate-molecular-weight DOC.