Long-term operated constructed wetlands: High organic carbon storage and autochthonous carbon dominance

Constructed wetlands (CWs) are engineered landscapes designed to replicate the functions of natural wetlands, serving as substantial sinks for sediment organic carbon (SOC). However, the seasonal dynamics of SOC composition and dissolved organic matter (DOM) characteristics, as well as their roles in autochthonous carbon cycling, remain poorly understood. This study investigated a typical long-term (over 13 years) operated CW in Chongqing, China, to elucidate the seasonal variations of sediment particulate organic carbon (POC) and mineral-associated organic carbon (MAOC). By integrating ultraviolet-visible spectroscopy, three-dimensional fluorescence spectroscopy, and microbial sequencing, we explored the relationships among DOM characteristics, greenhouse gas (GHG) emissions, and microbial communities. The results indicated that the CW sediments exhibited high SOC accumulation, which was dominated by MAOC (averaging 61.5% of SOC). MAOC exhibited significant seasonal fluctuations, reaching a minimum in autumn (96.9 ± 27.1 g·kg^-1) and peaking in winter (130.3 ± 17.6 g·kg^-1), whereas POC remained relatively stable. Spatial SOC variations were driven by plant species, with zones dominated by fibrous-rooted Hydrocotyle vulgaris accumulating higher levels than those with Acorus calamus. DOM primarily consisted of protein-like components (averaging a 69.4% contribution), exhibiting strong autochthonous characteristics and a low degree of humification. CO₂ was the dominant GHG emitted from sediment, and its emissions correlated strongly with protein-like DOM. The microbial communities displayed significant seasonal shifts, exhibiting higher relative abundances of denitrifying genera (e.g., Dechloromonas) in winter than in summer. These findings highlight the seasonal dynamics of MAOC as a critical regulator for carbon storage and turnover in CWs, offering insights into sediment organic matter stabilization within engineered landscapes.