Carbon removal mechanisms and microbial dynamics in constructed wetlands of differing depths

Climate change has intensified the mobility of dissolved organic matter (DOM) from land into aquatic ecosystems leading to increased brownification and hypoxia. Constructed wetlands (CWs) offer a potential mitigation strategy but optimal wetland design with respect to DOM removal remains underexplored. This study examined how depth and water residence time (WRT) affect DOM processing in experimental CWs during summer and fall. Organic matter was added to mimic brownification, and DOM changes were tracked using fluorescence spectroscopy and microbial activity measurements. A key finding was that labile DOM degrades rapidly within the first two days. At longer WRT shallow CWs released terrestrial-like fractions potentially increasing downstream brownification, while deep CWs showed sustained DOM degradation and slower internal production, potentially reducing downstream brownification. Based on spectral ratios it was found that microbial processes dominated DOM degradation, although photodegradation played a significant role during summer. Strong correlations between bacterial processes and DOM composition, highlight the critical role of labile carbon in driving microbial activity. Bacterial production correlated strongly with labile DOM fractions (Peaks T and M), while bacterial respiration, correlated with both labile and humic-like DOM fractions. Our results suggest that CWs can be optimized as tools for mitigating climate change impacts and improving water quality, ensuring long-term ecological sustainability. In addition, our findings advocate for integrating shallow and deep systems in series to maximize carbon removal, minimize brownification, and adapt to seasonal variability.