Organic matter accumulation and characteristics in tropical constructed wetlands

Constructed wetlands (CWs) are ecological engineering systems that integrate wastewater treatment with carbon retention through interactions among vegetation, soil chemistry, and redox processes. However, how pH, redox potential (Eh), and carbon stabilization vary across CW configurations remains poorly understood. Here, we characterized two tropical CWs receiving contrasting effluents: animal production wastewater (AP) and sanitary wastewater (IP), by combining physicochemical analyses, carbon (C) contents, and FTIR-based assessments of organic matter quality. The systems showed marked contrasts in soil conditions and carbon accumulation. AP exhibited neutral pH, predominantly oxidizing conditions, and the highest C stocks, likely driven by vegetation traits and high organic inputs. IP showed broader pH and Eh variability and lower C stocks, but more stabilized organic matter, consistent with a system undergoing maturation. A strong negative correlation between pH and Eh was observed in IP, while Eh was unrelated to total C, indicating that redox conditions are not controlled by bulk C stocks but by localized processes. In both wetlands, Eh decreased with depth, favoring carbon preservation and potentially limiting anaerobic greenhouse gas production. Organic matter displayed classical vertical stratification, with decreasing C contents and redistribution between particulate and mineral-associated fractions. FTIR spectra revealed strong organo-mineral interactions and a dominance of recalcitrant compounds, particularly in IP. Temporally, IP showed substantial C accumulation from 2022 to 2024, with mineral-associated sequestration rates exceeding those reported for natural wetlands. These findings indicate that CWs can function as effective C sinks, though their mitigation potential depends on organic matter quality and stabilization pathways.