Agricultural Straw-Derived Slow-Release Carbon Sources Enhance Denitrification of Low C/N Wastewater in Constructed Wetlands

Low carbon-to-nitrogen (C/N) tailwater from wastewater treatment plants severely constrains denitrification in constructed wetlands, yet sustainable external carbon supplementation remains a major challenge. Herein, we developed a series of biomass-derived slow-release carbon composites by coupling alkali-modified reed or corn straw with polybutylene succinate (PBS) and inert skeletal materials, and systematically evaluated their carbon-release behavior, nutrient-leaching risks, structural evolution, and denitrification performance. Corn straw-based composites exhibited substantially higher carbon availability and more favorable release kinetics than reed-based counterparts. Among the tested materials, AA−CC5 (corn straw:PBS = 10:2) achieved the optimal balance between sustained carbon supply and release control. In batch denitrification tests, AA−CC5 enabled rapid nitrate removal with a peak NO3−-N removal efficiency of 97.7% while suppressing excessive nitrite accumulation. When integrated into continuous-flow constructed wetlands treating tailwater at an influent C/N of 0.4, AA−CC5 increased average total nitrogen and nitrate removal efficiencies to 19.5% and 16.5%, respectively. High-throughput sequencing showed higher observed relative abundances of several genera commonly associated with denitrification, including Pseudomonas, Paracoccus, and Dechloromonas, in the AA−CC5 wetland. These community patterns were consistent with the additional organic carbon supplied by AA−CC5. These findings suggest that agricultural straw-derived composites have potential as supplementary carbon sources for carbon-limited constructed wetlands.