Long-term plant harvesting suppresses methane emissions by restructuring methanogenic pathways in constructed wetlands

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Abstract
Constructed wetlands (CWs) provide effective wastewater treatment but can emit methane (CH4), creating a climate-related trade-off that is increasingly relevant to their environmental sustainability. Although plant harvesting is routinely practiced to remove nutrient-rich biomass, its long-term influence on CH4 emissions and the microbial pathways governing methane production remains poorly understood. Here, we investigated an eight-year contrast between an annually harvested and an unharvested standing Phragmites australis CW cell under otherwise comparable operating conditions. Annual harvesting improved COD, total nitrogen, and total phosphorus removal while reducing annual mean CH4 flux by 38.16% (0.47 ± 0.75 vs. 0.76 ± 1.06 mg/m²/h). The lower CH4 flux occurred despite a higher organic-carbon concentration in the harvesting-cell substrate, indicating that carbon availability alone did not explain methane production. Instead, long-term harvesting was associated with marked restructuring of microbial communities, lower microbial diversity, reduced mcrA abundance, and increased pmoA abundance. Metagenomic analysis further revealed contrasting methanogenic pathway potentials: harvesting was associated with greater representation of genes involved in the early steps of hydrogenotrophic methanogenesis but reduced abundances of downstream genes, including mcr, whereas the standing cell exhibited enhanced potential for methylotrophic methanogenesis. We propose that long-term retention and decomposition of P. australis litter created a sustained source of methylated substrates that selectively favored methylotrophic methanogenesis in the standing cell, whereas annual biomass removal disrupted this substrate pathway. These findings demonstrate that long-term plant harvesting can simultaneously improve CW treatment performance and mitigate CH4 emissions by restructuring plant-derived carbon inputs and the microbial pathways governing methanogenesis.

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