Hydrologic regime drives greenhouse gas emissions and cross-assemblage convergence of soil protists and phototrophic microbiota along methane gradients in constructed mineral-soil wetlands

Abstract

Constructed wetlands are increasingly implemented as nature-based solutions for flood management, habitat creation, and climate mitigation. However, greenhouse gas (GHG) dynamics in these systems remain uncertain, particularly in mineral-soil wetlands where hydrologic conditions strongly influence carbon cycling processes. Understanding how hydrologic regime shapes greenhouse gas emissions and microbial community structure is therefore important for evaluating the climatic implications of wetland construction and management. Greenhouse gas fluxes and soil microbiota were examined across three emergent-vegetation wetlands in southern Ontario, Canada, including two constructed wetlands established on mineral soils with contrasting hydrologic regimes—one permanently flooded and one seasonally flooded—and a natural comparison marsh developed on organic soil. Carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) fluxes were measured using static chambers over three years (2023–2025). Soil surface microbiota assemblages were characterized from samples containing heterotrophic protists - primarily testate amoebae - and phototrophic microorganisms including diatoms, green algae, and cyanobacteria. Carbon dioxide fluxes exhibited strong seasonal patterns across all sites, increasing with temperature and peaking during summer months. In contrast, methane emissions were strongly structured by hydrologic regime. The permanently flooded constructed wetland exhibited sustained methane emissions throughout the year, including winter, whereas the seasonally flooded wetland maintained near-zero methane flux even during peak summer conditions. The natural comparison marsh showed intermediate methane emissions despite its organic substrate, reflecting shallower and more variable inundation. When expressed as CO₂-equivalent fluxes, methane accounted for most radiative forcing at the permanently flooded site but contributed minimally at the seasonally flooded site, indicating that hydroperiod exerts stronger control on climatic impact than wetland origin or substrate type. Microbial community analyses revealed that heterotrophic protist and phototrophic microbiota assemblages were structured along shared environmental gradients. Co-inertia analysis indicated significant cross-assemblage coupling, suggesting that both groups respond to integrated hydrologic and biogeochemical conditions within the wetlands. The dominant community gradient was positively associated with methane flux, indicating that microbial community organization reflects ecosystem states linked to methane production and transport, whereas no comparable relationship was observed with carbon dioxide flux. Together, the results demonstrate that hydrologic regime is a primary control on methane emissions and overall warming potential in mineral-soil constructed wetlands, while microbial community structure reflects underlying biogeochemical gradients associated with methane dynamics. These findings highlight the importance of hydroperiod in wetland design and management and contribute to understanding the climatic implications of constructed wetlands as nature-based solutions. Keywords: hydrologic regime, greenhouse gas fluxes, methane (CH₄), constructed wetlands, mineral-soil wetlands, carbon dioxide (CO₂), nitrous oxide (N₂O), testate amoebae, diatoms, co-inertia analysis, soil surface microbiota, variation partitioning