Increased snowpack enhances ecological functions of cold-region constructed wetlands via plant-microbe interactions

Snowpack variations in cold regions exert profound influences on the ecological functioning of constructed wetlands (CWs), particularly with respect to GHG emissions and nutrient removal. However, the underlying mechanisms have yet to be clarified. This study established pilot-scale vertical subsurface flow CWs in Northeast China, with Phragmites australis and Iris sibirica, and applied doubled snowpack (DS) and natural snow cover (CK) during winter. DS created highly moist, strongly reducing, and alkaline conditions during the dormant period, significantly inhibiting microbial aerobic respiration and denitrification, thereby reducing CO₂ and N₂O emissions. In contrast, elevated methanogenesis and suppressed methane oxidation led to increased CH₄ emissions. Additionally, DS accelerated the regrowth of P. australis, resulting in its dominance within the plant community. This shift enhanced community-level biomass and photosynthetic capacity during the operational period. Its well-developed aerenchyma of P. australis facilitated root radial oxygen loss, thereby improving substrate redox conditions. Although increased oxidation stimulated microbial aerobic respiration and CO₂ production, intensified plant photosynthesis offset this effect, resulting in no significant change in net CO₂ emissions. Furthermore, reduced nucleosides in root exudates mitigated oxygen competition with methanotrophs, enhancing methane oxidation, while increased ORP suppressed methanogenesis under DS. These processes ultimately reduced CH₄ emissions. Similarly, N₂O emissions decreased as DS increased the availability of labile carbon substrates, which served as electron donors and promoted the complete reduction of N₂O to N₂. Consequently, the CW system achieved an approximate 19.52 % reduction in annual GWP. Concurrently, NH₄⁺-N (77.87 % ± 2.58 %) and TP (71.80 % ± 2.74 %) removal rates significantly improved, primarily due to enhanced nitrification driven by reduced nucleoside concentrations and micro-oxygen optimization, as well as greater phosphorus uptake associated with increased plant biomass. Our findings demonstrate that snowpack variations, by regulating substrate-plant-microbe interactions, synergistically govern GHG mitigation and nutrient removal in cold-region CWs, highlighting their potential as ecological regulators.