Effects of hummock-hollow microtopography on CO2 and CH4 emissions from sedge peatlands in the Changbai Mountains, Northeast China

Peatlands are major global carbon pools and emit substantial amounts of greenhouse gases. Hummock-hollow microtopography, referring to small-scale mosaics of elevated hummocks interspersed with waterlogged hollows in peatlands, is widespread. However, its effects on CO₂ and CH₄ emissions remain unclear, especially in sedge-dominated systems. To investigate the effects of microtopography on CO2 and CH4 fluxes and their underlying mechanisms, we monitored CO2 and CH4 fluxes and associated environmental factors during the growing season of 2017–2020 in a sedge (Carex schmidtii) peatland in the Changbai Mountains, China. We found that hummocks exhibited higher soil temperatures, lower water levels, and greater soil organic carbon contents but lower nitrogen and phosphorus levels than hollows, creating better aeration and pronounced nutrient limitations. Both CO2 and CH4 fluxes showed similar seasonal patterns primarily driven by temperature and hydrological dynamics. However, spatial variation induced by microtopography exceeded temporal variation. Cumulative CO2 fluxes from hummocks were 3 times those from hollows, while cumulative CH4 fluxes from hollows were 1.8 times those from hummocks. Differences in water and thermal conditions were the primary drivers of these contrasts, with nutrients and vegetation exerting additional effects. Despite higher CO2 fluxes from hummocks, their porewater CO2 concentrations were lower, likely because nutrient limitation and diffusive loss reduced localized CO2 accumulation, while CO2 released from beneath hummocks contributed to greater total emissions. In contrast, enhanced aeration in hummocks created CH4 oxidation hotspots that markedly suppressed CH4 emissions. Consequently, the net CO2-equivalent greenhouse gas flux of hummocks was 36.1 % lower than that of hollows (p < 0.05). These results demonstrate that hummock–hollow microtopography exerts strong control over CO₂ and CH4 emissions in sedge peatlands and plays a pivotal role in regulating carbon cycling. Our findings highlight the need to incorporate microtopographic heterogeneity into global assessments of peatland carbon emissions.