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High-latitude climate change has impacted vegetation productivity, composition, and distribution across tundra ecosystems. Over the past few decades in northern Alaska, emergent macrophytes have increased in cover and density, coincident with increased air and water temperature, active layer depth, and nutrient availability. Unraveling the covarying climate and environmental controls influencing long-term change trajectories is paramount for advancing our predictive understanding of the causes and consequences of warming in permafrost ecosystems. Within a climate-controlled carbon flux monitoring system, we evaluate the impact of elevated nutrient availability associated with degraded permafrost (high-treatment) and maximum field observations (low-treatment), on aquatic macrophyte growth and methane (CH4) emissions. Nine aquatic Arctophila fulva-dominated tundra monoliths were extracted from tundra ponds near Utqiagvik, Alaska, and placed in growth chambers that controlled ambient conditions (i.e., light, temperature, and water table), while measuring plant growth (periodically) and CH4 fluxes (continuously) for 12 weeks. Results indicate that high nutrient treatments similar to that released from permafrost thaw can increase macrophyte biomass and total CH4 emission by 54 and 64%, respectively. However, low treatments did not respond to fertilization. We estimate that permafrost thaw in tundra wetlands near Utqiagvik have the potential to enhance regional CH4 efflux by 30%. This study demonstrates the sensitivity of arctic tundra wetland biogeochemistry to nutrient release from permafrost thaw and suggests the decadal-scale expansion of A. fulva-dominant aquatic plant communities, and increased CH4 emissions in the region were likely in response to thawing permafrost, potentially representing a novel case study of the permafrost carbon feedback to warming.

Plain Language Summary Over the past half century near the town of Utqiagvik (formerly Barrow) Alaska, plants growing in wetlands have expanded, over the same time period as increases in air/pond temperatures, permafrost thaw, and nutrient availability. Although circumstantial evidence suggests nutrients released from permafrost thaw may have influenced past vegetation expansion and land-atmosphere carbon exchange, direct evidence is lacking. We built a climate and environmentally controlled carbon flux monitoring system to evaluate the impact of nutrient availability on plant growth and CH4 emissions, associated with (1) permafrost thaw and (2) the maximum field-based observations. We found nutrients released from permafrost thaw/degradation to increase emergent plant biomass and CH4 emissions by 54 and 64%, respectively. While, nutrient concentrations similar to maximum field concentrations had no effect. Assuming permafrost thaw only occurs in aquatic tundra (similar to 9% of the land surface area), our estimates suggest that regional CH4 emissions may be enhanced by 30%. We conclude that long-term patterns of emergent vegetation expansion and increased CH4 emissions in this region were likely due to thawing permafrost, which may represent a novel well-documented case study of the permafrost carbon feedback to warming.