Nutrient Release From Permafrost Thaw Enhances CH₄ Emissions From Arctic Tundra Wetlands

Abstract

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 Utqiaġvik, 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 Utqiaġvik 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.

Arctic aquatic graminoid tundra responses to nutrient availability

“…However, K and Mg were lower compared to 1970. Increase in leaf nutrients are concomitant with long-term observations of nutrient increases in tundra ponds of nitrate, ammonia and soluble reactive phosphorus (Lougheed et al, 2011). Increased plant nutrients may be a result of nutrient release from long-term increases of active layer depth (Andresen and Lougheed, 2015), thawing permafrost (Keuper et al, 2012;Reyes and Lougheed, 2015) and nitrogen mineralization (Uhlířová et al, 2007;Weintraub and Schimel, 2003) leached from terrestrial inputs.…”

“…Increased plant nutrients may be a result of nutrient release from long-term increases of active layer depth (Andresen and Lougheed, 2015), thawing permafrost (Keuper et al, 2012;Reyes and Lougheed, 2015) and nitrogen mineralization (Uhlířová et al, 2007;Weintraub and Schimel, 2003) leached from terrestrial inputs. The remarkable increase in Ca observed between 1970 and 2013 is likely associated to accumulation from high transpiration (Chapin, 1980) and suggests enhanced C. aquatilis evapotranspiration rates compared to 50 years ago as a result of modern warmer temperatures in both air and water (Andresen and Lougheed, 2015;Lougheed et al, 2011). It is important to note that C. aquatilis has been shown to have phenotypical differences across moisture gradients (Shaver et al 1979).…”

“…In the lab, soil samples were air dried for 3 days after thaw, then analyzed for physical and chemical factors including pH, electric conductance (EC), and macronutrients (P, K, and Nitrate). Water chemistry was determined using standard methods as described in Lougheed et al (2011). In contrast to sediment, which was sampled for each plant type, water samples from open water mid-column were assumed to be representative of the whole pond, including both plant species given the relatively well mixed environment.…”

“…Nutrients have increased over the past 40 years in aquatic habitats (Lougheed et al, 2011) with parallel biomass increases of aquatic graminoids . This phenomenon will likely become more pronounced as increasing temperatures in Arctic soils continue enhancing nitrogen mineralization (Uhlířová et al, 2007;Weintraub and Schimel, 2003) as well as permafrost degradation and nutrient leaching (Frey and McClelland, 2009;Keuper et al, 2012;Reyes and Lougheed, 2015).…”

Lake Drainage in Permafrost Regions Produces Variable Plant Communities of High Biomass and Productivity

“…The mineral and peat permafrost soils of the Subarctic accumulate nutrients in the permafrost layer near the interface with active layer [ 39 , 40 ]. Thermokarst activity and active layer deepening exert strong control on the availability of nutrients in soils [ 41 , 42 , 43 , 44 ]. An increase in the average annual air temperature increases the intensity of organic matter decomposition, thus further enriching the ecosystem in nutrients [ 45 ].…”