Abstract
ABSTRACT Across the circumpolar north, the degradation of permafrost has resulted in an increase in the extent of thermokarst landforms. Within thermally disturbed ecosystems, climatically driven changes to hydrology and temperature regimes have the potential to modify nutrient cycling processes. To assess the impacts of changing moisture and thermal conditions on the mineralization of nutrients in the Hudson Bay Lowlands, subarctic Canada, soil cores were extracted along gradients of moisture and topographic position (peat plateaus, sedge lawns, channel fens, and thermokarst collapse scars). Soil subsamples were then subjected to a factorial design of temperature (4°C, 12°C, and 20°C) and moisture treatments (saturated, field moist, and air dried) in oxic conditions for three weeks. Nitrogen transformation rates were highly variable across landscape units (ranging from −1 to 96 μg N-NO3 − and from −53 to 73 μg N-NH4 + g−1 dry soil for the incubation period). Shoreline collapse scar material showed the greatest potential for nitrification, with rates two orders of magnitude higher than other landscape positions, under warm (20°C) and saturated conditions. This work shows the potential of increased plant-available nitrate for rapid vegetative colonization of thermokarst collapse scars, and provides novel insight into nutrient cycling processes in permafrost peatland landscapes.
Highlights
Many northern cold-region landscapes are favorable environments for the accumulation of peat (Kuhry and Turunen 2006), where the decomposition rate of organic matter is retarded by low mean annual soil temperatures and prolonged anoxia due to a perched water table overlying relatively impermeable permafrost (Woo 2012)
Climatic changes will affect different components of the landscape distinctly, and thermokarst CS material has the potential for unprecedented nitrogen turnover under warm conditions; net ammonification in surface material and net nitrification under moist conditions
As existing and newly thawed collapse scars become exposed to oxic conditions, pulses of plant-available nitrogen in the form of ammonium may be mineralized at a rate higher than undisturbed sites in warmer and either dryer or wetter conditions
Summary
Many northern cold-region landscapes are favorable environments for the accumulation of peat (Kuhry and Turunen 2006), where the decomposition rate of organic matter is retarded by low mean annual soil temperatures and prolonged anoxia due to a perched water table overlying relatively impermeable permafrost (Woo 2012). In particular warming global temperatures, are projected to be amplified at northern latitudes where permafrost peatland coverage is currently widespread (Callaghan et al 2010; Cohen et al 2014; Kaufman et al 2009). Long-term observational evidence has shown that permafrost has thawed extensively in the past century (Jorgenson, Shur, and Pullman 2006; Payette et al 2004) and is projected to continue to do so (Lawrence, Slater, and Swenson 2012; Schaefer et al.2011; Schuur et al 2013). Recent permafrost degradation has been shown to change both hydrologic pathways (Connon et al 2014; Walvoord and Kurylyk 2016) as well as carbon (C) cycling regimes (Schuur et al 2008). The potential for changes to nutrient cycling from climatic changes and permafrost thaw are less constrained (Vonk et al 2015), and have important implications for further modifying the ecological and C storage functions of northern ecosystems
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