Abstract
Nitrogen availability in the Arctic strongly influences plant productivity and distribution, and in permafrost systems with patterned ground, ecosystem carbon and nutrient cycling can vary substantially over short distances. Improved understanding of fine-scale spatial and temporal variation in soil N availability is needed to better predict tundra responses to a warming climate. We quantified plant-available inorganic nitrogen at multiple soil depths in 12 microhabitats associated with a gradient from low-center ice-wedge polygons to high-center polygons in coastal tundra at Utqiaġvik (formerly Barrow), Alaska. We measured vegetation composition, biomass, N content, and rooting depth distribution, as well as soil temperature, moisture, pH, and thaw depth to determine relationships between the spatial and temporal variability in N availability and environmental and vegetation drivers. Soil moisture varied across the microhabitats of the polygonal terrain and was the most important variable linked to distribution of both ammonium and nitrate, with ammonium predominating in wetter areas and nitrate predominating in drier areas. Total inorganic N availability increased as the soil in the active layer thawed, but the newly available N near the permafrost boundary late in the season was apparently not available to roots and did not contribute to plant N content. Nitrate in the drier sites also was not associated with plant N content, raising the possibility of N losses from this N-limited ecosystem. The strong relationship between soil moisture, inorganic N availability, and plant N content implies that understanding hydrological changes that may occur in a warming climate is key to determining nutrient cycling responses in complex polygonal tundra landscapes.
Highlights
Plant production is limited by low soil nutrient availability resulting from slow mineralization rates in cold, acidic, and often anoxic soils with low organic matter quality (Shaver and Chapin 1995; Jonasson and Shaver 1999; van Wijk and others 2004)
We investigated the nature and controls on finescale spatial and temporal variability in inorganic soil N availability in multiple microhabitats in polygonal tundra at the Generation Ecosystem Experiment (NGEE) Arctic site in the coastal tundra landscape of Barrow Environmental Observatory (BEO), Alaska
The microhabitats created by polygonal tundra and the seasonal variation in conditions created by thawing permafrost lead to a unique set of interactions between vegetation and plant-available N
Summary
Plant production is limited by low soil nutrient availability resulting from slow mineralization rates in cold, acidic, and often anoxic soils with low organic matter quality (Shaver and Chapin 1995; Jonasson and Shaver 1999; van Wijk and others 2004).An improved understanding of the spatial and temporal variability and controls on soil nutrient availability is needed to predict how a rapidly warming arctic climate will alter ecosystem structure and processes, such as plant community composition (for example, Chapin and others 1995; Jonasson and others 1999; Graglia and others 2001; Zamin and others 2014), and carbon balance and albedo (Hobbie and others 2002; Sturm and others 2005; Weintraub and Schimel 2005a; Loranty and others 2011). In arctic landscapes underlain by permafrost, determining trajectories of ecosystem change in response to global warming is a major challenge, as freeze–thaw processes frequently result in the formation of patterned-ground features, such as polygons, circles, nets, steps, and stripes (Washburn 1979). These fine-scale features result in spatial heterogeneity in abiotic conditions and plant communities, and significant spatial variation in ecosystem processes (Webber 1978; Bockheim and others 1999; Walker and others 2008; Zona and others 2011; Kelley and others 2012). Differences in forms of available N (for example, nitrate, ammonium, or organic N) are an important determinant of species distribution (McKane and others 2002), and an influence on vegetation community nutrient content and chemistry (Aerts and Chapin 2000; Welker and others 2005), and may determine the potential for N losses from ecosystems through leaching (Harms and Jones 2012)
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