Nitrogen dynamics in a small arctic watershed: retention and downhill movement of 15 N

• Published in: Ecological Monographs
• Main Authors: Yano, Yuriko, Shaver, Gaius R., Giblin, Anne E., Rastetter, Edward B., Nadelhoffer, Knute J.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2010
• Subjects: Tundra; Alaska
• Online Access: http://dx.doi.org/10.1890/08-0773.1; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1890%2F08-0773.1; https://esajournals.onlinelibrary.wiley.com/doi/pdf/10.1890/08-0773.1

We examined short‐ and long‐term nitrogen (N) dynamics and availability along an arctic hillslope in Alaska, USA, using a stable isotope of nitrogen ( 15 N), as a tracer. Tracer levels of 15 NH 4 + were sprayed once onto the tundra at six sites in four tundra types: heath (crest), tussock with high and low water flux (mid‐ and footslope), and wet sedge (riparian). 15 N in vegetation and soil was monitored to estimate retention and loss over a 3‐year period. Nearly all 15 NH 4 + was immediately retained in the surface moss–detritus–plant layer, and >57% of the 15 N added remained in this layer at the end of the second year. Organic soil was the second largest 15 N sink. By the end of the third growing season, the moss–detritus–plant layer and organic soil combined retained ≥87% of the 15 N added except at the Midslope site with high water flux, where recovery declined to 68%. At all sites, non‐extractable and non‐labile‐N pools were the principal sinks for added 15 N in the organic soil. Hydrology played an important role in downslope movement of dissolved 15 N. Crest and Midslope with high‐water‐flux sites were most susceptible to 15 N losses via leaching, perhaps because of deep permeable mineral soil (crest) and high water flow (Midslope with high water flux). Late spring melt season also resulted in downslope dissolved‐ 15 N losses, perhaps because of an asynchrony between N release into melt water and soil immobilization capacity. We conclude that separation of the rooting zone from the strong sink for incoming N in the moss–detritus–plant layer, rapid incorporation of new N into relatively recalcitrant‐soil‐N pools within the rooting zone, and leaching loss from the upper hillslope would all contribute to the strong N‐limitation of this ecosystem. An extended snow‐free season and deeper depth of thaw under warmer climate may significantly alter current N dynamics in this arctic ecosystem.