Nitrogen dynamics in a small arctic watershed: retention and downhill movement of15N

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 NH4 þ 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 NH4 þ 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.