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

We examined short- and long-term nitrogen (N) dynamics and availability along an arctic hillslope in Alaska, USA, using a stable isotope of nitrogen (N-15), as a tracer. Tracer levels of (NH4+)-N-15 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). N-15 in vegetation and soil was monitored to estimate retention and loss over a 3-year period. Nearly all (NH4+)-N-15 was immediately retained in the surface moss detritus plant layer, and >57% of the N-15 added remained in this layer at the end of the second year. Organic soil was the second largest N-15 sink. By the end of the third growing season, the moss detritus plant layer and organic soil combined retained >87% of the N-15 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 N-15 in the organic soil. Hydrology played an important role in downslope movement of dissolved N-15. Crest and Midslope with high-water-flux sites were most susceptible to N-15 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-N-15 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.