Additional nitrogen in arctic‐alpine soils and plants—a pilot study with 15NO $ _3^ - $ and 15NH $ _4^+ $ fertilization along an elevation gradient

Nitrogen (N) deposition has been increasing in alpine ecosystems, but its fate in soils and plants remains unclear. We assumed that the increased N load will be efficiently retained in alpine ecosystems but that the degree of N use efficiency changes with elevation. Thus, we performed a 3‐year ¹⁵N tracer experiment, in which we added 1 g m^?2 of either NH₄¹⁵NO₃ or ¹⁵NH₄NO₃ fertilizer to a plot of 1 m² in size at three elevations. Composite soil samples and aboveground plant material from lichens, dwarf shrubs, and graminoids were collected annually for three years and analyzed for their ¹⁵N accrual. We found a cumulative and plateauing rise in ¹⁵N concentration in soils and plants at all sites. However, overall recovery of the tracer decreased with time, amounting to 71% of fertilizer recovered in the soils in the first year, 69% recovered in soils and plants in the second year, and 37% in soils and plants in the third year. Moreover, the fertilizer use efficiency varied among fertilizer types and plant functional types. This utilization pattern appears to be modulated by elevation.