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

Abstract

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.