General range of the stable nitrogen isotope composition of the three major N-sources available for tundra plants: i) bird feces (N-birds), analyzed in fresh bird feces and the cryophilic alga <i>Chlamydomonas nivalis</i>; ii) N-atmospheric deposition, analyzed in <i>C</i>. <i>nivalis</i> at sites away from the bird colonies and mosses from nunatak Tuva; iii) N₂ primary fixation, analyzed in the N-fixing cyanobacteria <i>Nostoc</i> sp.

<p>Bird N-loads were recalculated from Zwolicki et al. [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136536#pone.0136536.ref015" target="_blank">15</a>], atmospheric N-deposition were based on Kühnel et al. [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136536#pone.0136536.ref002" target="_blank">2</a>] and atmospheric N₂-fixation as measured for Arctic by Chapin and Bledsoe [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136536#pone.0136536.ref040" target="_blank">40</a>]. Photos by G. Skrzypek and D. Richter.