Plant nitrogen retention in alpine grasslands of the Tibetan Plateau under multi-level nitrogen addition

Nitrogen (N) deposition might alleviate degradation of alpine grassland caused by N limitation on the Tibetan Plateau (TP). To determine such limitation and quantify the N-induced N retention in plant, a six-year fertilization experiment with six levels of N addition rates (0, 1, 2, 4, 8 and 16 g N m^-2 yr^-1) was conducted in the Namco alpine steppe and additional 89 experiments with multi-level N addition were also synthesized worldwide among which 27 sites were on the TP. In general, N addition promoted N retention in plants, and this increasing trend diminished at the critical N rate (N_cr). The maximum N retention capacity (MNRC) of plants at N_cr was strongly correlated with initial aboveground net primary productivity with a slope of 0.02, and the MNRC of grasslands globally ranged from 0.35 to 42.59 g N m^-2 yr^-1, approximately account for 39% of N_cr. Tibetan alpine grassland had a low average MNRC (2.24 g N m^-2 yr^-1) with distinct regional characteristic, which was much lower in the western TP (0.80 g N m^-2 yr^-1) than the eastern TP (4.10 g N m^-2 yr^-1). Our results inferred 0.33-1.21 Tg N yr^-1 (0.22-0.79 g N m^-2 yr^-1) can be retained and 5.65-20.11 Tg C yr^-1 (3.67-13.06 g C m^-2 yr^-1) can be gained by Tibetan alpine grasslands under current N deposition level. With the aggravation of N deposition, the alpine steppe ecosystem might continuously absorb N and C until N deposition reaches N_cr.