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.

Automated summary

Nitrogen deposition has the potential to alleviate degradation of alpine grassland on the Tibetan Plateau caused by nitrogen limitation. A six-year fertilization experiment was conducted in the Namco alpine steppe, along with 89 additional experiments worldwide. Results showed that nitrogen addition promoted nitrogen retention in plants with a decreasing trend at the critical nitrogen rate (N-cr). The maximum nitrogen retention capacity (MNRC) varied globally, ranging from 0.35 to 42.59 g N m(-2) yr(-1) and accounting for approximately 39% of N-cr. Tibetan alpine grasslands had a low average MNRC (2.24 g N m(-2) yr(-1)), with regional differences between the western TP (0.80 g N m(-2) yr(-1)) and the eastern TP (4.10 g N m(-2) yr(-1)). It was estimated that Tibetan alpine grasslands could retain 0.33-1.21 Tg N yr(-1) (0.22-0.79 g N m(-2) yr(-1)) and gain 5.65-20.11 Tg C yr(-1) (3.67-13.06 g C m(-2) yr(-1)) under current nitrogen deposition levels.