Abstract:

Nitrogen （N） is an essential element for plant growth and reproduction， and it exists in various chemical forms in soils. The location of alpine meadows on the Tibetan Plateau is unique， where the climate is severe with low temperature. The low temperature restricts decomposition of soil organic matter and leads to scare available N in the soil， which further limits plant growth. Moreover， overgrazing causes grassland degradation and succession， resulting in severe loss of available nutrients in soil and changes in the N use patterns of plants. In this study， a ¹⁵N labeling experiment was conducted in an alpine meadow in Zeku County， Qinghai Province to trace plant utilization of ammonium nitrogen （NH₄⁺-N） and nitrate nitrogen （NO₃^--N） in plant communities exhibiting different degradation stages. Six degradation stages， including lightly， moderately， and severely degraded grasslands， and eroded， severely eroded grasslands， and bare black soil， were selected based on prior knowledge on plant community succession during the degradation of the alpine meadows. The uptake rates of NH₄⁺-N and NO₃^--N by the dominant sedge species Kobresia pygmaea were measured in the six degradation stages. Additionally， we also quantified the uptake rates of NH₄⁺-N and NO₃^--N by forb species Ligularia sibirica and Elsholtzia ciliata， the dominant species in patches in the later degradation stages. The results showed that along the degradation stages， the uptake rate of total inorganic nitrogen by K. pygmaea showed a fluctuating downward trend. The uptake rate of ammonium nitrogen was consistent with the trend of total inorganic nitrogen uptake rate， while the uptake rate of nitrate nitrogen showed a fluctuating upward trend. Except for the erosion and bare black soil stages， the uptake rate of ammonium nitrogen by K. pygmaea is significantly higher in underground organs than in aboveground organs. Except for the severe degradation and bare black soil stages， the uptake rate of nitrate nitrogen by K. pygmaea in underground organs was significantly higher than that in aboveground organs in all other stages. In the later stage of degradation， there was no significant difference in the uptake mode of nitrate nitrogen between patches dominated by K. pygmaea or patches dominated by forb species. However， the uptake rate of ammonium nitrogen was significantly higher in the patches dominated by the forb species than in the patches dominated by K. pygmaea. In patches， the dominant forb species， L. sibirica and E. ciliata， have significant differences in the uptake of ammonium and nitrate nitrogen. L. sibirica has a significantly higher uptake rate of NH₄⁺-N than NO₃^--N， while E. ciliata has no significant difference in the uptake rate of the two inorganic N forms. In summary， at different stages of degradation in the alpine meadow， plant species adopt different inorganic nitrogen acquisition strategies to adapt to soil nutrient conditions， by which maintains plant species coexistence. A deeper understanding of the absorption and utilization of soil inorganic nitrogen by plants at different stages of grassland degradation can provide insight into the mechanisms of species coexistence and guide the restoration of degraded grasslands.

Key words: alpine meadow, degradation sequence, different chemical forms of inorganic nitrogen, nitrogen uptake rate, ¹⁵N tracing