Contributions of N sources to total N uptake in the degraded and non-degraded alpine meadow
The contributions of15NO3- to total aboveground N uptake were significantly greater than those of15glycine and15NH4+ in both meadow types (Fig. 1). For plant roots, the contributions of15NO3- to total N uptake were highest among the three N sources in the undegraded alpine meadow, while the contributions of 15glycine to total N uptake were highest in the degraded alpine meadow (Fig. 1). The results for the contributions of the three N sources to total N uptake by plants were consistent with the results obtained for the roots in both meadow types (Fig. 1).
We detected seven common dominant plant species in the undegraded alpine meadows, accounting for 89.41% of the biomass. Among these species,E. nutans , P. annua, and P. kansuensis were common generalist species in two meadows (Fig. 2a). In the degraded alpine meadow, the common dominant plant species of E. nutans , P. annua , P. kansuensis, and A. tenuifolia accounted for 49.75% of the total biomass (Fig. 2a).
The generalist species E. nutans and P. annua belonged to the family Gramineae and P. kansuensis was a forb species. ForE. nutans , the contribution of15NO3- to total N uptake was largest among the three N sources in the undegraded alpine meadow. Similar results were obtained in the degraded alpine meadow. ForP. annua ,15NO3- contributed most substantially among the three N sources in the undegraded alpine meadow, and differences among the contributions of N sources to total N uptake were not significant for the three N sources in the degraded alpine meadow. For P. kansuensis , the contributions of the three N sources to total N uptake did not differ significantly in the undegraded alpine meadow; however,15NO3- contributed most substantially among the three N sources in the degraded alpine meadow (Fig. 2b).
In the undegraded alpine meadow, the contribution of15NO3- was consistently largest among the three N sources for the three dominant unique species of S. distigmaticus , L. brachystachya andP. anserina , while for the other unique species,15NH4+ showed the largest contribution. In the degraded alpine meadow, for the unique species A. tenuifolia , the contributions of three N sources to total N uptake did not differ significantly. For the other unique species, the contributions of 15N-glycine to total N uptake were significantly greater than those of the other N sources (Fig. 2c).