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).