N limitation may inhibit the effectiveness of close-to-nature restoration measures for degraded alpine meadows on the northern Qinghai‒Tibet Plateau

As important terrestrial ecosystems on the Qinghai‒Tibet Plateau with important ecological and economic value, alpine meadows in some regions are in a degraded state due to climate change and unsustainable grazing. Due to the fragile ecological environment, exploring sustainable restoration models of degraded alpine meadows using close-to-nature restoration measures is the direction of ongoing efforts. Thus, in this study, three close-to-nature restoration measures, namely, manure application (M), Poa annua, Elymus dahuricus, and Puccinellia distans mixed with non-tillage reseeding (R), and the combined treatments (MR), were used to restore degraded alpine meadows from 2019 to 2021. The results showed that the M and MR treatments significantly enhanced plant biomass and reduced forbs dominance, thus improving community structure. Moreover, the M and MR treatments significantly affected plant and bacterial alpha diversity. However, the three restoration measures did not alter the correlation between the plant and bacterial communities. Furthermore, these three measures resulted in a significant increase in the plant carbon:nitrogen ratio and a significant decrease in the nitrogen:phosphorus ratio, while neither the plant carbon:phosphorus ratio nor the soil stoichiometric ratio changed significantly, indicating that nitrogen limitation was an important factor that limited the recovery process of the alpine meadows. The structural equation model indicated that the plant nitrogen:phosphorus ratio may regulate the response of plant and bacterial diversity to different restoration measures in alpine meadows. Our findings emphasize that the management of degraded alpine meadows using close-to-nature restoration measures should be a long-term effort, with more attention given to changes in the nitrogen content of meadow ecosystems.