Different sensitivity and threshold in response to nitrogen addition in four alpine grasslands along a precipitation transect on the Northern Tibetan Plateau

The increase of atmospheric nitrogen (N) deposition has resulted in some terrestrial ecological changes. In order to identify the response of sensitive indicators to N input and estimate the sensitivity and saturation thresholds in alpine grasslands, we set up a series of multi-level N addition experiments in four types of alpine grasslands (alpine meadow (AM), alpine meadow-steppe (AMS), alpine steppe (AS), and alpine desert steppe (ADS)) along with a decreasing precipitation gradient from east to west on the Northern Tibetan Plateau. N addition only had significant effects on species diversity in AMS, while had no effects on the other three alpine grasslands. Aboveground biomass of grasses and overall community in ADS were enhanced with increasing N addition, but such effects did not occur in AS. Legume biomass in ADS and AS showed similar unimodal patterns, and exhibited a decreasing tend in AM. Regression fitting showed that the most sensitive functional groups were grasses, and the...