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

Abstract

The increase in 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 multilevel 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 N saturation thresholds were 103, 115, 136, and 156 kg N hm−2 year−1 in AM, AMS, AS, and ADS, respectively. This suggests that alpine grasslands become more and more insensitive to N input with precipitation decrease. N saturation thresholds also negatively correlated with soil N availability. N sensitivity differences caused by precipitation and nutrient availability suggest that alpine grasslands along the precipitation gradient will respond differently to atmospheric N deposition in the future global change scenario. This different sensitivity should also be taken into consideration when using N fertilization to restore degraded grasslands.

Nitrogen economy of alpine plants on the north Tibetan Plateau: Nitrogen conservation by resorption rather than open sources through biological symbiotic fixation

“…The total community coverage in AMS and AS is 30%–40%. S. purpurea and Oxytropis microphylla are dominant plant species in ADS, with total community coverage approximately 20%–30% (Zong, Zhao, & Shi, ). Soils are characterized as alpine frost calcic soil and desert soil (Zhao et al, ).…”

“…Four types of alpine ecosystems are distributed and develop from east to west along the precipitation gradient, that is, alpine meadow (AM), alpine meadow steppe (AMS), alpine steppe (AS), and alpine desert steppe (ADS). The dominant plant species in AM is Kobresia pygmaea, with total community coverage more than 90% ( (Zong, Zhao, & Shi, 2019). Soils are characterized as alpine frost calcic soil and desert soil (Zhao et al, 2017).…”

Plants and Microbes Mediate the Shift in Ecosystem Multifunctionality From Low to High Patterns Across Alpine Grasslands on the Tibetan Plateau

“…Meanwhile, the AI index, which could reflect functions that drive the responses of plant nutrients to soils, plants, and eutrophication (Anderson et al, 2018), was positively correlated with high EMF with a higher slope than with low EMF (Figure 3). The plant biomass and species richness of alpine grassland generally increased from northwestern to southeastern Tibetan Plateau, which was closely related to the water-heat pattern (Zong et al, 2019;Sun et al, 2020b), and connected with the higher sensitivity response of low EMF. As for community functions, productivity, and soil properties, for example, AGB, CWM_LDMC, and SOC shaped EMF with higher sensitivity response under high EMF conditions (Figure 3; Table 1).…”

Dominance Structure Plays a Leading Role In Shaping Community Stability In The Northern Tibetan Grasslands

“…Northern Tibetan alpine grasslands are characterized by low temperature and cryophilic vegetation which shifts successively from alpine meadows to meadow steppes, steppes and desert steppes along a precipitation gradient ranging from over 600 mm in the east to less than 100 mm in the west (Zong et al 2019). These grasslands, with relatively poor species richness but diverse functional groups of sedges, grasses, forbs and legumes, are dominated by sedges in the meadows, forbs in meadows steppe, legumes in the steppes and some grasses in desert steppes (Wu et al 2013;Zhao et al 2017;Zong et al 2019). Dominants often act as nursery (or foundation species) and play a key role in determining functional structure and promoting biodiversity in alpine ecosystems (Cavieres et al 2014;Elumeeva et al 2017).…”