Different responses of multifaceted plant diversities of alpine meadow and alpine steppe to nitrogen addition gradients on Qinghai-Tibetan Plateau.

Qinghai-Tibetan Plateau (QTP), >4000 m known as the "third pole of the earth" and is highly sensitive to nitrogen (N) deposition, understanding the effects of N deposition on multifaceted plant diversity (taxonomy diversity, functional diversity and phylogenetic diversity) in the alpine grasslands of Qinghai-Tibet Plateau are vital for the conservation of alpine plant diversity and the sustainability of alpine grasslands ecosystem services. We added N of different gradients to test the effects of soil acidification, soil eutrophication, and phosphorus limitation independently, and interactively on the multifaceted plant richness and evenness in both an alpine meadow and an alpine steppe of the QTP. We found that all the p-value of taxonomy diversity, functional diversity and phylogenetic diversity were >0.05 and values of R2 of fixed factors by nitrogen addition gradients was low (<0.10). In contrast to the alpine steppe, diversity of alpine meadow is more sensitive to soil factors than alpine steppe. Soil acidification caused by nitrogen deposition changed taxonomic evenness (p < 0.05), while eutrophication associated with nitrogen deposition altered taxonomic richness and phylogenetic evenness (p < 0.05) in the alpine meadow and functional richness (p < 0.05) in the alpine steppe. These findings suggest that the effects of N deposition on the multifaceted plant diversity (taxonomic, functional and phylogenetic diversity) varied with N deposition gradients and ecosystem types. Rational adaptation and mitigation techniques should be considered for different types of alpine grasslands on the QTP according to their different responses to the nitrogen deposition gradients in the future.

Effects of simulated N deposition on photosynthesis and productivity of key plants from different functional groups of alpine meadow on Qinghai-Tibetan plateau.

Nitrogen (N) deposition may alter physiological process of plants in grassland ecosystem. However, little is known about the response mechanism of individual plants in alpine regions to N deposition. We conducted a field experiment, and three treatments including 0 kg Nha-1year-1 (CK), 8 kgNha-1year-1 (Low N), and 72 kg N ha-1 year-1 (High N) were established to simulate N deposition in alpine meadow of Qinghai-Tibetan plateau. Our objectives were to determine the influence of N deposition on photosynthesis of different functional types of herbage species in alpine meadow, and finally characterize the links of plant productivity and photosynthesis with soil nutrients. The results showed that responses of alpine plants were species-specific under N deposition. Compared with grass species Agropyron cristatum and forb species Thalictrum aquilegifolium, the sedge species Carex melanantha was much more sensitive to N deposition; a lower N load (8 kgNha-1year-1) can cause a negative effect on its photosynthesis and productivity. Additionally, N deposition can promote plant N uptake and significantly decreased the C (carbon)/N (nitrogen) ratio. Compared with CK and low N deposition, high N deposition inhibited the photosynthesis and growth of the forb species Thalictrum aquilegifolium and sedge species Carex melanantha. In all three functional types of herbage species, the grass species A. cristatum tended to show a much higher photosynthetic capacity and better growth potential; thus, suggesting that grass species A. cristatum will be a more adaptative alpine plants under N deposition. Our findings suggested that plant photosynthetic responses to N deposition were species-specific, low N deposition was not beneficial for all the herbage species, and N deposition may change plant composition by the differential photosynthetic responses among species in alpine grassland. Plant composition shift to grass-dorminant in alpine regions might be attributed to a much higher photosynthetic potential and N use efficiency of grass species.

Effects of Warming and N Deposition on the Physiological Performances of Leymus secalinus in Alpine Meadow of Qinghai-Tibetan Plateau.

Warming and Nitrogen (N) deposition are key global changes that may affect eco-physiological process of territorial plants. In this paper, we examined the effects of warming, N deposition, and their combination effect on the physiological performances of Leymus secalinus. Four treatments were established in an alpine meadow of Qinghai-Tibetan plateau: control (CK), warming (W), N deposition (N), and warming plus N deposition (NW). Warming significantly decreased the photosynthetic rate (Anet ), stomatal conductance (gs ), intercellular CO2 concentration (Ci ), and transpiration rate (Tr ), while N deposition and warming plus N deposition significantly increased those parameters of L. secalinus. Warming significantly increased the VPD and Ls , while N deposition and warming plus N deposition had a significant positive effect. Warming negatively reduced the leaf N content, Chla, Chlb, and total Chl content, while N deposition significantly promoted these traits. Warming, N deposition, and their combination significantly increased the activity of SOD, POD, and CAT. Besides, warming and warming plus N deposition significantly increased the MDA content, while N deposition significantly decreased the MDA content. N deposition and warming plus N deposition significantly increased the Rubisco activity, while warming showed no significant effect on Rubisco activity. N deposition and warming plus N deposition significantly increased the Fv/Fm, ΦPSII, qP, and decreased NPQ, while warming significantly decreased the Fv/Fm, ΦPSII, qP, and increased NPQ. N deposition strengthened the relations between gs , Chl, Chla, Chlb, Rubisco activity, and Anet . Under warming, only gs showed a significantly positive relation with Anet . Our findings suggested that warming could impair the photosynthetic potential of L. secalinus enhanced by N deposition. Additionally, the combination of warming and N deposition still tend to lead positive effects on L. secalinus.