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

Nitrogen rather than phosphorous addition alters the asymmetric responses of primary productivity to precipitation variability across a precipitation gradient on the northern Tibetan Plateau

Understanding the response of alpine grassland productivity to precipitation fluctuations is essential for assessing the future changes of ecosystem services. However, the underlying mechanism by which grassland productivity responds to wet and dry years after nitrogen (N) or/and phosphorus (P) nutrient addition remains unclear. In this study, we investigated the dynamics of plant communities based on eight-year N or/and P addition gradient experiments in four grassland types across a precipitation gradient on the north Tibetan Plateau. The asymmetry index (AI) was used to evaluate the responses of aboveground net primary productivity (ANPP) to precipitation fluctuations where AI > 0 indicates a greater increase of ANPP in wet years compared to the decline in dry years, and AI < 0 indicates a greater decline of ANPP in dry years compared to the increase in wet years. Our results showed that the AI values at community level in four natural grasslands were non-significant trend across the precipitation gradient, and showed slightly negative asymmetry, suggesting that the increase of ANPP in wet years was less than the decrease in dry years. N addition resulted in a significant decrease in community-level AI values with increasing mean annual precipitation (MAP), indicating that improved nutrient availability may favor the recovery of productivity in drier grasslands in wet years. At the functional group level, nutrient addition resulted in a significant decrease in the AI values of grasses and legumes and an increase in the AI values of forbs as MAP increased. Furthermore, the coupling of nutrients with precipitation can influence the productivity responses to precipitation changes by affecting soil nutrient availability and species richness. This research provides new insights into better predicting vegetation activity on N deposition rates and precipitation changes exacerbated in the context of climate change.

Diazotrophic abundance and community structure associated with three meadow plants on the Qinghai-Tibet Plateau

Symbiotic diazotrophs form associations with legumes and substantially fix nitrogen into soils. However, grasslands on the Qinghai-Tibet Plateau are dominated by non-legume plants, such as Kobresia tibetica. Herein, we investigated the diazotrophic abundance, composition, and community structure in the soils and roots of three plants, non-legume K. tibetica and Kobresia humilis and the legume Oxytropis ochrocephala, using molecular methods targeting nifH gene. Diazotrophs were abundantly observed in both bulk and rhizosphere soils, as well as in roots of all three plants, but their abundance varied with plant type and soil. In both bulk and rhizosphere soils, K. tibetica showed the highest diazotroph abundance, whereas K. humilis had the lowest. In roots, O. ochrocephala and K. humilis showed the highest and the lowest diazotroph abundance, respectively. The bulk and rhizosphere soils exhibited similar diazotrophic community structure in both O. ochrocephala and K. tibetica, but were substantially distinct from the roots in both plants. Interestingly, the root diazotrophic community structures in legume O. ochrocephala and non-legume K. tibetica were similar. Diazotrophs in bulk and rhizosphere soils were more diverse than those in the roots of three plants. Rhizosphere soils of K. humilis were dominated by Actinobacteria, while rhizosphere soils and roots of K. tibetica were dominated by Verrumicrobia and Proteobacteria. The O. ochrocephala root diazotrophs were dominated by Alphaproteobacteria. These findings indicate that free-living diazotrophs abundantly and diversely occur in grassland soils dominated by non-legume plants, suggesting that these diazotrophs may play important roles in fixing nitrogen into soils on the plateau.

Variation of plant CSR strategies across a precipitation gradient in the alpine grasslands on the northern Tibet Plateau

Identifying ecological strategies based on functional traits can help us better understand plants' adaptations and changes in ecological processes, and thus predict the impact of climate change on ecosystems, especially in the vulnerable alpine grasslands. Herein, we investigated the plant CSR strategies of four grassland types (alpine meadows, AM; alpine meadow steppes, AMS; alpine steppes, AS; and alpine desert steppes, ADS) and its functional groups (grasses, sedges, legumes, and forbs) along the east-to-west gradient of decreasing precipitation on the northern Tibetan grasslands by using Grime's CSR (C: competitor, S: stress tolerator, and R: ruderal) analysis. Although alpine grasslands were dominated by S-strategy, our results also indicated that AM with higher water, nitrogen (N) and phosphorus (P) availability had significantly lower S-strategy values and relatively higher C- and R-strategy values (C: S: R = 6: 63: 31 %) than those in AMS (C: S: R = 3: 94: 3 %,), AS (C: S: R = 3: 87: 10 %), and ADS (C: S: R = 1: 94: 5 %). The CSR strategy values of forbs and legumes showed greater variability compared with grasses and sedges in the environmental gradient. Furthermore, water variability on the precipitation gradient eventually affected plant traits and CSR strategies through soil N and P availability and pH. Our findings highlighted that plant CSR strategies were regulated by the availability of soil resources, and plants adopted more flexible adaptation strategies in relatively resource-rich environments. This study sheds light on the mechanisms of plant adaptation to the changing environment in the alpine grasslands.