Photosynthetic Carbon Reallocation to Nitrogen Metabolism Confers Adaptation Advantage of <i>Leymus secalinus</i> Under Elevated Nitrogen Deposition in Alpine Grassland

Nitrogen (N) deposition has driven a tendency towards graminoid monodominance in alpine grassland plant communities on the Qinghai-Tibetan Plateau (QTP), but the molecular mechanisms underlying these changes remain poorly understood. Here, we selected Leymus secalinus, the most dominant species in alpine grasslands of the QTP under N addition, to characterise its adaptation to N addition by measuring integrated morphological, physiological traits, transcriptomics, proteomics and metabolomics at different simulated levels of N addition of 0 (CK), 8 (N1), 40 (N3) and 72 (N5) kg N ha^- ¹ yr^- ¹. The results demonstrated that N addition significantly promoted the dominant growth of L. secalinus, enhancing its biomass and importance value. Under N addition, the expression of genes and proteins encoding key components of the photosystem (such as photosystem I and II proteins, antennae proteins, cytochrome b6f complex proteins, ferredoxin proteins) in L. secalinus was significantly up-regulated, enhancing its ability to compete for light resources. However, the enhancement of photosynthesis did not lead to the accumulation of soluble sugars and starch in L. secalinus. Instead, more carbon (C) skeletons and photosynthesis products were allocated to synthesise amino acids and their derivatives through the accelerated cyclic process of C and N metabolism to support the rapid growth of L. secalinus. Additionally, N addition obviously increased the antioxidant defence capacity of L. secalinus under the QTP's harsh environmental. These pathways might collectively contribute to the dominance of L. secalinus in alpine grassland on the QTP under N deposition, providing new insights into the response of alpine grassland plants to N deposition.