Nitrogen addition reduces the positive effect of Ligularia virgaurea on seed germination of alpine species on the Tibetan Plateau

Background and aims Nitrogen (N) deposition and native allelopathic plants may affect seed germination and growth of species through their effects on soil microbes and soil nutrient availability. However, our understanding of the interactions between N addition and allelopathic plants on the regeneration of alpine grasslands remains limited. Methods Here, we investigated the effects of N addition and the presence of the allelopathic plant Ligularia virgaurea (Maxim.) Mattf. on seed germination, survival, and growth of native herbaceous species ( Elymus nutans , Delphinium kamaonense and Tibetia himalaica ) on the Qinghai-Tibet Plateau. We used piecewise structural equation modelling to assess both the direct effects of N addition, allelopathic plants, and their interactions and indirect effects mediated by soil properties, soil microbial richness and diversity, and soil enzyme activity. Results We found that (1) L. virgaurea directly increased seed germination and early plant survival, and reduced plant root-to-shoot ratio; N addition directly increased early plant survival and biomass, (2) L. virgaurea indirectly increased plant biomass via bacterial richness, (3) N addition offsets the increase in seed germination promoted by the presence of L. virgaurea via soil acid phosphatase. Conclusion Our study suggests the importance of direct and indirect roles of allelopathic plants, N addition and their interaction on seed germination, survival and plant growth. Our results highlight the need to consider the interactions between environmental and biological factors as well as their direct and indirect effects to obtain reliable predictions and mechanistic understanding of the response of alpine plants to future climate change.