Response of Alpine Meadow Plant Diversity and Biomass to Nitrogen Addition and Growth Stage

Understanding the relationship between nitrogen addition and the diversity and productivity of alpine grassland plant communities is important for the sustainable development of grassland ecosystems. However, it is unclear how nitrogen addition, growth stage, and interactions between these factors affect plant biomass and diversity when regulating soil and plant nutrient factors. This study researched the temporal variations in plant community diversity and biomass after nitrogen addition in an alpine grassland from 2020 to 2021. Furthermore, this study used structural equation modeling to determine the direct and indirect pathways of plant and soil factors regulating plant biomass and diversity. Results showed that (1) nitrogen addition had significant effects on aboveground biomass, belowground biomass, and total biomass, while growth stage only had significant effects on aboveground biomass. Nitrogen addition, growth stage, and their interaction significantly affected the Shannon–Wiener diversity, the Margalef species richness, and the Pielou evenness. (2) Redundancy analysis indicated that total aboveground phosphorus, aboveground ammonium nitrogen, soil total nitrogen, and soil available nitrogen were the most significant explanatory variables for plant biomass variation, while soil available nitrogen, total soil nitrogen, and total belowground nitrogen were the key predicting factors for plant community composition variation. (3) Structural equation modeling revealed that plant biomass and diversity changed with nitrogen addition and growth stage. Nitrogen addition directly increased aboveground, belowground, and total biomass and indirectly affected plant biomass through the effectiveness of aboveground ammonium nitrogen, total aboveground phosphorus, and total soil nitrogen. Plant diversity was directly affected by the growth stage and indirectly by nitrogen addition through soil nitrogen and root nitrogen, which further affected plant diversity and evenness indices. The decrease in plant community diversity and species richness caused by nitrogen addition and different growth stages indirectly and negatively affected aboveground plant biomass. Overall, the results of this study indicate that plant biomass and diversity in alpine grasslands are affected by exogenous nitrogen addition and growth stage.