Exploring The Mechanisms By Which Nutrient Enrichment Reduces Plant Diversity In Grasslands With Modern Coexistence Theory

Numerous studies have shown that nutrient enrichment significantly reduces plant diversity in grasslands.To date,ecologists have proposed three hypotheses（the niche dimensionality hypothesis,the competitive asymmetry hypothesis and the soil acidification hypothesis）aiming to explain this general phenomenon.However,these hypotheses are still highly controversial and have not been tested rigorously and quantitatively,resulting in many blind spots in our understanding of the underlying mechanisms.To unravel the complex mechanisms involved,this thesis analyzed the Park Grass experiment（1856-）,the famous long-term fertilization experiment,within the framework of modern species coexistence theory.Specifically,I used time-series data from different treatments of this experiment（15 fertilization treatments fully crossed with four levels of soil p H）to fit the Lotka-Volterra competition model.I then quantified the intrinsic growth rate of each species and the niche differences and competitive differences between species to test the three hypotheses mentioned above and reveal why nutrient enrichment reduces plant diversity in grasslands.The results showed that（1）both nutrient addition and soil acidification led to a decrease in intrinsic growth rates（r）and the reshuffling of competitive differences（or hierarchies）among species,generally favoring grasses over legumes and forbs.Furthermore,the above changes in intrinsic growth rates（affecting species existence）and competitive differences（affecting species coexistence）effectively explained the loss of plant diversity due to nutrient enrichment and its recovery after withholding of nitrogen addition.This result supports the competitive asymmetry hypothesis and the soil acidification hypothesis.（2）nutrient addition significantly reduced intra-and interspecific competition strength but did not reduce niche differences as expected;soil acidification significantly increased intra-and inter-specific competition strength and niche differences.Moreover,the changes in niche differences could not explain the changes in species richness under nutrient enrichment.Therefore,this result does not support the niche dimensionality hypothesis.The above important findings can deepen our understanding of the responses of grassland plant communities to nutrient enrichment in nature,and also provide some theoretical basis for the conservation of biodiversity in grassland ecosystems and the formulation of related measures in the context of intensified global nutrient deposition across the globe.