Nitrogen addition alters multiple dimensions of stability in grassland communities

Qianna Xu and Xian Yang, School of Biological Sciences, Georgia Institute of Technology, Atlanta, GA, Shiqiang Wan, College of Life Sciences, Hebei University, Hebei, China, Lin Jiang, School of Biological Sciences, Georgia Institute of Technology, ATLANTA, GA

Background/Question/Methods
Anthropogenic increases in nutrient input through fertilization and increased atmospheric deposition are known to have substantial impacts on community structure and ecosystem functioning. However, the effect of increased nutrient input on the multidimensional stability (e.g., resistance, resilience, and recovery) of community structure and ecosystem functioning remains largely unexplored. Using data from a 3-year field experiment in the temperate semi-arid steppe of Inner Mongolia, China, we quantified three dimensions of stability (resistance, resilience, and recovery) at both community and ecosystem levels to explore the effects of nitrogen (N) and phosphorous (P) additions on the functional and compositional stability of plant communities in response to a natural drought. Functional stability was quantified by changes in plant cover and compositional stability was quantified by the Bray-Curtis dissimilarity between communities. Our study aimed to answer two specific questions. First, how do N and P additions affect multiple dimensions of functional and compositional stability of plant communities in the temperate steppe in response to drought? Second, what are the mechanisms that underlie the effects of N and P additions on different dimensions of stability?
Results/Conclusions
We found that N addition significantly decreased community functional resistance and recovery, but not resilience. On the other hand, N addition consistently reduced all three aspects of compositional stability By contrast, P addition alone did not impact community functional or compositional stability. Structural equation modeling (SEM) showed that species asynchrony, species richness, dominant species functional resistance, and compositional resistance jointly increased functional resistance of plant communities. N addition negatively affected functional resistance directly and indirectly through its negative effects on species richness and the dominant species compositional resistance, which increased community compositional resistance. Functional resilience was negatively affected by species asynchrony and compositional resilience, but not N addition. However, N addition decreased compositional resilience directly and indirectly through decreasing dominant species' compositional resilience. SEM also revealed that species asynchrony and dominant species functional recovery jointly increased community functional recovery; N addition decreased functional recovery indirectly through decreasing dominant species’ functional recovery. Our findings highlight the differential impacts of N addition on functional and compositional stability of communities in response to drought, emphasizing the need to investigate multiple dimensions of stability to gain a more comprehensive understanding of stability in the face of environmental changes.