| Summary: | How plant competition varies with the changes of biotic and abiotic factors has been debated for a long time in the field of ecology. The aim of this study was to explore how neighbors and soil fertility influenced the intensity and importance of competition on species Stipa grandis and S. krylovii. We conducted a target species (S. grandis and S. krylovii) × soil fertility (low and high fertility) × competition treatment (4 treatments: two target species grown in monoculture and in mixture with each other, and in mixture with Leymus chinensis or Agropyron cristatum) microcosm experiment. We calculated the intensity (NIntA) and importance (NImpA) of neighbor-effect with additive symmetry for each target species under different treatments, and explored how the NIntA and NImpA were influenced by the target species, neighbor identity and soil fertility. We calculated the fertilizer-addition effect index on each target species to explore how neighbor identity influenced the index. (1) When target species in mixture with L. chinensis, both the NIntA and NImpA decreased with the increase of soil fertility. The NImpA was significantly negative on S. grandis under both soil fertility treatments and on S. krylovii under the high soil fertility treatment. (2) When target species in mixture with A. cristatum, the NIntA significantly influenced by the interaction between target species and soil fertility. The NImpA was significantly negative on S. grandis under the low soil fertility treatment and on S. krylovii under the high soil fertility treatment. (3) When in S. grandis – S. krylovii mixture system, the NIntA increased with the increase of soil fertility, and the NImpA showed no significant effects on each target species. (4) The fertilizer-addition effect was higher than zero for all treatments except the one on S. grandis in mixture with L. chinensis. The effect on S. krylovii was relatively lower than that on S. grandis in mixture with A. cristatum but was relatively higher in any of the other three competition treatments. The present findings demonstrated that the identities of both target and neighbor species influenced the fertilizer-addition effect and the responses of intensity and importance of competition to the changes of soil fertility. The present study showed the complexity of target species and neighbor identities in influencing the outcomes of plant competition, and provided novel insights into how plant competition driving the community dynamics in the context of soil fertility changes.
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