The current rapid loss of biodiversity from the ecosystem to the species level at global regional and local scales is considered to be one of the major threats to the continued good functioning of ecosystems and the biosphere at large. Recent research has demonstrated that genotypic diversity within species also has important ecological impacts. Especially for dominant and constructive species, genotypic diversity within species may enhance plant population productivity and resistance to disturbance, reduce susceptibility to alien plant invasions, and affect associated arthropod community composition and diversity through several of mechanisms. The underlying mechanisms by which genotypic diversity within species may alter ecosystem processes are analogous to those proposed for species diversity. For both species diversity and genotypic diversity within species, the effects of diversity may be partitioned into "selection" and "complementarity" effects, where selection effect occurs if the community includes a genotype with a specific trait that becomes dominant over time. Complementarity effects occur when function increases or decreases as a result of interactions among conspecifics. Selection effects result in higher or lower functioning than expected, based on the average performance of the genotypes in monoculture, which is called non-transgressive over-yielding. Complementarity effects result in a diverse assemblage that performs better than its best performing member, which is called transgressive over-yielding. In northern China, typical steppe is one of the main steppe types. Leymus chinensis is the dominant and constructive species of the typical steppe, playing an important role in the community. Due to climate change and frequent human activities, the typical steppe has seriously degraded over half a century, but we found that L. chinensis in degraded steppe still plays an important role in maintaining the structure and functioning of typical steppe. Therefore, it is necessary to investigate how the genotypic diversity of L. chinensis affects the structure and functioning of an ecosystem. In this study, we examined the effects of L. chinensis genotypic diversity on the performance of the above- and below-ground biomass, number of tillers, number of rhizome buds, and root to shoot ratio in response to disturbance over four months. (1) The genotypic diversity and the intensity of disturbance had significant effects (P < 0.05) on the above- and below-ground biomass, number of tillers, and number of rhizome buds, but their interactions had no significant effects on these variables (P > 0.05). The values of these 4 response variables in polyculture (3, 6 genotypes) were significantly higher than those in monoculture (P < 0.05), and these response values decreased with increasing of disturbance. The intensity of disturbance had a significant effect (P < 0.05) on the performance of the root to shoot ratio, which was higher for the most serious disturbance condition (H5) compared to the other conditions. (2) The net diversity effects were positive for 25 out of 29 variables, and were significantly higher than 0 for 12 variables. After partitioning the net diversity effect by the Loreau & Hector method, the results showed that genotypic diversity effects were driven by both the complementarity and selection effects for 12 variables, by the complementarity effect for 3 variables, and by the selection effect for 5 variables;however, the complementarity effect contributed more to the positive diversity effect. These results indicated that genotypic diversity may improve the performance of L. chinensis by the complementarity effect, even under disturbance conditions, with this information being expected to contribute to the conservation and utilization of germplasm resources of this species.