Nitrogen enrichment affects and changes the community structure of terrestrial ecosystems. Recently, intransitivity competition networks have been widely considered to in maintaining coexistence and biodiversity. However, whether the structure and function of the species competition network changes under nitrogen enrichment remains unclear. In this paper, we applied the Apriori algorithm to find dominant species assemblies to simulate competition matrices. We found that (1) intransitive competition networks derived by a species-dominant assembly accounted for a relatively large proportion of the derived networks, and the species acting as mediators constituted a nested pattern; (2)Under N enrichment, the complexity of the networks varied from an intransitive network to a sub-competition structure, and transitive networks was a transition structure between these two structures; (3) the negative effect of N-enrichment on species network was counteracted by mowing and rainfall; and (4) increasing the number of loops enhances the diversity in a community, while high competition among species is a representation of an intransitive competition network.

Atmospheric nitrogen (N) deposition is a potential danger factor for grassland ecology, and will cause unpredictable consequences to plant communities. However, how plant species interactions response to N enrichment and then affect ecological functions are not fully known. We investigated how intransitive competition network was related to the functional attributes of plant community under a 13-years N-deposition experiment. Results showed that intransitive competition network was not a single structure, but a complexly interwoven structure of various simple structures. Nested work was more common, accounting for 76.96%, and gained new species at a higher colonization rate than short network did. The network had a long-term mechanism to maintain the small-scale Alpha diversity, and a significant lag effect on the large-scale Gamma diversity. Under the conditions of N ≥ 2 g N·m-2·year-1, without mowing and under high fertilization frequency, the increase of network complexity significantly decreased plot biomass gradually. The relationship between biomass and network complexity is quadratic curves, also between abundancy and the complexity, but with the opposite bending directions, which indicated that biomass and abundance were complementary to each other, which may be a mechanism of maintaining the relative balance of species competition. In addition, the decrease of species asynchronism changing with the increase of N-enrichment gradually destroyed ecosystem stability. However, at medium N enrichment, intransitive network counteracted the negative effects of N enrichment and maintained or even improved the biomass ecosystem stability. Our results suggested that intransitive competition network is an internal mechanism of self-restoration of a grassland ecosystem. Under nitrogen enrichment conditions, competitive networks complexity is reduced, leading to a reduction in species diversity. These analyses emphasize the important role of intransitive network structure to stabilize grassland ecosystem. In order to achieve sustainable development of grassland, it is indispensable to control nitrogen addition rate.