Nitrogen enrichment affects the competition network of aboveground species on the Inner Mongolia steppe

Abstract

Nitrogen(N) enrichment changes the community structure of terrestrial ecosystems. However, the patterns and structural functions of species interaction networks under N-enrichment conditions are still unclear. We compared and analysed data from 13 years of N deposition experiments in the Inner Mongolia temperate steppe in China. We found that spatial association effectively detected intransitive networks, the structure of which was occasionally changed by species with low biomass but high-frequency occurrence, as an especially vital intermediary for forming a large network. The peak value of the network complexity was maintained at 2 gNm-2⁢year-1. In the third year after fertilization at 50 gNm-2⁢year-1, the species interaction changed to transitivity, which lasted for 8 years. Then, the environment completely replaced species interactions as the main factor affecting community composition. Surprisingly, the negative effect of N enrichment on the species network was counteracted by mowing and precipitation, which increased the probability of intransitive network formation. In the intransitive network, species diversity was not only maintained but also increased under low N-addition rates from 0 gNm-2⁢year-1 to 3 gNm-2⁢year-1. These studies indicated that N enrichment reduced the complexity of the species network and increased hierarchical differences among species competition. Intransitive competition is highly prevalent and a key driver of species coexistence and diversity maintenance in the typical steppe. Moreover, environmental factors directly affect plant intransitive competition, resulting in changes in species diversity.