Figure 6 The relationships are given, (a) between species asynchrony and logarithm (with base 10) of N addition rate and (b) between the former and CV complexity (standard variance of network complexity divided by the mean. Solid line isv the linear least square regression. Note: * < 0.05.
Nitrogen enrichment significantly reduced species asynchrony (\(r^{2}=0.268,\ \ P<0.05\), in Fig. 6a), because it changed species competitive advantage and mobility, weakened complexity and network degeneracy. The increase of CV complexity had a tendency to reduce species asynchrony, but it was not significant (\(r^{2}=0.844,\ \ P>0.05\), in Fig. 6b). So, the external environment was the main factor affecting the species asynchrony rather than network complexity.

4. Discussion

4.1 Effects of network structure on plant community attributes under nitrogen enrichment

The theoretical and practical approaches on species interactions ignores the more complex chain of indirect interactions that may occur in different natural communities (Gallien 2017a). Gallien (2017b) defined four forms of interspecific interaction in theory and found that intransitive loops can both stabilize and destroy species coexistence, but the length and topological structure of intransitive loop significantly affect intensity and importance of intransitive competition. In reality, species-competition network often is not just a single structure, but multiple simple structures interweaving, controlled the vegetation community. We found that there were the five simple forms of intransitive loop, among which the nested-loop has a widest distribution range in the sample plots, accounting for 76.96%, always along with other forms except for the short loop.
Mathematical studies have shown that intransitive loops with an odd number of species stabilize species coexistence, while loops with an even number of species destabilize species abundance (Vandermeer 2011). Ulrich (2018) found a significant positive correlation between the nesting degree of network structure and the Alpha diversity in European salt marsh plant communities. Our results showed that, in terms of long-term effects on maintaining species diversity, nested network was always in an absolute advantage at the small (Alpha) and large-scale (Gamma) diversity, but had a certain delay effect on the Beta diversity, indicating that the effect of the ecosystem needed time to offset the negative effect of nitrogen deposition. In addition, nested network was also conducive to introduction of new species, suggesting that more complex competition can provide more chance to new species. There was no significant difference in species emigration between the two networks, while there may be yet other factors affecting on the difference.
Intransitive competition is likely to be the main reason for the construction of plant community. High intensity habitat disturbance results in less structured competition as well as less intransitive competition loops in plant communities (Ulrich et al. 2018). Our results showed that environmental filtering affects not only the attributes of the steppe communities, but also the competitive structure within the communities. With the increase of nitrogen enrichment, the complexity and degeneracy of network decreased simultaneously, indicating that high N-addition rates will destroy the soil environment on which vegetation depends for survival and then interfere with the interspecific interactions, which simplify the network structure and gradually clarify the competition levels, or expand the difference in the levels (Sun et al. 2021). Expansion of differences in the levels also lead to an easy natural selection among species, which generally existed in communities. The difference between the two kinds of networks was mainly characterized by colonization and emigration. The plots with nested networks had more resource utilization and stronger colonization potential for new species, resulting in the transition from the competition effect to the promotion effect.
The productivity of the plots treated with mowing was higher than that with no mowing (in Fig. S3be), so mowing management can partially offset the negative effects of nitrogen enrichment. This is mainly because mowing will increase soil nutrient loss and cause nutrient imbalance (Giese et al. 2013; Liu et al. 2015), thus intensify the competition for nutrients by lessening the competitivity of the dominant species. Mowing can also increase soil surface temperature by increasing ground irradiance (Wan et al. 2002), thereby increase the synchronicity of species responses to climatic conditions during growing season (Shestakova et al. 2016). In the temperate steppe, nutrients and light are important resources for plant growth, and mowing leads to redistribution of available nutrients and light, this may increase interspecific competition (Zhang et al. 2017). So, mowing can remove part of nitrogen enrichment, relieve the environmental pressure and promote plant growth. On the other hand, mowing disrupts the competitive balance of species networks, weakens the competitiveness of dominant species, resulting in reducing the differences in the levels of competition, promoting balanced growth of species, and maintaining the level of species diversity.

4.2 Effect of network complexity on plant community function under nitrogen enrichment

Empirical studies have shown that Intransitive competition, which usually occurs in communities along with direct interactions, has important effects on community structure and functions (Maynard et al. 2017a; Soliveres et al. 2015). Niche differences among species responds asynchronously to environmental fluctuations, resulting in asynchronous population dynamics and ultimately more stable overall ecosystem attributes (Loreau and De Mazancourt 2013). Species asynchrony can stabilize communities by compensating for growth in species or functional groups (Song and Yu 2015). Thus, species asynchrony is the main mechanism regulating the temporal stability of the community (Zhou et al. 2020; Taofeek et al. 2021).
The directly positive effect of nitrogen enrichment on species asynchronism is due to dynamic compensation, which increases interspecific competition for light resources (Schluter 1984; Micheli et al. 1999; Grman et al. 2010) and ultimately result in higher species asynchrony. The indirect effect of nitrogen addition on species asynchronism is due to the fact that nitrogen enrichment induces a decrease in species diversity, thus weakening the stabilizing effect of species asynchronism on the community (Yachi and Loreau,1999; Yang et al. 2012). High level of nitrogen enrichment may reduce intensity of competition for nutrient resources, thus reducing the asynchronous degree of species fluctuations (Song and Yu 2015). In high nitrogen enrichment environment, competitive order of dominant species changes and some rare species even disappear completely (Zhou et al. 2020). Our results showed that species asynchrony was synchronized with grassland ecosystem response to N-addition rates, which was consistent with previous findings (Liu et al. 2019). The effect of CV complexity on species asynchrony was not significant, thus the external environmental disturbance was the main factor affecting the species asynchrony rather than network complexity. Although the fierce interspecific competition easily leads to species replacement, a loop-structured network basically maintains the balance of species asynchrony. Thus, species asynchronous compensation has less effect on the stability in plot with loop structure.
In the sample plot of intransitive competition network, the damage of high nitrogen enrichment (≥15 g N·m-2·year-1) to the temporal stability of the ecosystem was basically the same as that of all samples. This indicated that under the high nitrogen enrichment, the absorption of nitrogen by plants reaches saturation and the positive effect on the ecosystem stability disappears (Zhou et al. 2020). Excessive nitrogen enrichment also accelerates vegetation transpiration, which leads to the decrease of water resources and thus aggravates the community instability (Chen et al. 2016). Under medium nitrogen enrichments (5, 10 g N·m-2·year-1), the temporal stability of the plots with loop structure had a significant increase, which inferred that the intransitive competition under the nitrogen enrichments is conducive to enhance the ecosystem stability and counteracts the negative effect of nitrogen enrichment apparently. Intransitive competition increased local diversity and small-scale species replacement, which may be the reason why network complexity did not have significant negative effect on species asynchronism. Therefore, in addition to the compensative effect of species asynchronism on ecosystem stability, we believe that the structural attributes of species-competition network may be another important internal mechanism to maintain the stability of the grassland ecosystem.
Community attributes have an obvious complementary relation in term of complexity. The relationship between network complexity and plot biomass was a significantly quadratic curve with down bending direction, while that between plant abundance and complexity was also a quadratic curve but with opposite direction. This shows that when plot biomass is low, it will be compensated by high plant abundance, and vice versa. The bending direction of the two curves also indicated that the competition structure may be a mechanism for self-recovery of grassland ecosystem, which can alleviate the high intensity of external interference.

5. Conclusion

In this paper, the response of plant communities to intransitive competition structure under nitrogen enrichment was investigated from the perspective of interspecific competition. We found that the overall structure of a real species-competition network was always very complex, weaving up of a variety of simple structures. Compared with short network, nested network introduced new species with higher colonization rate and had a long-term temporal mechanism to maintain the small-scale Alpha diversity, but had a significant lag effect on regulating the large-scale Gamma diversity. With change of network complexity, the biomass and the plant abundance showed quadratic curves with opposite bending direction, demonstrating a complementary relationship. The response of species asynchrony and grassland ecosystem stability decreased with the increase of nitrogen enrichment synchronously. Fortunately, at low and medium nitrogen enrichment, intransitive competitive network offset the negative effect of nitrogen. Intransitive competition structure is likely to be the internal mechanism to resist the external environmental disturbace and maintain the grassland ecosystem stability. In the future, we will focus on the changes in the intensity of species competition within network and analyze the ecological information behind them. We hope that the results of this study can enrich the theory of nitrogen deposition affecting interspecific interactions among species and provide theoretical support for coping with the effects of nitrogen deposition.

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