The microbial community is a diverse and complex structure that is difficult to understand (van Dijk et al, 2014). However, some researchers have proposed the use of molecular ecological networks to increase our understanding of microbial communities (Zhou et al, 2010). However, it is not common to use this method to explore the response of soil AMF communities to environmental change, and a 3-year trial of P addition treatments was conducted in an arid zone artificial grassland. We intend to test how the AMF co-occurrence network is affected by environmental influences under P circumstances, and the correlation between the nutrient contents in various organs of the model plant (Leymus chinensis) and the co-occurrence of AMF. The present study reveals that phosphorus affects phosphorus content in roots and nitrogen content in leaves of Leymus chinensis by altering the coexistence of different species of the AMF community.
Influence of environmental factors on the co-occurrence network of AMF
In this study, P was added to artificial Leymus chinensis in arid regions for three years, and four co-occurrence networks were built to look into the relationships between the various AMF species. The co-occurrence networks showed differences at different P levels (Fig. 2). The increase in the proportion of positively correlated edges under P conditions may be due to the addition of exogenous P to alleviate the competition for P among the different species of AMF, which shifted from a competitive to a reciprocal relationship among species, thus maximizing nutrient exchange with the plant to obtain C for maintaining life activities (Bennett et al, 2022; Whiteside et al, 2019). The modularity of the co-occurrence network increased with increasing P level and peaked in the P3 treatment (Schedule2), probably because of the well-developed root system of Leymus chinensis when soil nutrients increased, which led to the existence of overlapping ecological niches for different species of AMF (Johnson et al, 2010; Poudel et al, 2016). Higher network topology characteristics have been demonstrated to increase network complexity and stability (Coyte et al, 2015; Peng et al, 2016). Moreover, our results showed that the topological eigenvalues of the network increased under high P conditions (Schedule2). In addition, We calculated the stability of the co-occurrence network (Fig. 3), and the results further verified that P increased the stability of the co-occurrence network of AMF, which is in line with the results of previous studies on fertilization effects (Ma et al, 2021).
The AMF community mainly exists in the soil and plant root systems. Additionally, soil nutrients and plants dominate the assembly of AMF community in soil and root (Vályi et al, 2016; Vályi et al, 2014). The nodes in the constructed co-occurrence network were dominated by species of the genera Glomus and Paraglomus, with nodes ranging from 50.00–73.33% and 9.52–13.33%, respectively, followed by Claroideoglomus, with nodes ranging from 2.38–6.45%, and Diversispora (Table 2). Studies have shown that Glomus is a major genus of AMF under long-term fertilization conditions ((Higo et al, 2019; Ma et al, 2018; Zhu et al, 2020), which may be related to P tolerance and pathogen resistance (Melo et al, 2014; Veresoglou et al, 2013). In addition, species richness of Paraglomus and Claroideoglomus increased under high P conditions, i.e., P3 (Fig. 1d). Therefore, we speculated that the roots of Leymus chinensis showed strong selectivity and affinity for AMF under this condition (Schmitz et al, 2014; Vályi et al, 2014; Weber et al, 2019). Concomitantly, the root system that developed under high nutrient supply provided the AMF community constituents with more ecological niches (Arcidiacono et al, 2023; Nancy et al, 2010). The abundances of Diversispora and Paraglomus reportedly show opposite trends under P-rich conditions, consistently with our own results (Ducousso-Detrez et al, 2022).
The Mantel test results demonstrated that pH significantly affected the AMF community (Fig. 4a). Indeed, soil pH is considered an important factor affecting the activity and community composition of AMF (Davison et al, 2015; Davison et al, 2021; Geel et al, 2017; Hazard et al, 2013). In addition, pH can directly or indirectly affect soil nutrient content, thus indirectly affecting AMF communities (Ma et al, 2018; Xu et al, 2017).To explore the effects of environmental factors on the complexity and stability of the co-occurrence network of AMF, We analysed the relationship between soil environmental factors and topological features of co-occurrence networks based on Pearson’s correlation. The results showed that AK and some soil enzyme activities affected the complexity of the co-occurrence network, whereas pH, TP, AN, and MBP affected its stability (Fig. 4b). AMF play an important role in plant nutrient acquisition (Johnson, 2010; Lindahl et al, 2007); furthermore, some studies have demonstrated that soil AK, TP, and enzyme activities are important factors influencing the structure of fungal networks (He et al, 2017). These findings point at the factors that influence the co-occurrence network of clumping AMF and provide an important basis for understanding the interrelationships among different species of AMF communities.
Effects of AMF on nutrient uptake of Leymus chinensis
The co-occurrence network can further determine the interrelationships among different species of the microbial community, where the correlation of the edges of the co-occurrence network represents the relationship between two species (Banerjee et al, 2016b). Our results showed a significant (P < 0.05) relationship between P content in Leymus chinensis roots and the proportion of positively correlated edges of the co-occurrence network (Fig. 5), which in turn affected the P content of the roots by altering the relationship between species in the AMF community. In turn, leaf N content was significantly (P < 0.05) related to the number of nodes, edges, and positively correlated edges (Fig. 5). As an essential component of photosynthetic building blocks, chlorophyll, carotenoids, and photosynthetic enzymes, N plays a paramount role in plant growth and development. Further, AMF need to absorb soil N and P to exchange with the plant for photoassimilate, such as to maintain vital metabolic activities, and when soil nutrient content is sufficient, it affects the exchange of nutrients between AMF and their hosts (Jiang et al, 2017; Nancy et al, 2010).