Multiple global change factors weaken the benefits of AMF diversity
Our results showed that ecosystem multifunctionality decreased in both high and low AMF diversity treatments, under multiple GCFs (Fig. 1a). And the multifunctionality was often greater with the high AMF diversity than the average of the AMF inoculated individually, and greater than the best performing single AMF inoculated under the control, microplastics, drought and multiple GCFs. This indicates that there is complementary among AMF in supporting multifunctionality under some GCF scenarios. Our findings align with previous biodiversity-ecosystem functioning (BEF) studies, which generally suggest that ecosystems with high biodiversity exhibit greater niche differentiation and positive complementarity, thereby enhancing ecosystem multifunctionality and resilience to environmental changes (Craven et al., 2016; Godoy et al., 2020).
However, ecosystem multifunctionality under high AMF diversity was not the highest under nitrogen addition, pesticide pollution, and saline-alkali treatment. Some studies also reported that communities with higher diversity may exhibit lower productivity (Polley et al., 2003) or ecosystem functions (Becker et al., 2012; Jousset et al., 2011), which may be related to competition among species. In this study, under non-GCF conditions, plant root mycorrhizal colonization rate, HLD, and spore density were higher in high AMF diversity treatments compared to any single AMF inoculation treatment (Figure S2a-c). However, under conditions with multiple GCFs, the negative effects of GCFs on mixed AMF treatments were more sever compared to individual AMF like R. irregularis and G. etunicatum (Figure S2d-f). These observations suggest that the presence of multiple GCFs may intensify the competition among AMF. It has been indicated that the addition of nitrogen and phosphorus can intensify the competition among AMF (Ma et al., 2021; Qin et al., 2022; Tang et al., 2023).
Plant-derived carbon is the primary resource for competition among AMF species. Research has shown that host plants allocate more carbon to AMF species that supply more phosphorus in return (Kiers et al., 2011). Beyond resource competition, species can also release allelochemicals to directly affect competitors. In high-diversity communities, while positive functional complementarity exists, complementarity in allelopathic toxins can lead to a ‘negative complementarity effect’ (Becker et al., 2012). AMF can also release hyphosphere exudates, while current research primarily focuses on their role in recruiting hyphosphere microbes to explain AMF ecological functions. Studies on AMF allelopathy are still limited.
The diminished effect of AMF diversity on ecosystem multifunctionality under multiple GCFs, might also relate to specific GCF factor. For example, there was no significant difference in ecosystem multifunctionality among different AMF treatments under saline-alkali and pesticide pollution conditions. Plant root mycorrhizal colonization rates and HLD showed the most negative responses to saline-alkali treatment (Figure S2d, e). Davison et al. (2021) analyzed global soil samples and found that AMF virtual taxa are often associated with a particularly narrow range of pH and temperature conditions. Previous studies have investigated the mechanisms through which AMF mitigate plant saline-alkali stress; however, there is limited research on how pH influences the growth of AMF.