AM fungi can contribute positively to PMI due to their role in resource access and disease resistance, whereas pathogen give rise to negative PMI by causing root diseases (Schroeder et al. 2020). Our empirically results showed a statistical positive correlation between AM fungal colonization and PMI, as well as a negative correlation between pathogen disease and PMI. These observed relationships were largely caused by the difference between graminoids and forbs. Moreover, the correlativity between AM fungal colonization or pathogen disease and PMI varied for graminoids and forbs, indication that the relationship between PMI and its driving factors may vary depending on the plant functional group being considered. Additionally, interspecific competition and soil nutrient availability also play significant roles in this dynamic interaction. For example, the disease severity of graminoids was statistically negative related to their PMI effects in individual experiments in overgrazed soil, while the colonization of AM fungi was more positively related to the PMI of forbs in community experiment. Overall, we found that graminoids were suppressed by harmful microorganisms, particularly in resource limited context, whereas forbs benefited from AM fungi. These linkages between soil microorganisms and PMI can aid in the management and restoration of grassland vegetation by targeting specific plant function groups.
Various plant-microbial interactions among different plant species contribute fundamentally to community diversity (Stein and Mangan 2020). Our findings reveal that L. chin, which is overwhelmingly dominant in steppe grassland of northern China, suffered from a negative PMI, suggesting that soil microorganisms suppress the growth of L. chin. This suppression may facilitate community richness by creating low-competitive conditions for other plant species. This phenomenon aligns with the concept of negative PMI, also known as PSF, frequently occurs when plant seedlings grow in conspecific soil (Mangan et al. 2010). Considering that our soil samples were collected from a grassland perennially dominant by L. chin (biomass accounting for > 40%), it is an unsurprising result that strong negative PMI would be exerted on L. chin. For other graminoids, E. dahu and A. cris exhibited a propensity for experiencing negative PMI, whereas the biomass of S. capi appeared to be simply reduced by soil microorganisms in overgrazed soil. According to previous studies, plant species that are closely related are more likely to have natural enemies (Kamble et al. 2024) and show more similar feedback effects due to their longer shared evolutionary history (Münzbergová and Šurinová 2015). Moreover, graminoids generally release similar semi-polar metabolites to each other (Dietz et al. 2019, 2020). which are related to shaping soil microbial communities (Badri and Vivanco 2009). Our results showed that E. dahu and A. cris generally experienced a more comparable PMI with L. chin as compared to S. capi, possibly due to their close relation. Specifically, E. dahu, A. cris and L. chin were categorized as Triticeae while S. capi was classified as Stipeae. In contrast, eight plant forbs, with the exception of L. apet, exhibited a tendency to experience positive PMI, indicating that they were able to derive benefits from soil microorganisms. At the level of plant functional group, forbs suffered a more pronounced positive PMI compared to graminoids. This phenomenon could be attributed to the absence of specific pathogen that target forbs in the soil, as well as the relatively weaker nutrient competition ability of forbs (Li et al. 2020), which result in a greater reliance on mutualistic microorganisms for nutritional support.
We conducted measurements of AM fungal colonization and the percentage of root disease in order to evaluate the mutualistic or antagonistic relationships between plant and soil microorganisms, as previously mentioned. The results revealed a significantly higher colonization rate of AM Fungi in forbs compared to graminoids in community experiments conducted under low nutrient level. Furthermore, forbs showed a significant positive correlation between PMI and AM fungal colonization in all biological (individual and community experiments) and abiotic treatments (restored and overgrazed soil), whereas the PMI of graminoids was not affected by AM Fungi. These results statistically suggested a weak effect of AM fungi on the growth of graminoids, correspond to the patterns that C3 grasses exhibited smaller responses to mycorrhizal inoculation than C4 grasses and non-N-fixing plants(Hoeksema et al. 2010), with the graminoids selected in this experiment following the C3 photosynthetic pathway. This weak effect of AM fungi on the growth of graminoids could also potentially be attributed to differences in root structures and physiology (Güsewell 2004), and the resulting nutrient uptake strategies. Specifically, AM fungi could benefit plants by enhancing their nutrient access through the exploration of hyphae in plant-fungal symbiosis (Bueno de Mesquita et al. 2018; Li et al. 2019a). The lush root system of graminoids, particularly their fibrous root system, enables them to rapidly occupy soil space and efficiently absorb nutrients, whereas the root system of forbs is comparatively smaller in size, potentially limiting their nutrient uptake. Within the confined microcosmic system of our experiment, the expansive distribution of graminoids across a considerable soil has the potential to hinder the involvement of AM fungi in nutrient uptake processes. Conversely, forbs with their more restricted root range might exhibit a more pronounced increase in plant nutrient absorption facilitated by the hyphae.
In individual experiments, it was observed that graminoids exhibited a higher percentage of root diseases caused by pathogen compared to forbs. This could be attributed to the legacy of a grassland plant community that was dominated by L. chin, which made the graminoids more susceptible to these diseases. We detected a significant negative linear relationship between the PMI of graminoids and the percentage of pathogen-induced diseases under overgrazed soil conditions. However, the PMI of forbs did not show any changes with respect to diseases percentage. This finding suggested that the PMI of graminoids was more reliant on their resistance to pathogen, particularly when these plants were unable to obtain sufficient nutrients. Gramineous plants in our research could be regarded as being ‘fast’ species with a high root length (Cortois et al. 2016). When nutrients in the soil are insufficient, their growth may be limited, resulting in a weakened immune system and increased susceptibility. On the other hand, ‘slow’ species such as forbs showed slower growth but higher disease resistance, which explains the lack of correlation between the PMI of forbs and pathogen diseases. Further, we did not observe significant effects of soil nutrients on AM fungal colonization and pathogen diseases, nor did we observe significant effects of interspecific competition on pathogen diseases. In our research, restored soil was consider to be more resource-rich than overgrazed soil, with the presents of interspecific competition allowed graminoids to obtain more resources. On the other hand, forbs were subject to greater resource competition influenced by variations in plant competitiveness, supporting the notion that the intensity of competition is similar along a resource gradient. These findings contradict the expectations of Lekberg et al. (2018), who predicted that plant-soil microbial interaction would be more negative in high resource environments (Lekberg et al. 2018). Additionally, these finds do not align with the results of Klinerova and Dostal (2019), who suggested that nutrient-demanding species would experience less negative plant-soil feedback (Klinerová and Dostál 2020). Nevertheless, we did find that the relationship between PMI and AM fungi or pathogen, which indicates the dependence of plant growth on specific microorganisms, could be influenced by soil nutrients and interspecific competition. In the case of plants grown in overgrazed soil and with interspecific competitors, the relationship between the PMI of forbs and AM fungi became even stronger compared to plants grown in restored soil and grown alone. We attributed this result in Plant's dependence to mycorrhizal fungi is enhanced in low-nutrient environments (Schultz et al. 2001). Parallelly, the relationship between the PMI of graminoids and root diseases was found to be significantly only in overgrazed soil and in individual experiment. This suggests that plant resistance to pathogen is enhanced by higher nutrient levels.