Distinct effects of AM fungi and root pathogens drive differential plant-microbial interactions in steppe graminoids and forbs

Plant-microbial interactions (PMI), governed by both mutualistic and antagonistic soil microorganisms, are crucial for plant performance. However, empirical evidence directly linking PMI strength to the extent of arbuscular mycorrhizal (AM) fungal colonization or pathogen infection remains limited, particularly across varying biotic and abiotic contexts. Here, we quantified AM fungal colonization and root pathogen incidence across two plant functional groups (13 grassland species) under monoculture and interspecific competition in overgrazed and restored soils, and investigated their relationships with PMI. Results showed that the strength of plant-microbial interaction varied significantly among different plant species. Notably, graminoid species, such as Leymus chinensis, Agropyron cristatum, Stipa capillata and Elymus dahuricus exhibited negative PMI, whereas forbs exhibited positive PMI. Forbs had higher AM fungal colonization in overgrazed soil, and their PMI was positively correlated with colonization across all treatments. Conversely, graminoids had higher pathogen incidence, and their PMI was negatively correlated with pathogen incidence under overgrazed, monoculture conditions. Our findings demonstrate that PMI in steppe ecosystems is functionally dichotomous, being primarily mediated by AM fungi for forbs and by pathogens for graminoids, with direct implications for grassland management and restoration.