Arbuscular mycorrhizal fungi increase the interspecific competition between two forage plant species and stabilize the soil microbial network during a drought event: Evidence from the field

Arbuscular mycorrhizal fungi (AMF) have been found to help plants adapt to water deficit conditions. However, it still lacks filed evidence on how AMF affect plant physiology and soil microbial communities under natural extreme drought events. Here, we conducted an AMF-inoculation and legume-intercropping (Medicago sativa) field experiment in a forage (Broussonetia papyrifera) monoculture ecosystem, and assessed the effects of AMF on plants and soil microbes during a severe drought event lasting for 40 d. The results showed that the mycorrhizal colonization rate was significantly higher for M. sativa than B. papyrifera. In plots with both B. papyrifera and M. sativa, the physiological properties (e.g., diurnal range of water potential and malondialdehyde concentration) of M. sativa were better in the AMF-inoculation plots than in non-inoculation plots; the trend was opposite for B. papyrifera. AMF-inoculation significantly increased the aboveground productivity of M. sativa by increasing diurnal range of water potential and stem water content during the drought event, resulting in more severe soil water deficits. AMF-inoculation or/and legume-intercropping weakened taxonomic interactions (i.e., edges) and increased the modularity of co-occurrence networks compared with the control. The structural equation model analysis indicated that soil nutrients had a direct positive correlation with soil microbial community composition, whereas soil nutrients and moisture had an indirect negative correlation with soil microbial community diversity by inducing changes in aboveground biomass of B. papyrifera. Overall, AMF-inoculation helps M. sativa acquire water during the drought event, which exacerbates soil water deficits and thereby may increase interspecific water competition between B. papyrifera and M. sativa. AMF-inoculation or/and legume-intercropping maintain the stability of microbial networks by weakening taxonomic interactions and increasing the modularity under the extreme drought event. Our findings improve the understanding of AMF effects on plant-plant and plant-soil interactions under climate extremes.