The Diversity And Function Of Arbuscular Mycorrhizal Fungi Associated With Alfalfa Under Different Nutrient Conditions

Alfalfa is an economically and ecologically important pasture plant worldwide.As a legume,its roots often form symbiotic relationships with both rhizobia and arbuscular mycorrhizal(AM)fungi simultaneously.Existing evidence has proven that AM fungi are able to provide soil nitrogen to host plants through hyphae,but to date the relative amount of N contributed by AM fungi has not been quantitatively assessed.Since rhizobia are well recognized as a main nitrogen source to alfalfa,whether AM fungi could contribute nitrogen to this dual host is uncertain.If AM fungi play a role in alfalfa's nitrogen uptake,it remains unclear how much the relative contribution could vary under different soil nitrogen conditions and with the presence/absence of rhizobia.Answering these questions could not only further our understanding of AM fungal functions and plant-microbe interactions,but also provide scientific bases to practical practice in alfalfa management.In this study,I first analyzed AM fungal communities inside alfalfa roots and in its rhizosphere soil through field sampling and high throughput sequencing.Community variation in AM fungi under different phosphorus fertilization levels was examined in order to explore the key factors driving the dynamics of AM fungi associated with alfalfa.I then quantified the relative amount of plant nitrogen uptake contributed by AM fungi in a pot experiment where root uptake and hyphal uptake of nitrogen were differentiated by two types of meshes and N-15 labeling.The responses of fungal nitrogen contribution to background soil nitrogen levels and rhizobia were tested in a nitrogen fertilization and rhizobial inoculation experimentPreliminary results showed that AM fungal colonization in alfalfa roots declined with the increase in phosphorus fertilization levels.Phosphorus addition had significant effects on the relative abundance of Diversispora in roots.The relative abundance of Paraglomus within alfalfa roots had significant positive correlation with soil organic matter.Co-inoculation increased the plant height and biomass of alfalfa significantly.The relative nitrogen contribution of AM fungi to alfalfa was about 19.66%.When colonized by AM fungi,alfalfa with rhizobia had significantly lower nitrogen uptake than alfalfa without rhizobia.The relative contribution of N uptake by AM fungi did not vary among different levels of nitrogen fertilization or between plants with and without rhizobia.