Effects Of Degradation On Community Structure And Function Of Mycorrhizal Fungi In Tibetan Alpine Meadow

As a key component of underground biologic community,Arbuscular mycorrhizal（AM）fungi can form symbiotic relationship with most terrestrial plants.It is considered to be the result of coevolution of fungi and plants.AM fungi can obtain photosynthetic products from hosts by providing and transporting nutrients for plants as exchange,so as to interact between AM fungi and plants,and then establish a biological nutrient trading market.AM fungi play an important role in plant diversity,vegetation community succession and ecosystem stability.Grassland degradation leads to the loss of ecosystem function and productivity in the Qinghai Tibet Plateau,and the aboveground vegetation community has changed dramatically,accompanied by a large loss of soil nutrients.Therefore,the grassland ecosystem of the Qinghai Tibet Plateau is under great threat.However,there is a lack of research on the diversity,ecological function and driving community succession of AM fungi in degraded alpine grassland.In this study,we selected the Tibetan alpine meadow to study the symbiotic relationship between AM fungi and plants.Then,the Tibetan alpine meadow was divided into three degradation stages to study the community structure,ecological function of AM fungi and the nutrient exchange mechanism between AM fungi and plants along the degradation gradient.Finally,combined with the plant-soil feedback theory,we studied the feedback effect of AM fungi on plants in the process of degradation.The following research results were obtained:（1）We studied the AM fungal communities in 69 root samples from 23 plant species in a Tibetan alpine meadow using Illumina-Mi Seq sequencing of 18S r RNA gene.The sequences annotated as AM fungi were grouped into 121 OTUs,covering 3 orders,6 families and 9 genera.The number of Glomus sequences and OTU richness were the most.Different plant species had different AM fungal diversity and community structure,and Elymus nutans had the most OTU richness.The results showed a significant positive correlation between the phylogenetic distances of plant species and the taxonomic dissimilarity of their AM fungal community（P<0.05）.The plant-AM network was characterized by high connectance,high nestedness,anti-modular and anti-specialized.（2）In the process of degradation,the OTU richness and Shannon-Wiener index of AM fungi in soil and plant roots changed significantly（P<0.05）,and there were significant differences between the soil and root AM fungal community composition（P<0.05）.The composition of soil AM fungal community was significantly affected by available phosphorus（AP）（P<0.05）,and the composition of root AM fungal community was significantly affected by organic carbon（SOC）and total phosphorus（TP）（P<0.05）.The total driving effects of bareland patches,soil properties,plant richness and root/shoot ratio on the richness of soil and roots AM fungi were completely opposite.（3）Degradation significantly changed the AM fungal diversity and community composition of dominant and companion plants（P<0.05）.Degradation affected mycorrhizal growth response（MGR）and nitrogen uptake response（MNR）of dominant and companion plants.With the aggravation of degradation,the amount of ¹⁵N absorbed by plants through AM increased.The AM fungi ¹³C content of dominant species and companion plants varied with degradation gradient:severely degradation>moderately degradation>non degradation.Degradation stimulated carbon/nitrogen trading between AM fungi and plants,especially under the severely degradation,resulting in plants needed more ¹³C cost to obtain per unit of ¹⁵N from AM fungi.（4）Six representative plants showed different plant-soil feedback effects after inoculating AM fungi at the species level and community level.There was significant positive plant-soil feedback between Kobresia humilis and other plants（P<0.05）,and significant negative plant-soil feedback between Heteropapus hispidus and other plants（P<0.05）.In the non degraded stage,¹⁵N uptake by plants was significantly negatively correlated with plant-soil feedback（P<0.05）.In the severely degraded stage,the AM fungi ¹³C content and cost/benefit value were significantly positively correlated with plant-soil feedback（P<0.05）.