Effects Of Climate Warming And Precipitation Changes On Arbuscular Mycorrhizal Fungi And Soil Organic Carbon In Different Grasslands

Arbuscular mycorrhizal（AM）fungi form symbiotic associations with about 80%of terrestrial plants.They acquire their carbon（C）solely from the host plants and provide nutrients in return,thereby regulating plant community structure and soil C and nitrogen（N）cycling.Global change factors such as warming and precipitation changes can affect plant growth and plant-AMF interactions.However,few studies have examined how warming and precipitation changes may affect root-AM interactions,and mycorrhiza-mediated C dynamics,particularly in grasslands.Grasslands account for about 42%of China’s land area,containing a total soil C of 29.1Pg,i.e.,about 10%of the world’s total grassland carbon storage.Grasslands on Tibetan and Loess Plateaus in China play critical roles in forage production and environmental conservation.Plant productivity on Tibetan and Loess Plateau is mainly limited by temperature and water availability,and is sensitive to global change.As almost all the plants there are mycorrhizal,understanding how warming and precipitation may affect AM fungi and mycorrhizal mediated C dynamics is key to better predict the future plant community structure and soil C dynamics in these grasslands.Taking advantage of two long-term,field multi-factor manipulations in an alpine meadow on the Tibetan Plateau and a semi-arid grassland on the Loess Plateau,we directly examined how warming and precipitation alterations influenced roots and AM fungal traits,and their mediated C dynamics.In particular,we integrated field warming（+ca.1.4℃）and precipitation alterations（±30%ambient precipitation）manipulations with the Illumina Mi Seq sequencing and ¹³C isotope tracing to explore the linkages among root traits,AM fungal community and organic C sequestration in soil.The main results of this study are as follows:1.In the Tibetan alpine meadow,warming significantly increased the root biomass and plant-available N and P,but had no effect on the AM fungal colonization rate or the extraradical hyphal biomass,likely because increased N availability offset the positive effects of increased root production on AM fungi.Precipitation reduction enhanced mean root diameter and AM fungal abundance（including colonization rate and extraradical hyphal biomass）,suggesting a trade-off between fine roots and AM fungi as influenced by soil water variability.Warming reduced the evenness of AM fungal community and altered AM fungal community composition.Warming induced stronger environmental filtering of AM fungal community,likely due to reduced C allocation as a result of warming-increased root growth,and N and P availability.In contrast,precipitation change only increased the extraradical biomass of AM fungi,but did not change the diversity,composition or phylogenetic structure of AM fungi community.2.In the Loess semi-arid grassland,the trade-offs between fine root functional traits and AM fungal communities were also detected under warming and precipitation changes.While precipitation reduction and warming increased water stresses and root production,but decreased AM fungal abundance,precipitation increase alleviated the water stress and showed opposite effects on roots and AM fungi.Regression analysis showed that AM fungal richness was positively correlated with the root traits which represent the quantity of C investment to root construction（i.e.root production and root tissue）,but negatively correlated with traits for resource foraging（i.e.specific root length and specific root surface area）.Also,adjustments of root traits were detected influencing the response of AM fungal community composition to the fluctuation of soil water availability.However,neutralized by the effects of free-living microbial activity,neither AM fungal diversity nor overall community composition responded to either warming or precipitation alterations.In addition,the phylogenetic structure of AM fungal communities in all treatments were clustered,and neither warming nor precipitation changes shifted the community assembly of AM fungi.3.In the Tibetan alpine meadow,atmospheric warming significantly promoted soil organic C decomposition,likely by stimulating saprophytic microbes and altering their community composition.Although both precipitation increment and reduction significantly increased microbial biomass C,precipitation changes had no significant effect on soil organic C decomposition.AM fungi also promoted the decomposition of soil organic C,likely due to the priming effect of AM fungal exudates on soil microorganisms and their activities,and the microbial mineralization of soil organic matter in the AM fungal hyphosphere.In addition,the community evenness AM fungi positively correlated with mycorrhiza-mediated organic C decomposition.4.In the Loess semi-arid grassland,warming tended to suppress soil organic C decomposition,likely because our warming method with open top chamber slightly reduced the top soil temperature due to stronger shading in the growing season,which may decrease the activity of saprophytes.Altered precipitation had no effect on soil organic C sequestration.In addition,regression analysis showed that the negative effect of AM fungi on soil organic C decomposition increased with the increase of AM fungi abundance.Meanwhile,the AM fungal abundance was positively correlated with the community-level mean diameter of fine roots,indicating that warming and precipitation changes may induce tradeoffs between fine roots and AM fungal.In conclusion,in the Tibetan alpine meadow,AM fungal community composition and assembly,and soil organic carbon decomposition were more sensitive to warming,and AM fungi stimulated soil organic carbon decomposition.In the Loess semi-arid grassland,warming and precipitation changes affected the functional traits of plant fine roots and microbial activities through altering soil moisture,and then affected AM fungal community.And the fluctuation of AM fungal abundance resulted from the shifts in mean root diameter altered the function of AM fungi in soil carbon sequestration.These findings provide new insights into how plant-AM fungal associations may respond to climate change factors,and how these responses may modulate soil organic C dynamics under future climate change scenarios.