Effects Of Soil Nitrogen And Phosphorus Additions On Arbuscular Mycorrhizal Fungal Communities In Different Grassland Ecosystems

Arbuscular mycorrhizal(AM)fungi can form mutualistic mycorrhizal symbionts with more than 80%terrestrial plant species.Plants supply AM fungi with carbohydrates,and in return,AM fungi provide hosts with nutrients,particularly nitrogen(N)and phosphorus(P).It is well established that AM fungi play key roles in linking the aboveground and belowground ecosystems,regulating and influencing many ecosystem processes.Over the last several decades,atmospheric N deposition and chemical fertilizer addition have led to enrichment of soil N and P resources.Soil nutrient resources can directly and indirectly affect AM fungal communities.P additions often reduce AMF fungi.However,AM fungi responses to N additions were highly variable and inconsistent in previous studies,and the underlying mechanisms remain unclear.Grassland is the most extensive terrestrial ecosystem on the Earth,and the grassland plants are mostly mycorrhizal Alpine meadows on the Tibetan Plateau and the semi-arid grassland on the Inner Mongolia Plateau are the two most important grassland ecosystems in China.These ecosystems are very fragile,and sensitive to global change.Therefore,the responses of AM fungi to soil nutrient additions are important to maintain the stability and sustainable development of grassland ecosystems.We conducted field experiments of long-term N and P gradient additions in an alpine meadow on the Tibetan Plateau and in a semi-arid typical steppe in Inner Mongolia,and combined with greenhouse inoculation experiment to assess the effects of soil nutrient additions on the AM fungal communities in the roots of different dominant plant species,and explore the response mechanisms of AM fungal communities and functions as response to nutrient resource changes.The main results were as follows:1.Responses of AM fungal communities in the roots of Elymus nutans,a dominant plant species,to N and P gradient additions in an alpine meadow ecosystem on the Tibetan Plateau.The results showed that N and P additions had significant effects on the soil microbial community structures.Nitrogen additions significantly decreased the fungal to bacterial ratio,while P additions had no significant influence on it.AMF colonization of roots increased with the increasing N additions,and the external biomass of AM fungi increased under the low N addition,but reduced significantly with the high N additions.P additions significantly inhibited both the AM fungal root colonization and the external mycelia growth.The community structures of AM fungi were significantly affected by N and P additions,and the diversity of AM fungal communities decreased significantly with the high nutrient additions.2.Responses of AM fungal communities in the roots of Elymus nutans to nutrient gradient additions and recovery patterns in an alpine meadow ecosystem on the Tibetan Plateau.Bacterial community showed better resilience than fungal community,but excessive nutrient additions made bacterial community difficult to recover.After the cessation of nutrient additions,the decrease in the ratio of fungal to bacterial biomass caused by nutrient additions could not be restored,and the ecosystem was still unstable.Continuous N and P additions affected AMF infection and AMF communities.Four years after the N or P additions were stopped,AMF root colonization under the high nutrient additions significantly increased,and the difference of the AM fungal external biomass caused by nutrient addition gradients was basically disappeared.However,the inhibiting effect of nutrient additions on the AM fungal root colonization and external mycelia growth were existed.The community compositions of AM fungi showed a gradual recovery trend.Four years after N and P additions were stopped,the structure of AM fungal community became almost identical to the one in the control(no fertilizers applied),but the community diversity was not enough to recover to the level of initial state.3.Responses of AM fungal communities in the roots of two dominant plants(Artemisia grandis and Stipa grandis)of different functional groups to soil N and P nutrient additions.Nutrient additions significantly promoted the growth of A.grandis,while the growth of S.grandis was inhibited by competition.Responses of AM fungi to soil nutrient additions had similar trends in both host roots:P additions significantly decreased the AM fungal root colonization,while low to moderate level of N additions increased but high N additions reduced AM fungal colonization.N and P additions significantly affected the structure of AM fungal communities,and community compositions in roots were highly host-specific.4.Responses of AM fungal communities in the roots of two dominant plant species(Artemisia grandis and Leymus chinensis)to gradients of long-term N and P additions in a semi-arid typical steppe in Inner Mongolia.The results showed that distinct mechanisms could explain the effects of N and P additions on AM fungi.The impacting models of N and P additions were different.P additions significantly inhibited the AM fungal infection of host roots and external biomass under the annual and perennial species,while the effect of N additions on AM fungi varied with the amount of nutrient addition.The external biomass of AM fungi significantly increased under the moderate N additions.However,excessive N addition inhibited strongly the growth of AM fungal mycelia in soil.The effects of N additions on AM fungal colonization had certain host specificity.The AM fungal colonization in the roots of A.grandis decreased significantly with the gradients of N additions,and the AM fungal colonization in the roots of L.chinensis increased under the low level of N additions but decreased under the high level of N additions.Both N and P additions changed the AM fungal community structures in the host roots.P additions significantly reduced the diversity of AM fungal community,while N additions had no significant effect on AM fungal community.The dominant genera of AM fungal communities in the roots of two host plants were Glomus and Diversispora,and the responses of them to soil N and P additions were different in the two host plant roots.Based on the above findings,we proposed a new conceptual framework to explain the effect of N and P additions on AM fungal communities.Soil available N:P stoichiometry predominated AM fungi responses to P additions.However,N-induced soil acidity and possibly ammonia toxicity strongly impacted AM fungi under high N addition.The conceptual model can help us understand and explain the distinct N enrichment effects on AM fungi in previous studies,and provide a scientific basis for predicting the effects of soil nutrient enrichment on AM fungi and the symbiotic relationships with plants and AM fungi under future global change scenarios.