The Responses Of Arbuscular Mycorrhizae To Environmental Changes And Their Feedbacks On Host Plants

More than 80% terrestrial plants form mutualistic mycorrhizal symbionts with arbuscular mycorrhizal fungi (AMF), which change the competitive interaction between plants and mediate their coexistence. Environmental changes (such as the enrichment of atmospheric CO₂, increase of soil available nitrogen (N) due to N deposition and invasion of exotic species etc) would direcdy or indirecdy influence the interaction between plants and AMF, and potentially change the interrelationship between plants. The responses of arbuscular mycorrhizae to environmental changes (enrichment of atmospheric CO₂ simulated N deposition and invasion of exotic plant) and their feedbacks on host plants were investigated with pot and microcosm experiments in greenhouse. The main results obtained were shown as below:1. The responses of mycorrhizae to environmental changesThe responses of mycorrhizal colonization and spore numbers to elevated atmospheric CO₂ differed dramatically between plant species. Mycorrhizal colonization of C₃ legumes, C₃ forbs and C₄ forbs decreased (P<0.05), while that of Eleusine indica (C₄ grass ) increased (P<0.05). Spore numbers of Glycine max and Ecbinocbloa crusgalli (Linn.) Beauv. var. mitis (Pursh.) Peterm significantly decreased (P<0.01), but that of C₃ grasses increased (P<0.05). AMF displayed specificity to a certain degree, as some AMF colonized the majority of plants (common AMF group, CAG) while others only colonized a given plant (unique AMF group, UAG). Furthermore, they responded differently to elevated atmospheric CO₂: UAG decreased and CAG increased their abundance.Simulated N deposition resulted in invariant mycorrhizal colonization to most plants with the exception of Poa annua, Trifolium repens and Veronica didyma, which decreased 49.30%, 67.84% and 74.52% respectively (P<0.05). Spore numbers of P. annua, Avena fatua, E.crusgalli var.mitis and Plantago virginica increased, whereas that of Lolium perenne, Medicago lupulina, T. repens and V. didyma decreased significandy (P<0.05).Solidago canqdensis, the exotic invasive species, changed the spore composition of a constructed AMF community. Spore numbers of Glomus geosporum and Glomus etunicatum were much higher than that of Glomus diapbanum and Glomus mosseae. The colonizing pattern of the altered AMF community (A-AMF) and the original AMF community (O-AMF) exhibited great difference: mycorrhizal colonization of O-AMF to local species Kummerowia striata was lower than that of A-AMF, yet they had the same colonization to S.canadensis. Moreover, A-AMF and O-AMF showed different community composition and relative abundance in the roots of K. striata and S. canadensis.2. The responses of host plants to environmental changes Elevated atmospheric CO₂ promoted height and biomass of plants. Plant height of V. didyma, P. virginica and Gnapbalium affine increased 130.85%, 85.84% and 32.13%, respectively (P<0.01). Biomass enhancement of various functional groups followed the order as C₃ legumes>C₃ forbs>C₃ grasses>C₄ grasses>C₄ forbs. N and phosphorus (P) contents of most plants were increased under elevated atmospheric CO₂ but did not reach the significant level. Likewise, simulated N deposition showed great promotion on plant height and biomass. Plant height of P. virginica and V. didyma (C₃ forbs) increased 14.63% and 102.84%, respectively (P<0.01). C₃ legumes had the greatest biomass enhancement- among the 5 functional groups, followed by C₄ grasses and C₃ forbs (P<0.01). Plant N and P contents were also increased significantly.3. The feedbacks of mycorrhizae to plant interactions under different environmental changesTotal biomass of Oryza sativa and E.crusgalli vat.mitis, together with N and P contents of O.sativa were promoted by elevated atmospheric CO₂ under non-mycorrhizal treatments; however, mineral nutrition contents of E.crusgalli var.miti differed between years. On the contrary, elevated atmospheric CO₂ did not impact plant biomass and nutrition uptakes under mycorrhizal treatments. AMF inoculation reduced biomass as well as N and P contents of O.sativa but increased that of E.crusgalli var.mitis compared to non-mycorrhizal treatments, thus promoted the competitive ability of E.crusgalli var.mitis under elevated atmospheric CO₂.Simulated N deposition significantly increased total biomass of O.sativa and E.crusgalli var.mitis under mixture. Biomass ratio of O.sativa to E.crusgalli var.mitis and P content of O.sativa were decreased by simulated N deposition under non-mycorrhizal treatments. However, biomass ratio differed between years and N uptake of O.sativa was even higher than that of E.crusgalli var.mitis under mycorrhizal treatments. Nevertheless, AMF inoculation decreased all the tested parameters of O.sativa compared to non-mycorrhizal treatments, while that of E.crusgalli var.mitis differed between years. In conclusion, AMF tended to increase the competitive ability of O.sativa under simulated N deposition although its parameters decreased significantly.S. canadensis changed AMF spore composition, community structure and species relative abundance in plant roots. These changes affected on mycorrhizal colonization and functions. Canopy width, ramification, total biomass, total N content, aboveground ¹⁵N content and aboveground P content of K.striata under O-AMF treatments were increased by 15.58% (monoculture), 42.42% and 53.23% (monoculture and mixture), 55.82% and 110.14% (monoculture and mixture), 64.01% and 58.13% (monoculture and mixture), 157.19% and 113.65% (monoculture and mixture), 12.56% and 39.20% (monoculture and mixture) respectively compared to A-AMF treatments; whereas P content, belowground N content and aboveground ¹⁵N content of S. canadensis under A-AMF treatments were increased by 61.81%, 105.53% and 235.35% respectively compared to O-AMF treatments.