| Arbuscular mycorrhizal (AM) fungi can form mutualistic associations with the roots of most terrestrial plants. It is well established that the AM fungi play key roles in linking the aboveground and belowground ecosystems, and consequently, influence many terrestrial ecosystem processes. In addition, AM fungi can serve as an indicator of the ecosystem functioning, and the importances of AM fungal species and functional diversity are well appreciated in maintaining the sustainability of ecosystems. In recent decades, many ecologists have focused on the species compositions and functions of AM fungal communities in natural conditions, especially those anthropogenic disturbed ecosystems. Nonetheless, less is known about the AM fungal communities in the Loess Plateau and Qinghai-Tibet Plateau of China, where the ecosystems are unique and fragile, so that it is difficult to us to direct the sustainable management and restoration of disturbed ecoystems in this region. In this dissertation, I chose several anthropogenic disturbed ecosystems to investigate the effects of plastic film mulch, vegetation damage and fertilization on the species diversities and community compositions of AM fungi. The main results of this dissertation are listed below.(1) In the dry-land spring wheat field of Loess Plateau, AM colonization and spore density were increased significantly by the plastic film mulch (PFM). A total of nine AM fungal phylotypes was detected in wheat roots under PFM and no-PFM treatments, whereas the phylotype richness of AM fungi were similar between treatments (PFM:5±0.7; no-PFM:4.6±0.5). The PFM treatment could specifically activate or inhibit some AM fungal taxa to colonize wheat roots, resulting in distinct shifts of AM fungal community compositions. Changes of AM fungal variables under the PFM treatment were mainly attributed to the changes in soil properties such as available phosphorus and soil moisture.(2) In the regions with very high altitude (4,500-4,800m a.s.l.) along the Qinghai-Tibetan highway, a total of21AM fungal phylotypes were detected from the rhizospheres (based on roots and spores) of Dracocephalum heterophyllum (pioneer species) in sites disturbed30years ago and Astragalus polycladus (late-successional species) in undisturbed vegetation, including eight new AM fungal species and one new family-like clade. More AM fungal phylotypes colonized root samples of D. heterophyllum (5.4±0.49) than of A. polycladus (1.93±0.25); moreover, no arbuscules (nutrient exchange structure) were observed in the roots of A. polycladus, whereas the arbuscular colonization of D. heterophyllum was12.8%. These findings suggest that AM fungi would prefer to colonize pioneer species rather than late-successional species in this region. We also found that the vegetation coverage was an important factor in regulating AM fungal community, with a low AM fungal diversity detected in regions with serious vegetation degradation.(3) A total of38AM fungal phylotypes was identified from a long-term nitrogen and phosphorus fertilization site (3,500m a.s.1.) in the alpine meadow ecosystem of Qinghai-Tibet Plateau. With the increasing levels of fertilization, the plant species richness, phylotype richness of AM fungi in roots and AM colonization reduced from27.6to4.4, from13to4.4and from41.2%to12.7%, respectively. The highest level of fertilization caused the extraradical AM fungal hyphal biomass to decline by three-fold. High fertilizer inputs had strong negative effects on AM fungi, rusulting in dramatical loss of biodiversity and functions of AM fungal community. At the community-scale, response of AM fungal communities colonizing roots to long-term fertilization was strongly linked to the shifts of plant communities due to fertilization, suggesting the plant community composition is a key determinant in influencing the AM fungal assemblage.(4) The relationships among AM fungi, plants and soil environment are stabilied by long-term co-evolution and co-adaptation in a local scale, and consequently, any changes of plant and soil components will generate greatly effects on AM fungi. Anthropogenic disturbances such as fertilization, vegetation damage and agronomic practices will affect negatively the diversity and composition of AM fungal community, and influence the sustainability of ecosystems. |
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