| Organisms on the earth are dependent on various resource such as light and soil nutrients. Understanding how resource availability governs biotic communities is a crucial step towards predictive community ecology and ecosystem sustainability. Arbuscular mycorrhizal (AM) fungi, one of the most widespread root-associated microorganisms, can from mutualistic associations with most of the land plant species. Plants supply AM fungi with carbohydrates, and in return, fungi provide their host plants with phosphorus (P) and nitrogen (N). Moreover, it is well accepted that AM fungi play key roles in regulating plant communities, nutrient cycles, ecosystem productivity and stability, and consequently, influence many ecosystem processes. It has been well documented that plant communities as well as the AM associations would be very sensitive to changing light intensity and soil N and P fertility. However, how AM fungal communities respond to changing light and soil N and P availability, what the differences between plant and AM fungal responses to the changing resource availability, and which ecological process drives the shifts of both plant and AM fungal communities, are still poorly understood. To address these questions, I investigated the influences of changing light and soil N and P availability on both plant and AM fungal communities in two filed experimental sites with long-term shade and fertilization treatments in an alpine meadow ecosystem on the Qinghai-Tibet Plateau, and the primary ecological processes structuring both communities were inferred from the community phylogenetic analysis. The main results of this dissertation are listed below.1. In the first experiment with 4 yr of shade (blocked c. 70% light intensity) and fertilization [45 g (NH4)2HPO4 m-2 yr-1] interactive treatments (including:control, shade, fertilization and shade+fertilization), I found that the three experimental treatments caused plant species richness to decline by c. 54%-70% in comparison with that in control plots; however, only shade+fertilization treatment reduced significantly the AM fungal species richness inside roots (c.45%) and root length AM colonization (c.73%) compared with the untreated control. Shade and/or fertilization significantly changed the species and phylogenetic compositions of both plant and AM fungal communities, but it is clear that the changes of AM fungal communities were largely mediated by the shifts in plant species richness and shoot biomass that were caused by shade and fertilization. Plant community phylogenetic structure shifted from random in untreated control to overdispersion in other treatments, whereas AM fungal communities were phylogenetically clustered and random in unfertilized (control and shade treatments) and fertilized (fertilization and shade+fertilization treatments) plots, respectively. These results suggest that plant communities in treated plots were mainly determined by competitive exclusion, and that AM fungal communities in unfertilized and fertilized plots were determined by environmental filtering and random process, respectively. In addition, most variables of both plant and AM fungal communities were correlated significantly with the availability of light, N and P, but it seems that light availability might be more important in determining plant community, and soil N and P availability more important for AM fungal community.2. In the second experiment with 8 yr of fertilization gradient treatments [0,30,60, 90 and 120 g (NH4)2HPO4 m-2 yr-1; hereafter referred to as F0, F30, F60, F90 and F120, respectively], I found that across the fertilization gradient, plant species richness declined from 27.6 to 4.4, the relative abundance of Elymus nutans increased from 9.4% to 71%, and also the concentrations of both soil available N and available P were increased gradually. A total of 37 AM fungal phylotypes was identified in 25 soil samples, of which 17 phylotypes belonged to the genus Glomus. Fertilization significantly reduced the phylotype richness of AM fungi, especially the highest level of fertilizer input (F120) caused the phylotype richness within the genus Glomus to decline by 66%, but significantly increased AM fungal genus richness. Across the fertilization gradient, the relative abundance of Glomus declined from 81% to 14%, whereas the relative abundance of Diversispora increased from 5% to 43%. Both the species and phylogenetic compositions of AM fungal communities varied among treatments, and both were correlated significantly with soil available P and the relative abundance of Elymus nutans (all P< 0.001). AM fungal communities were phylogenetically clustered in unfertilized soil (F0), random in low fertilizer treatment (F30) and overdispersed in high fertilizer treatments (F60, F90 and F120), indicating that the primary ecological process determining AM fungal communities shifted from environmental filtering to stochastic process and finally to competitive exclusion across the fertilization gradient.In conclusion, I observed strong effects of light and soil nutrient availability on both plant and AM fungal communities. Reducing light intensity and increasing soil N and P fertility would result in community shifts and species loss of both plants and their associated AM fungi, and consequently influence the sustainability of ecosystems. The ecological processes structuring plant and AM fungal communities were different, but the shifts of both communities caused by reducing light and/or increasing soil nutrient availability were largely attributed to the increasing dominance of competitive exclusion. This study clearly reveals the patterns and underlying mechanisms of light and soil nutrient availability regulating both plant and AM fungal communities, and also the research findings in this study will help us to fully predict the ecological impacts of anthropogenic activities and global change such as fertilization, nutrient deposition and atmospheric haze. |
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