| Alpine marsh ecosystems on the Tibetan Plateau play an important role in protecting water resources,species diversity and environmental purification,as well as regulating the water system of the Yellow River Basin and regional climate.In recent years,global climate change and human activities have led to the alpine swamp wetlands are drying up and degraded gradually.The drying up of alpine marsh ecosytems can have a profound effect on the structure and function of terrestrial ecosystems.Nematodes are the main taxa of soil microfauna.Soil nematode communities are rich in species and abundance,occupying multiple trophic levels in the soil food web,and have important roles in promoting organic matter decomposition,material cycling and energy flow as well as maintaining soil ecosystem functions.Nematodes are sensitive to environmental changes and are indicator organisms reflecting environmental changes.The study area of this thesis is located in the alpine marsh in the eastern the Tibetan Plateau,and 23 different sites were selected in the spatial scale in this area as a long series of soil moisture change gradients.The direct effects of the drying process in the alpine marsh on the soil nematode community,and the indirect effects on the nematode community through the above-ground plant community and soil environmental factors,respectively,were systematically studied in this thesis.The main findings are as follows:1)Species richness,abundance and biomass of plant communities increased significantly with the drying of alpine marsh,and where there was no significant change in belowground root biomass.Meanwhile,soil water content,total nitrogen and total carbon decreased significantly and nitrate nitrogen significantly increased,while soil p H,ammonium nitrogen and total phosphorus did not change significantly with the drying of alpine marsh.2)In total,21,978 nematodes belonging to 42 genera were identified in 115 soil nematode samples.The richness and of abundance soil nematode genera increased significantly,the abundance of bacterivores,fungivores and herbivores increased significantly,the abundance of predators-omnivores did not change significantly,the soil nematode community-weighted mean of body size(CWM)decreased significantly,but nematode biomass did not change significantly with the drying of the alpine marsh.3)The species diversity of soil nematode species decreased significantly,no significant change in evenness,a significant decrease and increase in soil nematode maturity index(MI),and in enrichment index(EI)and basis index(BI),respectively,but no significant change in plant parasitism index(PPI)and structural index(SI)with the drying of alpine marsh.4)Plant communities(species richness and root biomass)and soil environmental factors(soil water content,total nitrogen and total carbon)were the main ecological factors driving soil nematode genus.Species richness of plant community was the most important predictor of soil nematode genus richness and abundance;neither plant communities nor soil environmental factors are effective predictors of the community-weighted mean(CWM)of soil nematode body size.5)The result of structural equation modeling(SEM)showed that alpine marsh drying directly increased nematode genus richness in terms of nematode genus richness and abundance.Drying of alpine marsh indirectly increased nematode genus richness and abundance by increasing species richness of plant community,and indirectly decreased nematode abundance by decreasing soil water content.O verall,the species richness and abundance of nematode genera in soil increased significantly with drying of alpine marsh.6)In terms of nematode trophic taxon abundance,alpine marsh drying directly increased fungivorous and herbivores abundance.Drying of alpine marsh indirectly increased fungivorous,bacterivores,and predators-omnivores abundance by increasing species richness of plant community.Alpine marsh drying indirectly decreased bacterivores and predators-omnivores abundance by decreasing soil water content,and it also indirectly decreased predatory-omnivorous nematode abundance by decreasing total carbon.Overall,the abundance o f bacterivores,fungivorous and herbivores increased significantly with drying of the alpine marsh,while the abundance of predators-omnivores did not change significantly.7)In terms of nematode body size,drying of alpine marsh directly reduced the community-weighted mean(CWM)of soil nematode body size,with no significant correlation with soil physicochemical factors and root biomass.Our results suggested that alpine marsh drying affects soil nematode communities through environmental filtering,specifically,it affects soil nematode communities by controlling food resources(plant community species richness)and regulating soil environment(soil water content and total carbon),and it is a strong driver of soil food web structure and function.O ur study revealed the direct and indirect mechanisms of alpine marsh drying on soil nematode communities,and it could contribute to our understanding of complex subsurface ecosystem processes and functions. |
