| Ecosystems worldwide are receiving increasing amounts of reactive nitrogen (N) through anthropogenic activities. Although the effects of increased N inputs on plant communities have been reasonably well studied, few comparable studies have examined impacts on whole soil bacterial communities, though they play critical roles in ecosystem functioning. Previous simulated nitrogen deposition experiments found it’s difficult to distinguish the direct effect of N addition from the soil carbon available changes along with the N enrichment. Given the important role of C input in determining the microbial activity and community structure in forest ecosystem, the research site of our research is located in a temperate steppe.In our study a split-plot designed experiment with3blocks and36plots was established in mid-May2009. There were6treatment combinations at two N levels (total12treatments):no input, no liter, no root, control, double litter. We collected in mid-Aug2012hopping to draw a whole map of soil bacterial diversity and community composition by sequencing bacterial16S DNA through Roche-454platform.1) N addition significantly improved soil microbial biomass by increasing the quality and quantity of aboveground plant carbon inputs. Soil organic carbon and total nitrogen content were significantly positive correlated with microbial biomass carbon, and Soil organic carbon was shown to be a mainly driver for soil microbial biomass.2) Of the classifiable sequences, we analyzed48soil samples, and34phyla were identified across all the soil samples. The dominant phyla were Actinobacteria, Acidbacteria, Protebacteria and Chloroflexi who accounted for about80%of the whole bacterial communities. The abundance of Acidbacteria was highest, up to25%-30%; Actinobacteria ranked right after reach19%-27%. The six sub-dominant groups accounted for about15%of the entire bacterial communities, only left less than5%for the remains.3) Results of structural equation modeling (SEM) analysis were as follows:at the ambient nitrogen condition, litter acted as an important role in maintaining the soil bacterial diversity, while SWC, DOC, TP and N:P were strongly inhibited bacterial diversity. Under enriched nitrogen condition this model significantly changed, litter was no longer a controlling factor in maintaining the diversity of soil bacteria, TP and N:P were not inhibition factors of bacterial diversity anymore. Plant roots don’t do any contribution in enhancing the soil bacterial diversity at any condition.4) Statistical differences among different treatments was assessed using multi-response permutation procedures (MRPP) in PC-ORD. Soil bacterial community composition significantly shift under N addition, which is mainly due to the change of carbon input rather than the variation of inorganic N concentrations or the acidification of soil. Generally, litter and plant roots jointly controlled soil bacterial community composition, showed an addition effect. Nitrogen deposition weakens the plant-microbe interactions, through the accumulation of litter still be a significant controlling factor of soil bacterial community composition.5) We examined shifts in the relative abundances of specific bacterial taxa. Acidobacteria, Verrucomicrobia, Firmicutes decreased, and α-Proteobacteria, Chloroflexi increased their relative abundance under N addition. The relationship between litter amount and the relative abundances of specific bacterial taxa generally show a liner model:The abundance of Actinobacteria decreased with the increase of litter inputs, while the abundance of Proteobacteria, Gemmatimonadetes, Bacteroidetes was a linear increase. Contrast to Verrucomicrobia, Firmicutes, and Nitrospirae, Actinobacteria is significant lower at the site with no root input.In conclusion, temperate has an extremely rich bacterial diversity, carbon input manipulation affected soil bacterial community significantly. Although the diversity of soil bacteria maintained by litter carbon input, soil bacterial communities was controlled by litter and root jointly; there was a weaker plant-microbe interaction under nitrogen deposition condition, through the accumulation of litter still be a significant controlling factor of soil bacterial community composition. |
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