| Grazing is one of the main ways of grassland management and utilization. It not only has a great influnces on the vegetation distribution, but also on soil microbial community structure. Therefore, soil microorganisms of grassland are generally taken as one of the important indicators during evaluation of grassland ecosystem health. In this paper, the response of grazing intensity by sheep on soil bacteria and fungi was studied in a arid natural grassland. The experiment was conducted at experiment site of Lanzhou University in Tianshuipu of Huan county in Gansu province. Grazing intensity was simulated by feeding different number of sheeps(0, 2.7, 5.3 and 8.7 sheep/hm2) with four grazing intensities. Soil samples were taken in May, July, September and November 2012, respectively. The clone library and 454-high throughput sequencing method were used to test and analyze the diversity of soil bacterial and fungal as well as communitiy structure under four grazing intensities. Additionally, physical and chemical properties of soil were measured. The main results were as follow:1. Grazing intensity has great influence on soil physical and chemical properties. Soil organic matter content was ranged from 8.09 to 14.09 g/kg under different grazing treatments, and that of grazing grassland was significantly higher than control(P<0.05) while no differece was found among treatments with grazed treatments. Total nitrogen content of soil was ranged from 6.55 to 14.83 mg/kg. Control and treatment of 2.7 sheep/hm2, 5.3 sheep/hm2 were significantly higher than 8.7 sheep/hm2(P<0.05). Soil total phosphorus, nitrate nitrogen and ammonium nitrogen content were 2.52-3.67 mg/kg, 0.02-1.19 mg/kg, 2.42-4.47 mg/kg, respectively, and no significant effects was observed. Soil p H values were changed from 6.98 to 7.25, although no significant differences between treatments, with the increment in grazing intensity, p H value showed an increasing trend.2. Grazing intensity has different degree of influnce on bacterial diversity, abundance and community structure. Under control and treatment of 2.7 sheep/hm2, 5.3 sheep/hm2, 8.7 sheep/hm2, the diversity index of shannon in the soil was 7.60, 7.72, 7.75, 7.70. And simpson index was 0.0012, 0.0008, 0.0008, 0.0012, respectively. In terms of soil bacteria abundance index of operational taxonomic units(OTUs), 3467, 3548, 3698, 3548 were measured for the four treatments, and Chao value were 7346, 7656, 8477, 7656, respectively. The results showed that bacterial diversity and abundance of grazing treatment were higher than control, the highest value appeared at 5.3 sheep/hm2.Study on bacterial community structure suggested that 7-9 predominant phyla were detected under four grazing treatments. The six phyla Actinobacteria, Proteobacteria, Acidobacteria, Gemmatimonadetes, Chloroflexi and Bacteroidetes were shared among all treatments. The difference of relative abundance of bacterial was changed between 0.61% to 3.57%. Relative small influnce was found on bacterial components and relative abandance. The phyla of unique bacteria were appeared at all treatments except 2.7 sheep/hm2 site. Control contained 4 special bacterial phyla(Fusobacteria, Spirochaetes, Tenericutes, Synergistetes); 5.3 sheep sheep/hm2 and 8.7 / hm2 treatments contained Aquificae and Tenericutes, respectively.Compared with 454- high throughput sequencing, less meaningful information was found at 16 S r DNA clone library method, therefore, it is hard to comprehensively and accurately analyze soil microorganisms.3. Grazing intensity has influnce on fungal diversity, abundance and community structure. Under the control and treatments of 2.7 sheep/hm2, 5.3 sheep/hm2, 8.7 sheep/hm2, shannon index was 2.86, 5.06, 4.38, 2.86, Simpson index was 0.19, 0.02, 0.08, 0.19, OTUs value was 572, 868, 708, 868, Chao value was 930, 1324, 1143, 1324, respectively. The results indicated that both soil fungal diversity and abundance were higher than control. The highest value appeared at the treatment of 2.7 sheep/hm2.The result of fungal community structure analysis showed that 3 predominant and shared fungi phyla(Basidiomycota, Ascomycota, Glomeromycota) were detected among all treatents. A special bacterial phyla(Neocallimastigomycota) was found. The difference of relative abandance of fungi phyla under different treatments was changed from 4.45 to 19.04%, indicating grazing has small influnce on soil fungi phyla while a significant effect on relative abandance.4. Season have an influnce on both diversity and relative abundance of soil bacteria and fungi. The results showed that in May, July, September and November of soil bacteria shannon index was 7.67, 7.68, 7.76, 7.67, Simpson index was 0.0010, 0.0008, 0.0007, 0.0010, the highest value appeared in September. In terms of soil fungi, shannon index was 3.09, 4.11, 4.28, 3.09, Simpson index was 0.24, 0.11, 0.13, 0.24, respectively from May to Novermber, and the highest value was witnessed in November. Among sampling months, difference of relative abundance of soil of bacterial phyla varied from 5.15 to 23.66%, while that of soil fungi phyla changed from 19.54 to 19.54%.5. Soil physical and chemical properties were closely related to community structure and relative abundance of soil bacteria and fungi phyla. Redundancy analysis showed that p H value and nitrate nitrogen have a great impact on soil bacterial composition and distribution. Positive relationship of relative abundance and total phosphorus were found at Aidobacteria, Spirochaeta, Fusobacterium, Firmicutes, Tenericute, Bacteroidetes and Chlorobi. Positive relatiship of total nitrogen, organic matter and grazing density observed at Elusimicrobia, Verrucomicrobia, Fibrobacteres, Proteobacteria, Gemmatimonadetes, Cyanobacteria and Planctomycetes. Another positive relationship of nitrate nitrogen and ammonium nitrogen monitored at Actinomycetes, Bacteroidetes, Caldiserica, Chlamydiae, Euryarchaeota and Chloroflexi. However, organic matter, total phosphorus and ammonium nitrogen played an important role for soil fungal composition and distribution. Positive relationship of totaol nitrogen and ammonium nitrogen appeared at Cryptomycota. Positive relationship with nitrate measured at Basidiomycete and Glomeromycota. Positive relationship of organic matter, total phosphorus and p H value observed at Neocallimastigomycota. |
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