| Featured by fast growth, growing into useful timber earlier, high yield, and easy to update, poplar is one of China’s important plantation cultivation tree species. In China, scientific cultivation and cultivation technology of poplar has been paid great attention by forestry industry. Poplar cultivation area has been more than 100 million mu in 2015, and ranked the first in the world. Poplar forest plantations embodiy continuous cropping obstacles and land capability decline at different levels, such as growing stock of stand declining generation by generation, severe plant diseases and insect pests, declining survival rate of afforestation in long-term continuous cropping operation mode. As study of chemical action of rhizosphere deepens increasingly and root exudate allelopathic effect reveals gradually, autotoxicity of rhizosphere gradually becomes the emphasis and hotspot of study of land capability decline mechanism of continuous cropping plantation. With poplar within Dashahe Forest Farm in Feng County, Xuzhou as the object, it explores the change of number and structure of community of bacterial and fungal populations of rhizosphere soil in multi-generation continuous planting of poplar forest from the view angle of autotoxicity, effect of aqueous extracts of rhizosphere soil of continuous cropping poplar plantation on lettuce seed germination and water planting of poplar cuttings. The study shows that: 1. Detect change of number of bacterium and fungus of rhizosphere soil of poplar plantation with real-time fluorescence quantification PCR(assume that each bacteria contains one 16 S r RNA gene copy, and each fungus contains one ITS r RNA gene copy). In rhizosphere soil of Generation I forest, there are 2.05×106~5.59×106 16 S r RNA gene copy in each gram of dry soil with the average of 4.18×106, and 3.33×103~9.73×103 ITS r RNA gene copy with the average of 6.79×103; in rhizosphere soil of Generation II forest, there are 3.37×106~7.53×106 16 S r RNA gene copy in each gram of dry soil with the average of 6.09×106 and 5.36×103~1.72×104 ITS r RNA gene copy with the average of 1.14×104; in rhizosphere soil of Generation III forest, there are 2.02×107~4.57×107 16 S r RNA gene copy with the average of 3.52×107 and 6.14×103~1.82×104 ITS r RNA gene copy with the average of 1.19×104. It shows that as the generation number of poplar continuous cropping increases, the number of bacterium and fungus populations increases significantly. However, the increase trend of the two is different. Bacterium and fungus of rhizosphere increase the most significantly in Generation III and I forest stage respectively. Increase of number of microorganism may be in that continuously cropped rhizosphere soil microorganism abstracts a lot of autotoxins and nitrate nitrogen gathering around the rhizosphere. 2. Detect the population structure of rhizosphere soil microorganism in poplar plantation with DGGE band analysis, DNA extract and PCR-DGGE analysis method. In respect of bacterial communities, the similarity between bacterial community of rhizosphere soil of Generation I forest and that of bare soil is about 91%; the similarity between bacterial community ofrhizosphere soil of Generation II and I forest is about 46%; the similarity between bacterial community of rhizosphere soil of Generation III and II forest is about 52%. While in respect of fungal communities, the similarity between fungal communities of rhizosphere soil of Generation I forest and that of bare soil is about 86%; the similarity between fungal community of rhizosphere soil of Generation II and I forest is about 52%; and the similarity between fungal community of rhizosphere soil of Generation III and II forest is about 40%. It indicates that under continuous cropping condition, change of population structure and number of rhizosphere soil microorganism in poplar plantation is non-synchronous. Population structures of rhizosphere bacterium and fungus change the most in rhizosphere Generation II forest stage, it may be in that the rhizosphere secretion and autotoxins selectively stimulate and inhibit bacterium and fungus in soil. 3. The sequencing results obtained by means of the comparative analysis to the bacterial community structure of rhizosphere soil of Generation II forest, which changes the most significantly as the generation number increases and bacterial community species of controlled bare soil with sequencing and phylogeny analysis methods show that in terms of the phylum level, the dominant bacterial communities of controlled bare soil is Acidobacteria, deltaproteobacteria and Betaproteobacteria; and the dominant bacterial communities of rhizosphere soil of Generation II forest are Acidobacteria, Gammaproteobacteria and Betaproteobacteria. The two are different in bacteria components, mainly embodying in that percentage of Gammaproteobacteria, Alphaproteob-acteria and Actinobacteria of rhizosphere soil of Generation II forest increased by 10.92%, 7.38% and 5.46% respectively than those of controlled bare soil, while the percentage of Acidobacteria decreased by 18.38%. 4. HPLC is employed to detect eight phenolic acids in the rhizosphere soil including hydroxybenzoic acid, vanillic acid, vanillin, ferulic acid, isovanillin, benzoic acid, cinnamic acid and coumarin. A cumulative rule is applicable for the aforesaid eight phenolic acids. With the increase of the number of generations, the accumulation rate does not show a trend of complete linear growth, but a trend of decrease appears. The soil is characterized in autotoxicity with the aforesaid eight phenolic acids, which can inhibit the activity of urease, sucrase and alkaline phosphatase; in addition, the inhibiting effect is strengthened with the increase of concentration and the activity is strengthened with increase of concentration of catalase, but the activity of polyphenol oxidase is not significantly influenced by the concentration of phenolic acid. Phenolic acids can produce obvious autotoxicity for the edaphon, which makes fungi increased with the increase of concentration and can strongly inhibit the growth of bacteria. 5. The analysis result obtained by means of bioassay shows that aqueous extracts of rhizosphere soil of continuous cropping poplar plantation has a significant inhibition action on lettuce seed germination, its not that the aqueous extracts of rhizosphere soil has no significant inhabitation effect on the lettuce seed germination. Compared with the germination rage of 84.5% of the pure water control group, the lettuce seed germination rage of the rhizosphere soil of Generation I, II and III forest decreases significantly with the maximum germination rage of no more than 60%, especially that the germination rage of Generation III forest is only 48.7%,reaching the extremely significant inhibition level(P<0.01). As mass concentration of the extract of rhizosphere soil increases, lettuce seed germination inhibition rate increases. It is able to promote the germination rate increasing when the generation II and III forest is at a low concentration of 0.1g/m L(the germination rage is higher than the control group). While the inhibition trend of the three generations of forest are different: the inhibition rate increasing range of Generation I forest is the smallest as the concentration increases; Generation II forest approaches to linear trend; the inhibition rate increasing range of Generation III forest is the largest as the concentration increases. Within the concentration of 0.2g/m L-0.5g/m L, the germination inhibition rates of the three generations of forest are the most approximate. 6. The analysis result obtained by means of bioassay shows that extract of rhizosphere soil of continuous cropping poplar plantation has a significant effect on the growth of water planted poplar cuttings. As the generation number increases, the poplar germ growing length and root extension length decrease. Extract of rhizosphere soil of Generation I forest has no significant difference; while the difference of Generation II and III forest is not affected by nutrition condition, and growing length and root extension length of germ in extract of rhizosphere soil of Generation III forest significantly decreased than Generation II forest. As the mass concentration of the extract of rhizosphere soil increases, germ growing length and root extension length gradually decrease, the inhibition rate gradually increase with the inhibition rate becoming negative when it comes to the minimum concentration of 0.1g?m L-1. Compared with the control group, the soil extract has a promotion role on the germ growing length and root extension length. The germ growing length is 4.88 cm when the soil solution concentration is 0.5g·m L-1, which is significantly different from those when the solution concentration is 0.1g·m L-1, 0.2g·m L-1 and 1g·m L-1, but has no significant difference when the concentration is 0.8g·m L-1. The germ growing inhibition rate is 14.7% when the soil solution is 0.5g·m L-1, significantly different from those when the concentration is 0.1g·m L-1, 0.2g·m L-1 and 1g·m L-1, but has no significant difference when the concentration is 0.8g·m L-1. The root extension length is 4.12 cm when the soil solution concentration is, which is significantly different from those when the solution concentration is 0.1g·m L-1, 0.2g·m L-1 and 1g·m L-1, but has no significant difference when the concentration is 0.5g·m L-1. The root extension length is 39.2% when the soil solution is 0.8g·m L-1, significantly different from those when the concentration is 0.1g·m L-1, 0.2g·m L-1 and 1g·m L-1, but has no significant difference when the concentration is 0.5g·m L-1. 7. Poplar plantation will generate autotoxicity after continuous cropping of 30 to 40 years, and continuous cropping obstacle will occur. Therefore, it is suggested to change the planting method with intercropping orcrop rotation method after continuous cropping of poplar plantation of 30 years to improve the environment of rhizosphere soil of microorganism, and reduce the occurrence of continuous cropping obstacles. |
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