| Polylactic acid(PLA)is the most productive bioplastic in the world,which has been used in human production and life instead of some traditional petroleum-based plastics.However,the effects of polylactic acid plastic on soil plants and microorganisms remains unclear.Therefore,this study selected two commercially available polylactic acid plastics as the research object,namely,PLA plastic from lunch box(main components of PLA)and PT plastic from handbag(main components of PLA and PBAT).Through pot experiment and16 S r RNA high-throughput sequencing technology.The effects of polylactic acid plastics(PLA and PT)and traditional plastics(PE and PP)on plant growth and rhizosphere microbial community were investigated.The " plastisphere " bacteria of polylactic acid were isolated by traditional culturable method,and the culture system with polylactic acid plastic as the only carbon source was established to screen the efficient degrading bacteria of polylactic acid plastic.The degradation performance of highly effective degrading bacteria in soil environment and its effect on plant growth were investigated,providing theoretical basis and practical basis for alleviating "white pollution".The main results of this study are as follows:(1)The effect of plastic on plant growth is related to the type of plastic,material and plant type.It was found that PLA plastic could increase the tillering number of wheat and increase the above-ground dry weight of tomato by 16.3% through the growth indexes of plants in pot experiment.PT plastics could reduce the dry weight of tomatoes above ground by 9.6% and below ground by 16.1%.Compared with traditional plastics,substances produced after degradation of bioplastics(PT)may have a greater negative impact on plant biomass.However,traditional plastics are not easy to degrade and may increase soil porosity and water content in the short term,thus promoting plant growth to a certain extent.(2)Polylactic acid(PLA)plastics had significant effects on the diversity and richness of microbial communities in plant rhizosphere soil.PLA plastics have a significant effect on Proteobacteria and Actinobacteriota.Adding PLA plastics increased the relative abundance of Actinobacteriota in wheat rhizosphere soil by 36.5%.Proteobacteria decreased by 22.6%.The addition of PT plastics increased the relative abundance of Actinobacteriota in wheat rhizosphere soil by 3.4%,and Proteobacteria decreased by 5.5%.The addition of PLA plastics reduced the relative abundance of Actinobacteriota by 0.5%,the addition of PT plastics reduced the relative abundance of Actinobacteriota by 8.0%and the addition of Proteobacteria by 6.5%.LEf Se analysis showed that the evolutionary distance of different species in wheat rhizosphere microorganisms under polylactic acid plastic treatment was far from that of traditional plastics in the control group,and the species of different species in tomato rhizosphere microorganisms was less than that of traditional plastics.Under PLA plastic treatment,Actinobacteriota was significantly enriched in wheat rhizosphere soil.Chloroflexi was significantly enriched in tomato rhizosphere soil,polylactic acid plastics have a significant effect on Proteobacteria and Actinobacteriota.Adding PLA plastics increased the relative abundance of Actinobacteriota in wheat rhizosphere soil by 36.5%.Proteobacteria decreased by 22.6%.The addition of PT plastics increased the relative abundance of Actinobacteriota in wheat rhizosphere soil by 3.4%,and Proteobacteria decreased by 5.5%.The addition of PLA plastics reduced the relative abundance of Actinobacteriota by 0.5%,the addition of PT plastics reduced the relative abundance of Actinobacteriota by 8.0%and the addition of Proteobacteria by 6.5%.LEf Se analysis showed that the evolutionary distance of different species in wheat rhizosphere microorganisms under polylactic acid plastic treatment was far from that of traditional plastics in the control group,and the species of different species in tomato rhizosphere microorganisms was less than that of traditional plastics.Under PLA plastic treatment,Actinobacteriota was significantly enriched in wheat rhizosphere soil.Chloroflexi was significantly enriched in tomato rhizosphere soil.(3)A total of 19 strains of bacteria were isolated from the "plastic sphere" niche by enrichment culture,among which 5 strains were highly degradable.The 19 isolates were belonging to Bacillus,Chungangia,Streptomyces,Cytobacillus,Kocuria,Brevibacterium,Microbacterium,Arthrobacter,Janibacter and Georgenia.By establishing a culture system with polylactic acid plastics as the only carbon source,the weight loss rate,SEM and FT-IR of degraded plastics were measured.After comprehensive analysis,5 strains of efficient degrading bacteria of polylactic acid plastics were screened out.They were L1(Pseudomona aeruginosa),L5(Streptomyces exfoliatus),P7(Bacillus zanthoxyli),W1(Kocuria salina),and W2(Kocuria assamensis).L5,P7 and W2 could reduce the weight loss of PLA plastics by 4.8%,4.0% and 3.7% within 8 weeks,respectively.L1,L5 and W1 could reduce the weight of PT plastic by 5.9%,5.1% and 5.0% within 8 weeks,respectively.SEM and FT-IR tests showed that the surface roughness of plastics in L1,L5,P7,W1 and W2 treatment groups increased,the size and number of holes and cracks formed were large,and the characteristic peaks of the samples’ oxygen-containing functional groups and unsaturated bonds changed,and hydroxyl functional groups were introduced.(4)Five strains of highly degradable bacteria could accelerate the degradation of polylactic acid plastics in soil without harming plant growth.SEM and FT-IR results showed that the microstructure and functional groups of the surface of polylactic acid plastics were changed after 30 days of application of highly degradable bacteria in soil,and all five strains of bacteria had the ability to degrade polylactic acid plastics in soil.It was found that Bacillus zanthoxyli P7 strain did not have negative effects on wheat growth,and Bacillus zanthoxyli P7 strain had growth-promoting ability.It can increase the dry weight of wheat seedling by 25.8%.The results of this study enriched the database of the effects of plastic pollutants on plants and microorganisms,and provided theoretical and practical basis for the actual mitigation of polylactic acid plastic residual pollution in soil environment. |
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