| Plastic mulching film is an important agricultural material which could increase soil temperature and conserve soil moisture,thereby improving crop yield.Low density polyethylene plastic film has been widely used due to its low price,high transmittance,and high ductility.However,the ‘white pollution’ caused by the plastic film has seriously harmed the ecological environment of farmland surroundings and soil health which is intensively concerned recently.Although there are many studies focusing on plastic residues and its effects on soil properties,the research working on small particles such as microplastics generated from weathering and aging by mechanical wear,is still ongoing,especially the insufficient concerns working on microplastic behaviors in soil-vegetation system.Therefore,based on field investigation,this study conducted a pot experiment under field condition and investigated soybean growth,soil properties,as well as rhizosphere microbial community characteristics in treatments with different levels of low-density polyethylene film fragments(PE,0.5 cm×0.5 cm and 1 cm×1 cm)and microplastic particles(MP,150～500 μm)added in black soil.The main findings are as follows.(1)Plastic contamination strongly affected soybean growth(p<0.05).Compared with the control(CK),high level of microplastics inhibited plant height,plant diameter,root length and root area,reduced biomass,but yield slightly increased without any significant difference.The C/N/P contents in soybean leaf and stem showed that in treatments with low and medium levels of microplastics their contents were boosted while inhibited in the treatments with high level of microplastics.The C/N/P content in soybean root decreased with the increase of microplastic level.However,in the treatments with plastic fragments addition,biomass increased with the increase of plastic fragments level,plant height and diameter were smaller than that in CK,and the yield decreased slightly without any significant difference.The C/N/P contents in leaf stem and root of soybean decreased with the increase of PE concentration,while the C/N/P contents in biomass increased with the increase of PE level.In different sampling periods,the plant height,plant diameter,root length,root area biomass and root C/N/P contents of soybean in treatments with microplastics at harvesting stage were higher than those at flowering stage.Unlikely,in the treatments with plastic fragments,plant height,plant diameter,root length,root area,root biomass and root C/N contents of soybean at harvesting stage were higher than those at flowering stage,while aboveground biomass and root P content were lower than those at flowering stage.(2)The effects of plastic accumulation characteristics on soil p H,soil aggregates and soil C/N/P varied significantly.In different microplastics’ treatments,mechanical stable aggregates showed that the contents of >2 mm and <0.25 mm aggregates increased with the increase of microplastics concentration.Water-stable aggregates showed that the content of >2mm aggregates decreased with the increase of microplastic concentration,while the content of <0.25 mm aggregates increased with the increase of microplastic concentration(except in the treatment with 1% MP).The fractal dimension(FD)of the mechanical stable aggregates did not change with the microplastics concentration,while the FD of the water-stable aggregates increased with the microplastics concentration.To be contrast,in the treatments with plastic fragments,mechanical stability and water stability of aggregates showed that >2mm and <0.25 mm aggregates increased with the increase of PE concentration.The FD of mechanical stable aggregates increased with the increase of PE concentration,while the FD of water-stable aggregates did not change with the increase of PE concentration.Soil p H in treatments with plastic addition was higher than that in initial soil,and the p H in treatments with plastic contamination was lower at harvesting stage than that at flowering stage.Soil p H in treatments with PE was higher than that in treatments with MP,and the value decreased with the increase of PE concentration.Compared with CK,high concentration of microplastics significantly increased soil organic carbon,significantly decreased soil total nitrogen(p<0.05),and nitrate and ammonia nitrogen contents while the total phosphorus content did not change.Accordingly,soil organic carbon,total nitrogen and water-soluble carbon increased with the increase of PE concentration;the total phosphorus content slightly changed;and the available phosphorus of ammonium nitrogen decreased with the increase of PE concentration.In different sampling periods,soil organic carbon content,nitrate,and ammonia nitrogen contents at harvesting stage of microplastic treatments were higher than those at flowering stage,while the total nitrogen content,total phosphorus content,water soluble organic carbon and available phosphorus content were lower than those at flowering stage.However,in the treatments with plastic fragments,the soil organic carbon,total nitrogen,nitrate and ammonia nitrogen in the harvesting stage were higher than those in the flowering stage,and total phosphorus,water soluble organic carbon,available phosphorus were lower than those in the flowering stage.(3)The effects of plastic contamination on non-rhizosphere soil differed significantly(p<0.05).Compared with CK,high concentration of microplastics significantly promoted the activity of C-/N-/P-source enzymes(p<0.05),but significantly inhibited the microbial biomass C,N and P(p<0.05).However,the activity of C-/N-/P-source enzymes and microbial biomass nitrogen increased with the increase of PE concentration,while the microbial biomass carbon and phosphorus decreased with the increase of PE concentration.In different sampling periods,the activities of C-/N-/P-source enzymes and microbial biomass P at harvesting stage were lower than those at flowering stage,while microbial biomass C and N were higher than those at flowering stage.Accordingly,in treatments with plastic fragments,the activities of C-/N-/P-source enzymes and microbial biomass nitrogen and phosphorus were lower than those at flowering stage,while microbial biomass carbon was higher than that at flowering stage.(4)Plastic contamination in soil significantly altered microbial communities in rhizosphere soil(p<0.05).Compared with CK,C-/N-/P-source enzymes activities and microbial α-diversity in treatment with high level of microplastics were significantly changed(p<0.05).Principal coordinate analysis(PCo A)showed that β-diversity of microbial community in treatment with high level of microplastics was significantly different from CK(p<0.05).Meanwhile,Proteobacteria and Ascomycota were the dominant groups of bacteria and fungi,respectively in those treatments.However,Compared with CK,activities of N-/Psource enzymes were significantly inhibited in different treatments(p<0.05),and the diversity of C-source enzymes and microbial α-diversity varied.PCo A showed that β-diversity of microorganisms in agricultural film residue treatment was significantly different from in CK treatment(p<0.05).Proteobacteria and Ascomycota were the dominant groups of bacteria and fungi respectively.In different sampling periods,the activity of C-/N-/P-source enzymes in treatments with microplastics harvesting stage was higher than that at flowering stage at high concentration,while the diversity of bacteria was higher than that at flowering stage,while the diversity of fungi was lower than that at flowering stage.Proteobacteria and Ascomycota were higher than at flowering stage in low concentration of microplastics,while high concentration of microplastics was lower than that at flowering stage.Accordingly,the activity of C-/N-/Psource enzymes,bacterial diversity and fungal diversity of agricultural film residues at harvesting were higher than those at flowering stage,but lower than those at flowering stage.The number of Proteobacteria was higher than that of flowering stage,and the number of Ascomycota at 0.1% concentration was higher than that of flowering stage,while that of other treatments were higher than that of flowering stage.In conclusion,soil(micro)plastic characteristics have significant effects on soil aggregates,p H,C/N/P and microbial communities.Microplastic treatments inhibited the growth of soybean,but increased yield,destroyed soil aggregates,reduced soil p H,increased soil organic carbon content,reduced soil phosphorus content,increased non-rhizosphere soil C-/N-/P-source enzymes activity,inhibited microbial biomass carbon,nitrogen and phosphorus,and changed the rhizosphere soil microbial diversity.However,in the treatments with plastic fragments,it promoted the growth of soybean,reduced yield,and did little damage to soybean aggregates,reduced soil p H,increased soil C/N content,reduced soil P content,increased non-rhizosphere soil C-/N-/P-source enzymes activity,reduced microbial biomass nitrogen,reduced microbial biomass C/P,and changed the rhizosphere soil microbial diversity.It can be seen from the above results that there is a great difference between the cumulative pollution of microplastics and debris in soil.The reason may lie in the particle characteristics of microplastics themselves,which aging process in soil is relatively slow,and thus have relatively little effect on plant growth in the short term.However,agricultural film residue may be continuously fragmented and granulated under the interpenetration of soil particles and crop roots,and release harmful substances such as plasticizers or additives,which have negative effects on plant growth and yield.Therefore,the characteristics of agricultural film residue have more influence on soil-plant system than microplastic characteristics,while the aging-degradation mechanism of microplastics in soil remains is unclear.The effects of granulation process on plant soil properties and microbial feed-back remain to be studied.In the future,the effects of microplastics on soil function and health should be further studied.so as to provide a data basis for microplastics pollution in farmland soil and soil health evaluation. |
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