Evaluation Of Microecological Effect Film And Screening Of Degrading Bacteria Of PE And PPC

Black soil is one of the most fertile soils in the world,with good properties,high fertility,and suitability for cultivation.However,the northeast black soil region is located in the semi-humid and semi-arid climate zone,with frequent seasonal drought,mainly in spring,which seriously affects spring sowing and crop emergence.At present,low temperature and lack of water are the main factors restricting the early growth and development of crops in Northeast China.Plastic film mulching can effectively alleviate spring drought in the Northeast black soil region by increasing temperature and retaining moisture,promoting growth and development,and increasing yield and income.Different types of plastic film have different effects on the soil environment.Polyethylene film is thin and fragile,and is difficult to degrade under natural conditions,remaining in the soil for a long time and causing"white pollution".Polypropylene carbonate is a new type of biodegradable plastic that can be degraded under natural conditions,and the residual film in the soil is easy to degrade and can reduce the negative impact on the soil environment,thus effectively improving and alleviating the problem of using polyethylene film pollution.This study was based on field experiments and included three treatments:（1）no mulching control（CK）;（2）polyethylene film mulching（PE）;and（3）polypropylene carbonate film mulching（PPC）.The effects of film mulching（PE and PPC）on the activity of C,N,and P cycling-related enzymes at different aggregates of black soil,as well as their ecological stoichiometry characteristics,microbial community structure and function were studied.In addition,plastic-degrading bacteria were screened,and the main research results are as follows:1.The response of soil enzyme activity to PE and PPC varies greatly among different aggregates.In the carbon cycle,compared with CK,the PE treatment reduced polyphenol oxidase（PPO）activity in 0.25-2 mm（MA）and<0.25 mm（MI）fractions（p<0.05）,by 73.2%and 26.3%,respectively,and significantly decreasedβ-1,4-glucosidase（βG）activity in>2 mm（ME）and MI fractions（p<0.05）,by 27.8%and 4.5%,respectively.The PPC significantly reduced PPO activity in ME by 26.7%（p<0.05）,while significantly increasedβG activity in MA by 34.5%（p<0.05）.In the nitrogen cycle,compared with CK,both PE and PPC treatments significantly decreased nitrogenase（NITS）and nitrous oxide reductase（NOS）activities in MA fraction（p<0.05）,while significantly increased key denitrification enzymes nitrite reductase（NIR）,nitrate reductase（NAR）,and nitric oxide reductase（NOR）in MA fraction（p<0.05）;the PE and PPC treatments significantly decreased N-1,4-acetylglucosamine glucosidase（NAG）activity in ME and MA（p<0.05）;the PE treatment significantly increased ammonia monooxygenase（AMO）and leucine aminopeptidase（LAP）activity in ME fraction（p<0.05）,by29.2%and 25.4%,respectively.In addition,compared with CK,both PE and PPC treatments significantly increased alkaline phosphatase（ALP）activity in soil MA and MI（p<0.05）.Therefore,plastic film mulching（PE and PPC）has consistent effects on PPO,but opposite effects onβG,with different effects on PPO in different aggregates.Plastic film mulching reduces soil nitrogen fixation fraction in MA,increases N₂O emission in MA,and promots organic phosphorus decomposition in MA and MI fractions.2.Studies on the ecological enzymatic stoichiometric characteristics show that compared to CK,both PE and PPC treatments significantly increased the ratioβG/（NAG+LAP）,βG/ALP,（NAG+LAP）/ALP,with a greater increase observed in the PPC treatment.Therefore,N and P cycles are restricted under both PE and PPC treatments,with a stronger restriction observed in the P cycle,especially under the PPC treatments.In addition,both PE and PPC treatments significantly increase the geometric mean of soil enzyme activity（GMea）in MA,with higher GMea observed in all aggregates fractions under PPC treatment than under PE treatment.The trend of GMea among different aggregates under the CK treatment is MI>ME≥MA,while the trend under both PE and PPC treatments is MI<ME<MA.Therefore,plastic film mulching enhances the overall soil enzyme activity capacity in MA,and replacing PE with PPC increases the overall soil enzyme activity capacity;plastic film mulching also changes the GMea trends among different aggregates fractions.3.Plastic film mulching may affect the structure and function of soil bacterial community.Compared with CK,the PE and PPC treatments significantly increased the abundance of beneficial Deinococcus by 9.2%（p<0.01）and 7.5%（p<0.05）,respectively.As compared to PE,the PPC treatment significantly increased the abundance of Tropicibacter by 20 times（p<0.01）,Aneurinibacillus by 5.3 times（p<0.01）,and Ohtaekwangia by 3 times（p<0.05）,which promote soil phosphorus cycling and nitrification.4.Six potential PE plastic-degrading bacterial strains were selected through enrichment culture experiments,including Enterobacter（PE1）,Enterobacter（PE2）,Chryseobacterium（PE3）,Bacillus velezensis（PE4）,Pseudomonas plecoglossicida（PE5）,and Bacillus subtilis（PE6）.A composite bacterial community was constructed using these strains.Based on the results of scanning electron microscopy（SEM）,bacterial growth OD values,and the degree of film fragmentation observed,single strains PE1,PE4,and PE5 as well as the optimal composite bacterial community PE1-PE2-PE4-PE5-PE6 were finally selected for their highest ability to degrade PE plastic film.In summary,plastic film mulching may change soil enzyme activity in different aggregats,and the type of plastic film can also affect its effect on enzyme activity in different aggregats.Plastic film mulching would limit soil nitrogen and phosphorus cycling,but increase the comprehensive ability of soil enzyme activity in MA particle size.Plastic film mulching would shift the microbial activity particle size from MI to MA mainly.Replacing PE with PPC may increase the abundance of beneficial bacterial genera and the comprehensive ability of soil enzyme activity,and promote soil nitrogen and phosphorus cycling.This study screened the optimal microbial group PE1-PE2-PE4-PE5-PE6,with good application potential for degrading PE films,and further attention is needed to address the negative environmental effects of plastic film mulching.