| Mulch technology will remain an important tool to ensure a stable agricultural harvest in China for a longer period of time to come.Under the action of light,temperature,water,air and biological factors,the residual mulch residue left in the soil will gradually crack into small fragments or macromolecules until it is completely decomposed.This thesis uses the root cellar experimental equipment(referred to as root cellar)and organic matter composting experimental equipment(referred to as composting unit)as the research platform,based on the foundation of the supervisor’s research team(started from 2010 to present),to carry out research on the separation and extraction,characterization and physicochemical properties of polyethylene mulch residue microplastics in different soils under natural conditions and under composting conditions,based on the soil cultivation environment treated under natural conditions for twelve consecutive years We analyzed the effects of polyethylene film residue microplastics on soil ecology under natural conditions and revealed the changes in the physicochemical properties and soil microenvironmental effects of film microplastics in soils.The main research results are as follows:(1)Establishment and optimization of a new method for the extraction of mulch microplastics in soil: By comparing the sieve flotation method,Soxhlet extraction method and density separation method,the results showed that the technical extraction rate of the sieve flotation method for the separation of mulch microplastics could reach 74.08%～95.58%,the extraction rate of the density separation method for the separation of mulch microplastics could reach58.91%～89.60%,and the extraction rate of the Soxhlet extraction method for the separation of mulch microplastics The extraction rate of plastic was only 41.13%～86.10%.Scanning electron microscopy(SEM)observation showed that the surface of the mulch microplastics separated by the Soxhlet extraction method had fine and tight fish-scale texture,flakes and uneven structure,and the surface of the mulch microplastics separated by the density separation method had a large number of micro particles attached,while the surface of the mulch microplastics separated by the sieve flotation method contained only a small amount of impurity particles,and most of the surface was the same as the original sample,and the surface microscopic shape of the mulch microplastics was not was not destroyed.The sieve flotation method was superior to the density separation method and the soxhlet extraction method in terms of extraction rate and retention of the microfine interfacial properties of different types of mulch microplastics in different soils.In the optimization of the sieve flotation method,it was found that the optimum steel sieve aperture size was 30 μm(500 mesh)for LDPE,and the optimum ethanol concentration was 100% to remove high-density impurities and 55% to remove low-density impurities;the optimum steel sieve aperture size was 43 μm(325 mesh)for HDPE,and the optimum ethanol concentration was 100% to remove high-density impurities and 60% to remove low-density impurities;the optimum steel sieve aperture size was 43 μm(325 mesh)for LLDPE,and the optimum ethanol concentration was 43 μm to remove high-density impurities and 60% to remove low-density impurities.The optimum steel sieve aperture is 43 μm(325 mesh),and the optimum ethanol concentration is 100% to remove impurities of high density and 70% to remove impurities of low density.(2)Analysis of physicochemical properties of polyethylene film residues in different soil environments: sieve flotation method has no extracts for Z1(shallow pit,sandy soil,LDPE(low density polyethylene film residues),1-year accumulation,same below)and Z8(deep pit,loamy soil,LDPE,100-year accumulation,same below),Z2(shallow pit,loamy soil,HDPE(high density polyethylene film residues),50-year accumulation,same below)The extraction rates of Z3(shallow pit,clay,LLDPE(linear LDPE film residue,100-year accumulation,same below),Z5(medium pit,loamy soil,LLDPE,1-year accumulation,same below),Z5(medium pit,loamy soil,LLDPE,1-year accumulation,same below),Z7(deep pit,sandy soil,LLDPE,50-year accumulation,same below),Z9(deep pit,clay,HDPE,1-year accumulation,same below)were 66.15%,61.33%,64.56%,respectively.The mean molecular weight of HDPE extracts decreased by 63.27%,the mean heavy molecular weight decreased by 69.84%,and the PD value increased from 1.6134 to 5.1171;the mean molecular weight of LLDPE extracts decreased by 97.33%,the mean heavy molecular weight decreased by 93.33%,and the PD value increased from 1.6134 to 5.1171.A new absorption peak was observed at1800-1600 cm-1 for each treatment extract,1300-1150 cm-1 for LDPE residue,and 1400-900cm-1 for HDPE and LLDPE residue.The increase of absorption peak area was observed for HDPE and LLDPE residues at 1400-900 cm-1.Thermogravimetric analysis of the extracts of each treatment revealed that the pyrolysis temperature of the polyethylene film residues extracted from the treatment group decreased by 3-7 ℃ compared to CK.The extracts of each treatment either had rough surfaces with different degrees of pores and gullies appearing,or flaky structures that were about to fall off,or had uneven surfaces with honeycomb-like pores appearing and structures similar to those formed by wind erosion.The strength of degradation of polyethylene film residues in soil is influenced by soil type and polyethylene film residue type factors,sandy soil > loamy soil > clay,LDPE > HDPE > LLDPE,change in molecular structure of polyethylene film residues,formation of new oxygen-containing functional groups or other groups,and change in surface of polyethylene film residues,indicating degradation of polyethylene film residues in soil.(3)The extraction rates of common mulch residue in compost by sieve flotation method ranged from 30.78% to 48.27%,and those of photothermal degraded mulch residue ranged from 20.94%to 32.90%.Under the same residue accumulation,the number average molecular weight and heavy average molecular weight of photothermal degraded mulch residue were lower,and the molecular weight distribution of photothermal degraded mulch residue was narrower.A new absorption peak appeared at 1800-1600 cm-1 for both photothermally degraded film residues and common film residues;the absorption peak area increased at 1400-900 cm-1;and the peak values decreased significantly at 1500-1300 cm-1 and 750-720 cm-1.The pyrolysis temperature of photothermally degraded mulch residue was reduced by 9-10 ℃ compared with the control;the pyrolysis temperature of common mulch residue was reduced by 6-7 ℃ compared with the control.The surface of ordinary mulch residue either showed holes and cracks,or cracks and stripes on the surface;the surface of photo-thermal degraded mulch residue either showed uneven pits and small holes,or larger holes,and the erosion was more serious.Both regular and photothermal film residues degrade in soil,and the addition of photothermal degradation masterbatch to photothermal film residues accelerates the degradation process of film residues.(4)The lowest mean urease activity was 161.78 μg/(g-d)in the Z4 treatment group,the highest mean urease activity was 228.21 μg/(g-d)in the Z3 treatment group,soil type had a highly significant effect(P < 0.01)on urease activity,and the effect of each factor on urease activity was: soil type > soil depth > mulch residue type > mulch residue accumulation.the Z1 treatment The lowest mean phosphatase activity was 285.05 μg/(g-d)in the Z1 treatment group,the highest mean phosphatase activity was 415.26 μg/(g-d)in the Z8 treatment group,soil type had a highly significant effect on phosphatase activity(P < 0.01),and the effect of each factor on phosphatase activity was: soil type > mulch residue accumulation > soil depth > mulch residue type.The lowest mean enzyme activity of sucrase was 200.78 μg/(g-d)in Z1 treatment group,the highest mean enzyme activity of sucrase was 340.53 μg/(g-d)in Z2 treatment group,soil type had a highly significant effect on sucrase activity(P < 0.01),soil depth and type of mulch residue had a significant effect on sucrase activity(P < 0.05),and the effects of various factors on sucrase activity were: soil type > soil depth > mulch residue accumulation > mulch residue type.The effects of each factor on sucrase activity were: soil type > soil depth > type of mulch residue > mulch residue accumulation.Soil type was the main factor influencing enzyme activity in soil,with polyethylene type and polyethylene accumulation contributing much less to changes in enzyme activity than soil type. |
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