Study On The Mechanism Of The Influence Of Residual Plastic Film In Cotton Field On Soil Environment And Crop Growth

Agriculture in arid and semi-arid countries has benefited from irrigation with plastic film mulching,but the plastic film that remains in the soil has become "white pollution." The presence of residual film disrupts the original structure and environment of the soil,which has a negative impact on crop growth.This thesis takes the residual plastic film in cotton fields in Xinjiang as the research object.The residual amount of plastic film in different years and soil depths is obtained through fixed-point observation and regional investigation,and the spatial distribution characteristics of large-scale residual plastic film pollution are evaluated.The impacts of residual film on soil water transport were explored by assessing horizontal diffusion,wetting front migration,cumulative infiltration,and cumulative evaporation using a combination of the indoor soil column and soil box simulation and field experiment;Through physical and chemical experiments,high-throughput sequencing,scanning electron microscopy,Fourier transform spectroscopy and gas chromatography,and mass spectrometry,the effects of residual film on soil salinity,nutrients,and microorganisms as well as the accumulation characteristics of heavy metals and phthalates were investigated.The mechanism of influence of residual film on soil physical and chemical properties and soil environment was evaluated.The main conclusions are as follows:(1)The spatial and temporal distribution and accumulation patterns of residual mulch film and microplastics were elucidated.For five years in a row,the density of mulch film residue in cotton fields has been above the threshold for mulch film residue on farmland and the threshold for residual film in the measuring standard(75 kg/ha).Currently,the mulch film in the research region is decomposing at a rate of 15.002 kg/ha and 148,250pieces/ha.From the perspective of residual film quality,as the age of lamination increases,the mass ratio of each layer between 0-30 cm tends to become increasingly consistent.The new residual film resides mainly in the 0-20 cm soil layer,whereas the residual film produced in previous years progressively degrades and sinks into the earth’s depths.Zero to twenty-five square centimeter residual film accounts for more than85% of the total residual films.With an increase in the number of years of mulching,the residual film exhibits a geographic variation of migration to deep soil and cumulative features of progressive fragmentation and concentration in a 30 cm soil layer.The abundance value of microplastics increases with the age of lamination.Under the same mulching age,the 10-20 cm soil layer had the highest abundance value.In the studied region,film-based microplastics constituted the biggest fraction.Microplastic particle size decreases with increasing film age and soil depth.With the increase of lamination age,the proportion of small particle size microplastics smaller than 0.5 mm increased significantly.(2)The effects of residual film on soil water,salt,nutrients,microorganisms,heavy metals,and phthalates were analyzed.With an expansion in residual film,the residual film’s obstructing impact on water migration in horizontal and vertical directions grows continually,as does the infiltration uncertainty.Cumulative evaporation is negatively associated with residual film thickness.When the amount of residual film exceeds710 kg/ha,its influence on water penetration is minimal,but its effect on evaporation is pronounced.The horizontal direction of soil salinity is manifested as follows: the larger the density of the residual film,the greater the spatial variability of the salinity,and the "segmentation layer" emerges between 30 and 40 cm in the treatment of high residual film.At various soil depths,the variable degree of soil salinity initially increased and subsequently dropped,and the soil salt content increased by 0-40 cm with the increase in residual film quantity.The content of nitrate nitrogen and ammonium nitrogen in soil deteriorated with the increase of residual film quantity.A certain amount of mulch film(<1030 kg/ha)in the soil increases microbial diversity and species richness.A small amount of residual film stimulates soil enzyme activity,but an abundance of residual membranes inhibits enzyme activity.The residue of mulch film rose,as did the levels of lead and cadmium in the soil and crops.There is a direct linear link between heavy metal accumulation,residual film density,and soil mulching age.The Pb in cotton field soil in the research area met the limit of soil pollution risk management and control in China when the amount of residual film reached 1050 kg/ha,while Cd hit the limit at 955 kg/ha.The potential ecological hazard of Cd in the study region is significantly greater than that of Pb.When the residual film density exceeds 1030 kg/ha,the combined potential ecological risk index of Pb and Cd released by the remaining film will approach a level of considerable biological hazard.The release of DEHP and DBP in soil increased with the quantity of residual film,and the growth rate of DEHP was strongly positively linked with the residual film density.(3)The influence mechanism of residual film on crop growth and the threshold of influence on yield were investigated.Increased levels of the residual film resulted in varying degrees of changes in plant growth index,biomass,and yield at different growth stages,as expressed by a lower growth index,a drop in chlorophyll content,inhibition of root growth,a decrease in biomass,and a loss in yield.The presence of residual film in soil decreases cotton yield due to the overall embodiment of the negative effect of physical barrier,soil structure,moisture,salt,nutrients,microorganisms,heavy metals,and phthalates on crops.When the density of the residual film reaches 1210 kg/ha,the yield increase rate of the film begins to be less than the yield decrease rate of the film,and after 42 years,even if the film is no longer planted,its negative influence will be higher than its positive effect.Simultaneously,the cultivation and planting of cotton types with established roots and the use of biodegradable mulch film are effective methods for ensuring land’s sustainable usage and cotton’s substantial economic advantages.