Effects Of Microplastics Addition On Soil Carbon And Nitrogen Emissions And Nitrogen Leaching In Citrus Orchard

In recent years,plastic products have been widely used in various fields,convenient for people’s life but also caused serious environmental pollution.Microplastics（MPs）are characterized by small particle sizes（<5 mm）,difficult to decompose in the environment,which has attracted extensive attention from scholars at home and abroad.However,most of the studies focused on the marine environment,there were few studies in the soil.Soil is the source and sink of microplastics,which has much more microplastics in soil than in the ocean.Therefore,it is necessary to study the interaction mechanism between soil microplastics pollution and soil properties.In this study,the soil of Dangyang Orange Garden was taken as the research object,and three laboratory experiments were carried out from effect of microplastic on three aspects of soil（carbon and nitrogen transformation,earthworm activity and nitrogen leaching）.The main results are as follows:（1）Only adding microplastics had no significant effect on soil CO₂emission,but significantly affected soil N₂O emission.Compared with the control,the cumulative emission of N₂O increased by 13.20%with the addition of low amount of microplastics（0.25%PP）,while the cumulative emission of N₂O decreased by 6.67%with the addition of high amount of microplastics（7%PP）.The addition of straw significantly increased CO₂and N₂O emissions by 6.93 and 3.02 times,respectively.Mixed application of microplastics and straw had significant effects on soil CO₂and N₂O emissions,and different levels of microplastics had different effects.Low amount of microplastics and straw added promoted soil CO₂and N₂O emissions,while high amount of microplastics added inhibited soil CO₂and N₂O emissions.（2）The application of straw enhanced the contents of DOC and MBC,while high amount of microplastics significantly decreased the contents of DOC,MBC,NH₄⁺-N and NO₃^--N in the soil.The addition of microplastics had no significant effect on protein-like substances（tyrosine,soluble microbial by-products,tryptophan）in DOM components,but the low amount of microplastics significantly increased the fluorescence intensity of humic acids,increasing by 20.61%.At the end of incubation,the activities ofβ-glucosidase,urease and peroxidase in soil were significantly inhibited by three concentrations of microplastics,the high amount of microplastics treatments decreased by17.79%,17.01%and 34.12%,respectively.Soil active organic carbon,mineral nitrogen（N）and enzyme activity are crucial factors affecting the conversion of carbon and nitrogen.Microplastics change their properties and further affect the carbon and nitrogen cycle.（3）Adding microplastics significantly inhibited earthworm growth,the mortality rate increased with the application of microplastics added.The death rates of earthworms were22.22%and 44.44%in the treatment with medium（2%PP）and high amounts（7%PP）of microplastics,no earthworm death with no microplastic treatment.At the end of incubation,the biomass of earthworm treated with low,medium and high microplastics decreased by 28.64%,32.17%and 43.18%,respectively.The addition of earthworms promoted soil CO₂and N₂O emissions,while the addition of microplastics significantly reduced soil CO₂and N₂O emissions under the action of earthworms,and the greenhouse gas emissions decreased with the increase of the amount of microplastics added.（4）In the nitrogen leaching experiment,the addition of microplastics changed the structure of soil aggregates,increased the water loss,and accelerated the nitrogen leaching.The volume of leachate solution in low,medium and high amounts of microplastics treatments increased by 3.39%,4.65%,4.16%（7 d）and 4.29%,5.81%,8.10%（14 d）,respectively.Compared with the control,the total nitrogen leaching loss of low,medium and high microplastics treatments increased by 8.53%,15.13%and 15.88%,respectively,and there was no significant difference in the total nitrogen leaching loss among different microplastics addition levels.Mixed application of microplastics and straw reduced the total nitrogen leaching,medium and high application of microplastics and straw significantly reduced the total nitrogen leaching by 29.40%and 32.88%,respectively,and low application of microplastics reduced the total nitrogen leaching by9.88%but not significant.The addition of straw improved the soil water retention capacity,significantly reduced the leaching volume and nitrogen leaching,and the total nitrogen leaching loss was significantly reduced by 25.68%.The leaching loss of nitrogen was mainly NO₃^--N leaching,accounting for 64.53%-84.96%of the total leaching loss of nitrogen.Compared with the control,the NO₃^--N leaching amount of low,medium and high microplastics treatment increased by 12.38%,19.04%and 15.14%,respectively,while the nitrate leaching amount of low,medium and high microplastics and straw mixed application significantly decreased by 25.73%,33.73%and 27.11%,respectively.Adding microplastics only had no significant effect on the leaching loss of total phosphorus,but the mixed application of straw and microplastics significantly increased the leaching loss of total phosphorus,which was contrary to the leaching result of nitrogen.Low,medium and high amount of microplastics increased by 31.34%,59.27%and 87.56%,respectively.