| In recent years,the degradation of ecological functions caused by microplastic pollution has attracted widespread attention,microplastic pollution has been considered one of the greatest threats to the functionality of the Earth system caused by human activities.Nitrogen is the main nutrient element required for plant growth and development,and the continuous accumulation of microplastics in soil will have a significant impact on nitrogen nutrient cycling.In order to explore the effects of polyethylene microplastics addition on the basic physical and chemical properties of soil,the functional gene abundance and community structure of microorganisms related to nitrogen cycle,two groups of soil culture experiments were set up in this study.(1)Different polyethylene microplastics(MP)addition concentration tests:CK(not added),MP0.1(0.1%,microplastics to soil mass ratio),MP0.5(0.5%),MP1(1%),MP2(2%),MP4(4%),and MP8(8%);(2)Comparison test of polyethylene microplastics,polyethylene resin raw materials(PE),and plastic additives(A): CK(not added),MPL(0.5%),MPH(2%),PEL(0.5%),PEH(2%),AL(added to soil at the amount of additives contained in 2% microplastics),AH(10 times the dosage of AL added).The main research results obtained are as follows:(1)With the increase of the concentration of polyethylene microplastics added,except for MP0.1 treatment,the soil bulk density showed a decreasing trend,with a reduction range of 3.82%～12.21%(compared to CK,the same below).Except for MP0.1 and MP0.5 treatments,soil porosity showed an increasing trend,with an increase range of 3.82% to 7.01%.The field water capacity of MP2 and MP4 treatments increased by 16.96% and 6.07%,respectively;Except MP0.1 treatment,the average weight diameter of aggregates increased,while the geometric mean diameter increased first and then decreased.The soil p H value shows a decreasing trend with the addition of microplastics.The content of total carbon,organic matter,and soluble organic carbon in soil increased from 6.95% to 73.52%,25.63% to92.96%,and 18.20% to 56.58%,respectively;The soil microbial biomass carbon shows a decreasing trend,with a reduction range of 22.99% to 50.06%.There was no significant change in soil soluble total nitrogen;The soluble organic nitrogen of MP4 and MP8 treatments decreased by 22.26% and 41.93%,respectively.The soil nitrate nitrogen and microbial biomass nitrogen showed an increasing trend,with different concentrations of microplastics increasing in the range of 24.73%～208.70%and 40.19%～123.70%,respectively;However,soil ammonium nitrogen showed a decreasing trend,with a reduction range of 18.32% to 56.25%.The overall trend of soil net nitrification rate and net nitrogen mineralization rate is increasing,with an increase range of 239.19%～973.56% and 53.98%～160.31%.The above results indicate that different concentrations of microplastics can significantly alter soil physicochemical properties,as well as different forms of nitrogen content and conversion rate.In addition,this study found that the changes in soil total carbon,organic matter,total nitrogen,soluble organic nitrogen,nitrate nitrogen,and ammonium nitrogen content were consistent under the treatment of polyethylene microplastics,polyethylene resin,and plastic additives.The effect of individual additive treatment on soil total nitrogen,soluble organic nitrogen,nitrate nitrogen,and ammonium nitrogen was significantly lower than that of microplastic treatment and polyethylene resin treatment.(2)The addition of polyethylene microplastics has a significant impact on the microbial community structure of soil nitrogen cycle function.The results of principal coordinate analysis show that MP8 treatment has the greatest change compared to CK,indicating a significant concentration effect.Compared with CK,different concentrations of polyethylene microplastics(especially MP8 treatment)cause the relative abundance of microorganisms related to nitrogen fixation(Rhizobia,Mesorhizobia),nitrification(Conexibacter,Anaeromyxobacter)and denitrification(Streptomyces,Micromonospora,Methylobacterium)to decrease,The relative abundance of microorganisms related to assimilative nitrate reduction(Nocardioides,Nocardia,Sphingomonas,Agromyces)increased.There was no significant difference in microbial community structure between polyethylene microplastics and polyethylene resin,but there was a significant change in AH treatment.In addition,compared with polyethylene micro plastic treatment and polyethylene resin treatment,plastic additive treatment has more obvious effects on soil denitrification microorganisms(Streptomyces,Cysticercus,Hydrophage,Usitatibacter).(3)The addition of polyethylene microplastics has a significant impact on the abundance of nitrogen cycle functional genes in soil.The addition of polyethylene microplastics at different concentrations significantly reduced the relative abundance of nif DHK,nir K,and nor B genes in soil,with decreases ranging from34.21% to 62.54%,2.34% to 9.77%,and 8.16% to 26.34%,respectively.However,it increased the relative abundance of ure ABC,nx RB,and nas A genes,with increases ranging from 13.47% to 40.37%,28.37% to 55.46%,and 2.86% to 11.59%,respectively.In addition,compared with polyethylene micro plastic treatment and polyethylene resin treatment,plastic additive treatment has more obvious effect on soil denitrification gene(nos Z),followed by soil genes related to organic nitrogen synthesis and degradation(nmo and asnb).Compared with CK,the relative abundance of nitrogen limiting response gene ntr B in AH treatment significantly decreased by 7.95%.It can be seen that the release of additives will affect the process of soil denitrification and organic nitrogen metabolism.(4)There was a close correlation between soil physical and chemical properties,soil nitrogen cycle functional gene composition and nitrogen content under the addition of microplastics.Redundancy analysis showed that soil nitrate nitrogen,soluble organic carbon and microbial biomass nitrogen were significantly positively correlated with the relative abundance of soil nitrogen cycle functional genes,while soil ammonium nitrogen,soluble organic nitrogen and microbial biomass carbon were significantly negatively correlated with the relative abundance of soil nitrogen cycle functional genes.In addition,soil nitrogen cycle functional microbial community diversity has a significant positive linear relationship with soil net nitrification rate and net nitrogen mineralization rate,and soil nitrogen cycle functional microbial community composition has a significant nonlinear relationship with soil net nitrification rate and net nitrogen mineralization rate.It can be seen that microplastics affect nitrogen cycling by acting on soil nitrogen cycling functional microorganisms.By analyzing the basic physical and chemical properties of soil,the functional genes of nitrogen cycle and the changes of microbial community structure,this study will help to understand the microbial mechanism of microplastics affecting soil nitrogen cycle,and provide a theoretical basis for exploring the impact of microplastics on soil nitrogen. |
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