Effects Of Microplastics And Nitrogen Deposition On Soil Multifunctionality

Fuel combustion and human production activities have caused a sharp increase in atmospheric nitrogen deposition（ND）and had negative impacts worldwide generally.As a new type of global pollutants,microplastics（MPs）derived from plastic have gradually attracted people’s attention for their impacts on soil ecosystems.Soil is the main place for human activities and cultivation,the various services and functions provided by soil are the foundation for the sustainable and healthy development of human society.Currently,there have been many studies on the effects of MPs and ND alone on soil ecosystems,but the studies on their impacts on soil multifunctionality,especially the composite effects of their interactions in soil are still insufficient.Therefore,the soil incubation and corn pot experiment were conducted to investigate the effects and mechanisms of MPs and ND pollution on soil multifunctionality.Soil incubation experiment was conducted by two types of MPs,polylactic acid（PLA）（0.1%and 1%,w/w）,polyethylene（PE）（0.01%and 0.1%,w/w）and four levels of ND concentration(0,50,100 and 150 kg N ha^–1 yr^–1).The results showed that the interaction between ND and MPs affected the multifunctionality of soil.ND increased soil nutrient content（available nitrogen and phosphorus）significantly.The impacts of MPs on them were relatively small when compared to ND,and related to the type and concentration of MPs,and level of ND.Bacterial FAPROTAX function prediction indicated that ND increased some C-cycling processes,such as cellulolysis and plastic degradation.MPs showed stimulating effects on these processes too,particularly in the soil with ND.In addition,ND significantly reduced the abundance of functional genes for nitrogenase（Nif H）,monooxygenase（amo A）,and nitrite reductase（Nir K）,leading to inhibition of soil nitrogen fixation,nitrification,and denitrification.The addition of MPs affected N-cycling processes:0.1%PE enriched some bacterial groups for denitrification,and 1%PLA enriched N-fixation bacteria at all ND levels.In the aspect of interaction mechanism,ND caused lower soil p H but higher soil N,decreased bacterial diversity and richness,and changed the composition and activity of functional bacteria,which explains why ND changed soil functions and regulated the impact of MPs.The results of corn pot experiment based on soil incubation showed that ND increased the content of soil nutrient,chlorophyll content in corn leaves and promoted corn growth,while MPs reduced corn biomass at high levels of ND(100 and 150 kg N ha^–1 yr^–1).ND enriched some bacterial communities with C and N cycling functions in the corn-soil ecosystem,but significantly reduced the relative abundance of saprophytic fungi with the function of degrading refractory organic substances such as lignin.Moreover,ND increased the relative abundance of plant and animal pathogens in bacterial and fungal communities significantly.Bacterial function prediction analysis showed that 1%PLA inhibited the urea decomposing bacteria and enrich nitrogen fixation functional bacteria in the corn-soil ecosystem significantly,while PE（especially at high concentrations）increased the relative abundance of denitrification functional bacteria.Analysis of fungal community function prediction showed that 1%PLA treatment had the highest organic matter degradation ability and the lowest relative abundance of animal and plant pathogens,but PE resulted in the lowest organic matter decomposition and the highest relative abundance of animal and plant pathogens.These meant that in the context of ND,PLA had higher nitrogen fertilizer retention capacity and lower ecological risk compared to PE,which had a guiding role for healthy agricultural production.In conclusion,MPs and ND had significant interactive effects on soil functions.Although ND reduced the overall microbial activity and diversity significantly of soil,it promoted the growth of corn.MPs at high concentrations of ND inhibited the growth of corn.Furthermore,in the context of ND,PLA-MPs were more ecologically beneficial than PE-MPs.Therefore,this study provides valuable information for assessing and predicting the ecological impacts of the combined effect of MPs and ND,and contributes to the management of MPs and ND pollution in the environment.