Effect Of Microplastics On Plant N Uptake And Soil Gaseous N Losses

Microplastics（MPs）have become one of the common pollutants in the environment.Microplastics can enter the soil and continue to accumulate through a variety of pathways and have important impacts on soil ecosystems,such as causing changes in soil structure and erosion processes.Nitrogen（N）fertilizer application is a key basis for improving crop yields.However,excessive application of chemical N fertilizers not only leads to an increase in agricultural production costs,but also causes a sharp increase in ammonia volatilization（NH₃）and nitrous oxide（N₂O）emissions,which causes atmospheric degradation and surface source pollution to occur.It has been shown that soil properties and plant growth conditions can be significantly affected by fertilizer application and exogenous additions.However,little is known about how microplastics affect soil properties,crop growth,N utilization and its loss through NH₃volatilization and N₂O emissions at different levels of N supply.In this study,polyethylene（PE）microplastic pellets(60 g pot^–1)were added to the test soil in a soil column（30 cm in diameter and 28.5 cm in height）to study the effect of microplastic on the growth,NH₃ volatilization and N₂O emissions,and soil properties of rice and poplar cuttings at two levels of N supply(rice:low N–1.4 g N pot^–1,high N–1.9 g N pot^–1;poplar cuttings:low N–0.9 g N pot^–1,high N–1.8 g N pot^–1).The main results and conclusions of this study are as follows:（1）Microplastics did not significantly inhibit plant growth at both levels of N application.When N application was reduced from high N to low N,microplastic addition resulted in a 9.1～10.7%reduction in rice seed yield and 2.3～7.2%reduction in biomass of poplar cuttings,however,there were no significant differences among the treatments.And the inhibitory effects of microplastics were higher at low N levels than at high N levels.There was no significant effect of microplastic addition on rice spike number,spike grain number,thousand grain weight and amino acid content at both N application levels.At high N levels,microplastic addition resulted in a significant 8.4%reduction in physiological nitrogen utilization in rice.There was no significant effect of microplastic addition on the height,ground diameter and nitrogen utilization of poplar cuttings at both levels of N supply.（2）The NH₃ volatilization accumulation during the rice season and poplar cuttings was significantly reduced by 34.4%and 60.2%（P<0.05）,respectively,when the N rates ranged from high to low.However,microplastic addition at both N supply levels had no significant effect on soil NH₃ volatilization accumulation in both seasons.The cumulative NH₃ volatilization from soil is mainly related to the level of N supply.（3）Microplastic addition suppressed soil N₂O emissions in the rice season,and this effect was correlated with the level of N application.Among them,microplastic addition resulted in a significant reduction of cumulative soil N₂O emissions by 14.4%（P<0.05）in the rice season at low N levels.In contrast,the inhibitory effect of microplastics on soil N₂O emissions was not significant at high N levels.Microplastics at low N levels reduced soil nir S and nir K gene copies by 80.4%and 50.3%,respectively,but increased nos Z gene copies by 47.9%,which was the main reason for the decrease in soil N₂O emissions in the rice season after microplastic addition.（4）Microplastic addition affects soil properties,and this effect is related to the level of nitrogen application.At low N levels,microplastics had no significant effect on soil properties in the rice season,but significantly increased soil fast-acting potassium by 7.3%（P<0.05）in poplar cuttings at seedling stage.At high N levels,microplastic addition resulted in a significant decrease of 45.3%and 25.0%（P<0.05）in soil NH₄⁺-N and fast-acting potassium content in the rice season,respectively.However,there was no significant effect on the soil properties of poplar cuttings at the seedling stage.In summary,microplastic addition had a potential inhibitory effect on plant growth;no significant effect on soil NH₃ volatilization,but at low N levels would reduce soil N₂O emissions in the rice season by affecting functional gene enrichment;overall,microplastic field effects on soil nutrient content were not significant.