Effects Of Microplastics On Soil Organic Carbon Mineralization And Greenhouse Gas Emissions In Coastal Wetlands

Coastal wetland is one of the important carbon sinks in global ecosystem,and its small changes may cause significant climate change effects.Microplastics（MPs）,as a new type of pollutant,has been widely found in coastal wetland.However,the impact of MPs on soil organic carbon（SOC）mineralization and GHG emissions in coastal wetland soil is still unknown.In addition,biochar（BC）has been widely used in soil pollution remediation and improvement（including coastal wetland soil）due to its excellent physical and chemical properties.Then,when MPs and BC are co-exposed to the soil,does MPs have a significant effect on SOC mineralization behavior in a single BC presence system,Are there any differences in the impacts originated from different types of MPs（non-degradable and degradable）,Unfortuntely,these issues have not been explored in previous studies,and the underlying mechanism is unclear.Elucidating the effects and mechanism of MPs on SOC mineralization and GHG emission in coastal wetland soil under different conditions is of great significance for a comprehensive assessment of the potential ecological risks of MPs.Foucsing on the shortcomings aforementioned,the main content of this research includes two aspects:First,a 60-day laboratory soil incubation experiment was designed to explore the influence of fibrous polypropylene MPs（FPP-MPs）exposure on the GHG emission from a coastal wetland soil,in which the rule and mechanism of aging time and addition rate on the above influence were studied in particular.Second,by establishing soil incubation systems（100d）with different existence conditions of reed straw biochar（RBC）and MPs,the effect of MPs on BC-mediated SOC mineralization and its underlying mechanism were explored.The main research results are as follows:（1）The exposure of FPP-MPs under different conditions had significant differences in the emissions of N₂O,CO₂,and CH₄from coastal wetland soil.Compared with the control,the additions of un-aged FPP-MPs with both two rates（0.2 and 2%）and aged FPP-MPs with a low rate（0.2%）showed an insignificant effect on N₂O emission,while the aged FPP-MPs added with a high rate（2%）resulted in a remarkably increase in N₂O emission,and a longer aging time resulted in a greater increase.A significant increase in CO₂emission was only observed in the 30-day-aged FPP-MPs treatments,compared with the control,and a higher addition rate produced a higher increase of CO₂emission.Regarding CH₄emission,it was significantly increased by adding aged FPP-MPs,and a longer aging period or/and a higher addition rate generated a higher degree of promotion of CH₄emission.However,compared with the CO₂emission,the quantity of CH₄emission was extremely low.（2）Through the determination and analysis of soil basic chemical properties（p H and salinity）,the content of carbon/nitrogen related substances（such as soil organic matter（SOM）,dissolved organic carbon（DOC）,NH₄⁺-N,NO₃^–-N and NO₂^–-N）,and microbial communities,combining with statistical analysis methods such as linear regression,generalized estimation equation,and structural equation model,the main mechanism of aging FPP-MPs promoting GHG emission was revealed.The mechanism of enhanced N₂O emission are as follows:i)enhanced SOM decomposition provided more effective carbon sources（such as DOC）for denitrification-related microorganisms;ii)the rapid conversion of NH₄⁺-N to NO₃^–-N or NO₂^–-N provided a substrate for generating more N₂O;iii)the aged FPP-MPs provided a suitable habitat for denitrification-related microorganisms and acted as electron donors or shuttles like the aged BCs for microorganisms.The mechanism of enhanced CO₂emission enhancement are as follows:i)the elevated the relative abundance of carbon mineralization-related microorganisms by providing suitable habitats and improving soil physical structure properties;ii)the mutual promotion of inorganic nitrogen transformation and carbon mineralization.The enhanced CH₄emission was mainly due to the rapid consumption of O₂and the formation of a local anaerobic zone during the fast process of SOM mineralization.（3）The CO₂emission in RBC treatment significantly increased compared with control（CK）（coastal wetland soil without MPs and BC）,indicating that the addition of RBC alone promoted SOC mineralization.When PLA-MPs or PE-MPs co-existed with RBC,CO₂emission significantly reduced compared with the RBC treatment,and the reduction degree of PE-MPs was higher than that of PLA-MPs.This suggested that the coexistence of MPs can alter the behavior of SOC mineralization mediated by RBC alone.The soil physical（soil aggregate characteristics）and chemical（p H and salinity）characteristics,nutrient（NH₄⁺-N and NO₃^–-N）contents,carbon related substances（SOC,DOC）contents and their distribution characteristics in aggregates of different particle sizes,and microbial community,were measured and analyzed for different treatments.Spearman correlation,redundancy analysis and structural equation model analysis were combined to further reveal the main mechanism of changing the single RBC-mediated SOC mineralization behavior when MPs co-existed:i)by improving the stability of soil aggregates,the protective effect on SOC is enhanced,and the content of organic carbon available to microorganisms（enzymes）in the soil was reduced;ii)PLA-MPs and PE-MPs may adsorb nutrients and occupy adsorption sites when they adsorbed onto RBC,thereby inhibiting the activity of the carbon decomposition-related microorganisms and enzymes,and finally inhibiting the mineralization of SOC and CO₂emission.（5）Based on the above results（1）–（4）,it was found that aged FPP-MPs significantly promoted the emission of soil GHG,while unaged granular PLA-MPs and PE-MPs coexisted with RBC greatly changed the effect of RBC alone on SOC mineralization behavior（inhibited SOC mineralization）.The differences between the above results may be related to the type of MPs,aging treatment,or the interaction between MPs and RBCs.These issues are worthy of clarification in future studies.Finally,the results of this study can provide a more comprehensive and in-depth understanding of the environmental effects of MPs in coastal wetlands,and provide useful scientific theoretical support for the prevention and control of MPs pollution.