Combined Biological Effects Of Microplastics And Carbon Nanotubes In The Soil

Microplastics（MPs）and carbon nanotubes（CNTs）,as emerging pollutants,have received high attention worldwide due to their potential adverse effects on ecosystems.Soil is one of the main sinks of MPs and CNTs,but the combined biological effects of MPs and CNTs in soil are still unclear.Accordingly,we selected three types of MPs:non-degradable high-density polyethylene（HDPE）,polystyrene（PS）,and biodegradable polylactic acid（PLA）.Soil incubation and plant pot culture experiments were conducted to investigate the effects on soil properties,microbial communities and peanuts of single and combined contamination with MPs and multi-walled carbon nanotubes（MWCNTs）.The purpose is to clarify how the coexistence of MPs and MWCNTs affects soil health and crop growth.The results of soil incubation tests showed soil pH was increased by 10%HDPE and PLA treatments,but decreased by increasing MWCNTs.Soil dissolved organic carbon（DOC）content was increased only by 10%PLA treatments.NO₃^--N content was decreased by MPs,with a decrement of 99%by 10%PLA.Similarly,available P content was reduced by 10%MPs.Urease and alkaline phosphatase activities increased under10%PLA treatment,but did not change significantly under HDPE and PS treatments.Fluorescein diacetate hydrolase（FDAse）activity increased under HDPE treatment and decreased under 10%PLA treatment,but did not change significantly under PS treatment.In addition,invertase activity decreased with increasing MWCNTs.Both MPs and MWCNTs altered the diversity of the soil bacterial community.Co-exposure to 10%MPs and 1 or 10 mg/kg MWCNTs generally resulted in the lowest species richness and Shannon index.MPs,especially at 10%,altered bacterial community composition and associated metabolic pathways,leading to enrichment of specific taxa and functional genes.In conclusion,non-degradable and biodegradable MPs differentially alter soil geochemical properties and microbial community structure and function,and can be further modified by co-occurring MWCNTs.The results of plant pot culture tests showed that the presence of plants did not alter the trend of the effects of MPs and MWCNTs on NO₃^--N,NH₄⁺-N and available P,but altered the effects of MPs and MWCNTs on soil urease,alkaline phosphatase and FDAse.For example,HDPE and PS significantly increased alkaline phosphatase activity,but 10%PLA had no significant effect on FDAse activity,100 mg/kg MWCNTs reduced alkaline phosphatase activity.In addition,PS reduced soil pH instead.Meanwhile,MPs or 100mg/kg MWCNTs single contamination reduced the levels of available K,exchangeable Ca and Mg in the soil.Theαdiversity of soil nitrogen-fixing bacteria was not significantly altered under exposure to MPs,but was significantly decreased under exposure to 100 mg/kg MWCNTs.MPs and MWCNTs together shaped the structure of the rhizosphere nitrogen-fixing bacterial community.Among them,the low inorganic nitrogen environment induced by 10%PLA MPs promoted the growth of unclassified＿o＿＿Rhizobiales,while 1 mg/kg MWCNTs reduced the relative abundance of unclassified＿o＿＿Rhizobiales under 10%PLA MPs treatment.The results of plant indicators showed that MPs promoted peanut growth.HDPE,PS and PLA increased peanut plant height,and 10%HDPE increased peanut below-ground dry weight.MWCNTs had no significant effect on peanut plant height,but reduced peanut biomass to some extent.In addition,MPs reduced the total nitrogen content in peanut,and both HDPE and PS reduced the chlorophyll content of peanut leaves,but PLA increased the chlorophyll content of peanut leaves.It is worth noting that the severe deficiency of soil inorganic N under 10%PLA exposure promoted root nodulation in peanut,and the presence of root nodule in large numbers helped to mitigate to some extent the adverse effects of soil N deficiency on peanut.Compared to HDPE and PS,PLA increased the production of malondialdehyde（MDA）and hydrogen peroxide（H₂O₂）,activating the antioxidant system in plants.In addition,the effect of MWCNTs on plant antioxidant systems depended on the concentration of MWCNTs,where 100 mg/kg MWCNTs increased plant peroxidase（POD）activity.The physiological and biochemical responses of plants suggested that MPs and MWCNTs may pose a risk to the healthy growth and development of peanut.In conclusion,the type and dose of MPs and the concentration of MWCNTs are important factors affecting the soil microenvironment and the growth of peanut seedlings.The presence of MPs significantly changed the soil physicochemical properties and microbial community and thus threatened the health of legumes,especially the soil NO₃^--N content.But,the presence of the legume-nitrogen-fixing bacteria symbiosis system weakened the adverse effects of MPs on plants on terms of soil nutrients to some extent.Not to be overlooked,the effects of MPs and MWCNTs on photosynthetic and antioxidant systems of peanut may pose a potential risk to its healthy growth in the maturity stage.Therefore,the environmental risks of MPs and MWCNTs on soil health,growth and development of farm crops,and food security cannot be ignored.