Effects Of Microplastics On Enzymatic Activity And Microbial Community Structure In Agricultural Soil

As a new type of persistent soil pollutants,microplastics have attracted extensive attention in recent years due to their increasing environmental pollution.Farmland soil is considered to be the main collection site of microplastics,but the effects of microplastics on microbial diversity and ecosystem function in farmland are still largely unknown,and the effects of straw returning with microplastics on soil ecology are rarely reported.Based on this,through soil column culture test,colorimetric method and high-throughput sequencing technology,this paper explores the effects of microplastics on farmland enzyme activities and microbial community structure,providing theoretical basis for deepening the understanding of the effects of microplastics accumulation and migration on farmland microbial ecological environment and soil biogeochemical cycle.The article includes the following two aspects: 1.By adding 0.5 % and 2.0 %w/w low density polyethylene film(4×4×0.01 mm)and polystyrene microspheres(particle size of 2 mm)to soil at 0～20 cm and 20～40 cm depth,the changes of enzyme activity and bacterial community succession characteristics in farmland under exposure to common microplastics were simulated.The relationship between soil physical and chemical properties,enzyme activities and bacterial communities was further analyzed by redundancy analysis.2.The effects of mixed application of low density polyethylene microplastics(0.5 %,1.0 % and 2.0 %w/w)and straw(0 %,1.0 % and2.0 %w/w)at different concentrations on the changes of enzyme activity and bacterial community succession in farmland were studied.The main conclusions of the study are as follows:(1)Low density polyethylene microplastics(LDPE-MPs)increased the activities of urease and fluorescein diacetate(FDAse)in soil,but inhibited the activities of alkaline phosphatase.With the increase of soil depth,the effect of soil depth on enzyme activity decreased gradually.In addition,high-throughput sequencing results showed that LDPE-MPs could not only increase the abundance of functional bacteria involved in soil nitrogen cycle such as Burkholderiaceae,Pseudomonas and Nitrospira.Bacillus,Nocardioidaceae and Comamomnas related to plastic degradation were also induced to enrich.In different soil layers,only the abundance of plastic-degrading bacteria was significantly different,and the abundance was higher in the 0～20 cm soil layer.The redundancy results showed that soil p H,available nitrogen and phosphorus content and related dominant bacteria were important factors that caused the change of soil enzyme activity after the addition of LDPE-MPs.(2)Polystyrene microplastics(PS-MPs)significantly increased the activities of urease and fluorescein diacetate in soil.High concentration of PS-MPs significantly promoted the activities of urease and FDAse,while alkaline phosphatase activity was inhibited,and the effect of concentration on enzyme activity was not significant.The effect of soil depth on soil enzyme activity after adding PS-MPs was reflected in that the enzyme activity in 0～20 cm soil layer was significantly higher than that in 20～40 cm soil layer,and the change trend was stronger.In addition,high-throughput sequencing results showed that the diversity and richness of soil microbial community structure decreased significantly after the addition of PS-MPs,among which Proteobacteria(29.81 %～37.44 %)and Actinobacteriota(5.52 %～11.90 %)were significantly increased in relative abundance;Acidobacteria(18.28 %～12.81 %)and Bacteroidetes(11.90 %～5.92 %)decreased significantly.PS-MPs improved the relative abundance of some functional dominant bacteria.For example,Bacillus and Pseudomonas related to soil nitrogen cycling(Burkholderiaceae)and plastic degradation(including Bacillus and Pseudomonas),the redundancy results showed that soil available nitrogen and phosphorus content and related dominant bacteria were correlated with the addition of PS-MPs.The important factors causing soil enzyme activity change.(3)At the same time,the addition of microplastics and low amount of straw promoted the increase of soil β-glucosidase activity,but with the increase of microplastics concentration,the promoting effect of enzyme activity was gradually weakened.The addition of straw significantly increased the urease activity in soil.Compared with the high amount of microplastics,the addition of medium and low amount of microplastics at the same straw concentration had a more obvious promotion effect on the urease activity.In addition,high-throughput sequencing results showed that straw supplementation significantly increased the diversity and richness of microplastic-contaminated soil bacterial community,and the promotion effect was more obvious with the increase of straw concentration.After mixed application of microplastics and straw,the relative abundance of Bacteroidota and Proteobacteria and Actinobacteriota increased significantly.In conclusion,microplastics significantly affect the changes of soil enzyme activities in farmland,and induce the directed succession of bacterial community structure by increasing the relative abundance of related dominant functional bacteria.This study provides valuable insight into the interactions between microplastics,soil enzymes,and microbial communities,and highlights the potential of straw as an effective strategy for mitigating the negative impacts of microplastics on agrosoil ecosystems.