内蒙古河套灌区农田土壤中微塑料的赋存特征

为阐明内蒙古河套灌区农田土壤中微塑料的赋存特征及其与覆膜年限、灌溉类型等的响应关系,该文采用田间取样与室内试验相结合的方法进行了系统研究,分析了研究区地膜覆膜现状,并考虑覆膜年限(覆膜5、10、20 a)及灌水方式(滴灌和畦灌)2个因子,探索了河套灌区农田土壤中微塑料的丰度、类型、颜色、粒径等赋存特征,并通过扫描电镜、能谱仪等对微塑料表面特征及表面附着物进行观察与分析。结果表明:内蒙古河套灌区不同覆膜年限下(5、10、20 a)土壤中微塑料平均丰度分别为2526.00、4352.80、6070.00个/kg,单层土的微塑料含量最大值(2133.50 ind/kg)出现在覆膜20 a的0~10 cm土层,最小值(678.00个/kg)出现在覆膜5 a的>20~30 cm土层;不同覆膜年限和灌溉类型影响土壤微塑料丰度,滴灌农田微塑料丰度值略大于畦灌农田,在不同土层深度上微塑料丰度值随土层深度增加逐渐减小;微塑料类型主要有纤维类(23.34%)、碎片类(26.31%)、薄膜类(38.57%)和颗粒类(11.78%)等4种,且薄膜类在不同土层占比都较高,微塑料颜色包括黑色、透明、绿色、红色、蓝色等,比例分别为24.56%、23.83%、19.34%、16.52%和15.75%,其中0~10 cm土层中微塑料以黑色为主,占比30.25%,在>10~20、>20~30 cm土层中微塑料以透明为主,占比30.15%和29.23%,粒径则随覆膜年限增加而呈逐渐减少趋势,粒径小于1 mm的微塑料居多,随着覆膜年限的增加,微塑料粒径在0~10、>10~20、>20~30 cm土层间的差异逐渐减小,且粒径大小与灌溉类型无显著关系(P>0.05);微塑料样品表面特征粗糙,呈现不规则孔隙,纤维类、薄膜类和碎片类微塑料均具有较多规则微小孔隙,而颗粒类微塑料表面孔隙则不规则且呈凹凸状,土壤中微塑料的多孔特性造成微塑料比表面积增大,进而增加对土壤中其他污染物和微生物的吸附;微塑料的表面孔隙附着有机体和污染物,表面存在稳定的铁氧化物、稀土元素等,并容易形成有机-无机复合污染效应。研究对于明晰微塑料在河套灌区土壤中的分布现状及危害具有重要意义。

Occurrence characteristics of microplastics in farmland soil of Hetao Irrigation District, Inner Mongolia

In order to study the characteristics of microplastics and its response relationship with mulching years, irrigation methods in the farmland soil of Hetao Irrigation District, field sampling and indoor experiments were used in combination for in-depth study. This paper mainly analyzed the current situation of plastic film mulching in Hetao Irrigation District by considering the effects of mulching years (5, 10, 20 a) and irrigation methods (drip irrigation and surface irrigation under film mulching). The abundance, type, color, particle size and other distribution characteristics of microplastics in the farmland soil were analyzed and other microplastic surface features and attachments also were observed and discussed, which was supported by scanning the electric microscope and energy spectrometer. The results showed that the average abundance of microplastics in soil under the different mulching years (5, 10, 20 a) in Hetao Irrigation District was 2 526.00, 4 352.80, 6 070.00 pieces/kg. The maximum microplastic was 2133.50 pieces/kg in every layer of soil appeared at 0-10 cm soil layer by mulching 20 a, and the minimum microplastic was 678.00 pieces/kg at 20-30 cm soil layer by mulching 5 a. The average annual increasing rate of microplastics was 14.46% in the early stage (5-10 a), while the increasing rate in the later stage (10-20 a) was 3.95%. The abundance of soil microplastics was affected by different plastic-mulching years and irrigation methods. The abundance of microplastic in drip irrigation farmland was slightly larger than the surface irrigation farmland, and it decreased with the increase of soil depth. By further analysis, the major categories of microplastics were fibers (23.34%), fragments (26.31%), films (38.57%) and grains (11.78%); the colors of microplastics included black, transparent, green, red, blue, etc., with the proportions 24.56%, 23.83%, 19.34%, 16.52% and 15.75% respectively. In the 0-10 cm soil layer, the main color of microplastics was black and it accounted for 30.25% of the total microplastic; in the >10-20 and >20-30 cm soil layer, the main color of microplastics were transparent, accounted for 30.15% and 29.23% of the total microplastic. The particle size exhibited a downward trend with the increase of the mulching years, and the majority of particle size less than 1 mm. In addition, with the increase of mulching years, microplastics particle size in >0-10, >10-20, >20-30 cm soil layer was gradually decreased, and there was no significant relationship between size and irrigation method (P>0.05). The surface features of microplastic samples were rough and irregular pores existed on it, and fiber, thin film and fragments microplastics had regular micropores, while the grain microplastic surface pores were irregular and concave. The porous properties of microplastics in soil made the characteristic of greater specific surface area carried by it. This characteristic could increase the adsorption of other pollutants and microorganisms from the soil; and there were stable iron oxides and rare earth elements on the surface. The research is of great significance to clarify the distribution and harm of microplastics in the soil of the Hetao Irrigation District.