玛丽·贝丝·柯克汉姆（Mary Beth Kirkham）被邀请在环境中合着一本关于微塑料的新书时，没有研究微塑料，但是她对现有研究的态度很突出。
"I had read in the literature that ... cadmium and other toxic trace elements [are] increased when we have these particulate plastics in the soil. So that was of concern to me," Kirkham, a plant physiologist and distinguished professor of agronomy at Kansas State University, told EHN.
柯克汉姆（Kirkham）的专业知识是在水与植物关系和重金属摄取方面，因此她决定进行自己的研究，在其中培养了暴露于微塑料，镉，微塑料和镉的小麦植物。然后，她将这些植物与无添加剂生长的植物进行了比较。她之所以选择镉，是因为它有毒，致癌性和无处不在的环境中，由于人类活动而脱落 - 它是从电池和汽车轮胎中脱落的，并且在用于制造的磷酸盐岩石中自然发现农业肥料。
"Cadmium is everywhere," said Kirkham.
在实验结束时，她将小麦植物送去了分析，并验证了先前的报道，用微塑料生长的植物更受镉污染。她说：“塑料确实是吸收镉的矢量。”
Her experimentbecame a chapter in the new book“陆地和水生环境中的颗粒塑料。”
"I think people just haven't felt that microplastic uptake by plants is an issue," said Kirkham. "It just hasn't been in the public eye."
Microplastics, loosely defined as plastic pieces smaller than 5 millimeters across, or roughly the size of a small grain of rice, have made their mark on both the global ecosystem and the popular consciousness, famously killing seabirds andraining down on wilderness areas。And while the impacts of ocean microplastics have been the subject of significant media and scientific attention, researchers say that most microplastics actually accumulate on land, including agricultural areas. One estimate suggested that 107,000 to 730,000 tons of microplastics could be dumped onto agricultural soils in the United States and Europe every year, compared to the 93,000 to 236,000 tons that enter the oceans.
Microplastics arrive on farms through processed sewage sludge used for fertilizer, plastic mulches and even are intentionally added as slow-release fertilizers and protective seed coatings. In just the last few years, an uptick in research has uncovered alarming potential impacts of this contamination on all aspects of agricultural systems from soil quality to human health.
污水污泥，覆盖物和缓释肥料
挪威水研究所的研究科学家卢卡·尼泽托（Luca Nizzetto）开始研究农业土壤中的微塑料，因为他注意到大多数关于微塑料的研究都集中在海洋上。
"Most of the marine sources are actually land-based," Nizzetto told EHN. "No one was looking at what was happening close to the source." When his team began evaluating potential land sinks for microplastics they "immediately [identified] agriculture as one of the hot areas."
Microplastics can enter agricultural lands via sewage sludge, the solids filtered out of wastewater, commonly used to fertilize agricultural fields. Microplastics get into the wastewater originally through laundry, personal care products and urban runoff.
Nizzetto said that most of the microplastics are retained in the sludge as the water is cleaned in treatment facilities and, in a 2016paper, his team estimated that between 125 and 850 tons per million people annually are dumped on European agricultural lands via sewage sludge. Nizzetto also reported in the same paper that roughly 50 percent of sewage sludge is processed for agricultural application in both Europe and the U.S.
Microplastics have beenreportedin U.S. sewage sludge as early as 1998, and in 2020 researchers estimated roughly 21,249 metric tons of microplastics are released to U.S. agricultural lands from sewage sludge annually. Because of their recalcitrance in soils, U.S. researchers even调查the possibility of using the contemporary microplastics profile of soils as an indicator of past sewage sludge application.
微塑料还可以通过农民使用的塑料材料降解进入农业土壤。柯克汉姆（Kirkham）说，在1950年代，塑料覆盖在温室中取代了玻璃。塑料覆盖物也得到了普及，在世界上大部分地区变得司空见惯。这些覆盖物，铺有塑料片，以抑制杂草，温暖土壤，保留水分，并具有挑战性地回收和昂贵的处置。
According to Kirkham, farmers may end up piling them up on their land or burning them to avoid disposal costs. Nizzetto said that in some areas, the mulches simply are left to break down into the soil.
根据Nizzetto和Ehn交谈的其他研究人员的说法，故意生产的微型塑料是农业土壤的微塑料排放的另一个来源。这些可以包括塑料封装的慢释肥料和旨在保护种子免受微生物的塑料涂料。
A 2017report欧盟委员会的汇编估计，慢释肥料中多达8,000吨的塑料每年被广播到西欧农业土壤上（尽管他们说其中一部分可能不是微塑料）。2019年欧洲化学局report列出的排放量为慢释肥料的10,000吨，每年用于处理的种子500吨。美国的数字不可用。
Microplastics alter the physical and biological properties of soils
Sixteen days into Kirkham's microplastics and cadmium experiment, her plastic-treated wheat plants began to yellow and wilt. Water had been pooling on the top of the soil in the plastic treated plants, but to keep her experiment consistent, she had to give all the plants the same amount of water.
"The particulate plastic appeared to clog the soil pores, prevent aeration of the soil, and cause … the roots to die," said Kirkham. Plants without microplastics, even the cadmium-contaminated ones, were in much better shape. "It was the plastics that were controlling the growth more than the cadmium."
Another team of researchersreported相似的结果。他们发现接触塑料会导致体重，高度，叶绿素含量和根生长的降低Arabidopsis thaliana,卷心菜和西兰花的亲戚。在这项研究中，研究人员使用了纳米塑料，塑料块的大小小于100纳米。对于比例，新型冠状病毒的测量为60至140纳米。
The full impact of microplastics contamination in agricultural soils, particularly as concentrations increase with time, is unknown. However,studieshave shown that microplastics possess physical and chemical characteristics that have the potential to alter soil bulk density, microbial communities, water holding capacity and other properties that influence plant development.
穿过植物和人类组织
Earlier this year, Yongming Luo, a professor at the Yantai Institute of Coastal Zone Research and the Nanjing Institute of Soil Science in China, and colleaguesreportedmicroplastics accumulation in wheat and lettuce plants exposed to microplastics in a laboratory setting. The researchers grew the plants in hydroponic and soil systems with microplastics laced with fluorescent dyes. The researchers analyzed cross sections of the plants under a microscope outfitted to detect the fluorescence. The roots, stems and leaves lit up.
"For decades, scientists have believed that plastic particles are too large to pass through the physical barriers of intact plant tissue. But our new study disproves this assumption," Luo told EHN.
Luo's team reported that the microplastics seemed to be entering the plants through cracks in the roots where lateral branching occurs as well as diffusing through cells at the developing root tips.
A team of scientists alsoreportedearlier this year that they had detected microplastics in Italian supermarket produce including carrots, lettuce, broccoli, potatoes, apples and pears. The researchers wrote that they found the most microplastics contamination in apples and the least in lettuce, and speculated that the perennial nature of a fruit tree allowed microplastics to accumulate more than in annual crops.
[微塑料]的大多数海洋来源实际上都是陆基的。
Luo说：“如果微塑料进入我们的蔬菜，它们就会进入吃蔬菜的所有食物……这也意味着它们也在我们的肉和乳制品中。”
以前在蜂蜜，啤酒和海鲜中发现了微塑料。
With clear and uncontrolled pathways into human food systems, ingestion of microplastics by humans is practically unavoidable, but the consequences of ingestion are as yet unknown.
塑料超细纤维已经成立in malignant lung tissue biopsies of cancer patients. These plastics probably were inhaled rather than swallowed, but the concern remains that microplastics can become lodged in tissue and cause dangerous inflammation. Studies of mammals forced to ingest microplastics in laboratories also have provided evidence that microplastics can pass through cell walls, move through the body, accumulate in organs and impact the immune system.
微塑料是化学活性材料，能够吸引并结合已知会损害人类健康的化合物。除镉外，还显示微塑料会积累铅，PCB和农药。此外，塑料是用自己的一组有毒化合物制造的，其中可能包括内分泌干扰器BPA。研究人员已经提出，无论是土壤还是人体组织，获得的和内源性化合物都可以从降解塑料中浸出。
"Because we are concerned that microplastics may harm our health … we find it interesting that the precautionary principle is not applied," Sophie Vonk, a researcher at the Plastic Soup Foundation in the Netherlands, a group dedicated to ending plastic pollution, told EHN. "So as long as there's no proof, we just find it OK that we're being exposed to these particles every single day, by our food, water, the air we breathe."
What to do?
Because microplastics enter agricultural systems through a variety of means, addressing this issue would require a multi-tiered approach.
The Plastic Soup Foundation has a longstanding campaign to eliminate the use of plastic microbeads in personal care products. This likely would reduce the amount of plastic that ends up in sewage sludge. The group also supports limiting single-use plastics generally, as these ultimately will break down to microplastics that end up polluting both ocean and terrestrial environments. "We're not anti-plastic," said Vonk. "We feel like plastic can be very useful for certain purposes, but the way we're using it now is just really, really not clever."
The European Chemicals Agency has proposed an EU-wide ban on intentionally introduced microplastics, including those in personal care products as well as the slow-release fertilizers and seed coatings used in agriculture. Some U.S.statesalso have moved to ban microbeads from personal care products.
To address the plastic mulch issue, Nizzetto said that one helpful step would be to make companies that manufacture plastic mulch films responsible for their recycling and disposal. This would help reduce inappropriate disposal at farms.
这是一种不可逆的污染。
The use of biodegradable plastics for mulch also has been proposed, but these polymers potentially come with their own set of problems. For instance, one of Huerta Lwanga'sstudies发现可生物降解的塑料对小麦的生长产生了比研究中使用的常规塑料更大。同样，关于某些“可生物降解”塑料是否确实降解为无害化合物，或者它们是否只是更快地分解成微塑料的速度，也存在争议。这种争议围绕着氧降解的塑料，欧盟将其移至禁止在2019年。
由宾夕法尼亚州Rodale Institute的研究人员开发的塑料覆盖物的另一种替代方法涉及种植营养序列的覆盖作物，然后将其滚动以形成厚的垫子。然后，农民将其种植到垫子中，垫子本身持续并抑制杂草，让水通过并增加营养素，而不是微塑料到土壤中。根据研究人员的说法report，该技术可以代替每英亩90磅以上的塑料覆盖物。
Gladis Zinati, director of the vegetable systems trial at Rodale and co-author on the report, told EHN that the strategy is scalable to large farms and emphasized the importance of building up healthy soils on farmlands to support long term resiliency.
Much more research is needed to paint a complete picture of the scope and impacts of microplastic pollution of agricultural soils. In the meantime, the plastics will continue to accumulate.
"This is a kind of irreversible contamination," said Nizzetto. "There's no way to remediate this kind of contamination at the scale of agricultural soils."
