Research has shifted from ecotoxicological effects to ecosystem effects and associated earth system feedback
Pollution of microplastic
The pollution of microplastics (plastic particles <5 mm) in different environmental media has become an environmental problem and health threat that people are paying more and more attention to it. The initial microplastics research focused on marine and aquatic ecosystems, but recently it has begun to involve terrestrial systems.
Ecotoxicological effects are the field of early research on microplastics in water bodies and terrestrial ecosystems. Ecotoxicological effects include indoor testing of individual organisms (usually model organisms), which is necessary for establishing toxicological baselines and understanding mechanisms.
Many effects are likely to be related to the physical properties of microplastics (such as shape and size) rather than chemical properties, and most of these effects are non-fatal, even nominally positive. The research on global change factors focuses on the research on the ecosystem level, and the research on the impact of microplastics on the ecosystem level has just begun.
The carbon element in microplastics
The carbon element in microplastics is the main constituent element. Therefore, the entry of microplastics into the ecosystem means a source of carbon input that has nothing to do with photosynthesis and net primary productivity. Due to the inertness of microplastics in the environment, the carbon turnover rate of this polymerized form is very slow.
Moreover, the environmental behavior and residence time of microplastics in the soil and the input rate of this polymer carbon into the ecosystem are not yet known. This is because the research so far has mainly focused on quantifying the number and types of microplastics, rather than the polymer carbon of the microplastics themselves.
Most of the carbon source of microplastics comes from petroleum and due to the non-degradable characteristics of microplastics, it accumulates in the soil, so it should be considered when assessing the carbon storage of the main functions of the ecosystem.
Effect of microplastic on soil
Other effects of microplastics on the carbon cycle are indirect and may depend on the shape and size of the particles. The effect of microplastic fibers on soil agglomeration in the key process of controlling soil structure has been well studied.
Soil aggregates are an important structure that constitutes soil and play an important role in shaping the habitat of soil organisms. Carbon compounds are stored in aggregates, where they are physically protected from rapid decomposition.
Soil aggregates also determine the pore space of the entire soil, which in turn affects the movement of gas and water and the activities of related microbial communities. The plastic fibers in the soil form a lower soil density and have different indirect effects.
This change may result in faster plant growth. However, additives in microplastics can harm plants in which type of microplastics can promote or inhibit plant growth is an important research scope in the future.
Impact of microplastics on the cycle of other elements
The impact of microplastics on the cycle of other elements (nitrogen, phosphorus, etc.) is more uncertain. The direct impact may be insignificant because the impact of nitrogen and phosphorus in microplastics is almost negligible (even with exceptions, such as polyamide). But microplastics cause changes in soil structure, which will affect many microbial ecological processes, such as the nitrogen cycle.
Microplastics affect the anaerobic denitrification process (reduction of nitrate and nitrite to the gaseous form of nitrogen, including nitrous oxide and nitrogen) occurring in the center of soil aggregates.
The influence on the phosphorus cycle is still in the inference stage. For example, a study found that microplastics in the soil can cause an increase in arbuscular mycorrhizal fungi. This is a key symbiotic flora associated with plant roots, which may affect the cycle of phosphorus because these symbiotes will include phosphorus. Nutrients are transported to their plant host.
The presence of plastic film and fibers may change the movement and circulation of soil moisture, including evaporation. Therefore, microplastics are likely to affect soil hydrodynamics and energy balance by changing soil properties directly or indirectly through plant mediation.
The input of microplastics will also have other effects at the level of the soil ecosystem, including changes in the erosion rate due to microplastics affecting the stability of soil aggregates.
Key issues of microplastics
Before fully understanding the impact of microplastics on terrestrial ecosystems and the feedback mechanism of ecosystems on microplastics, some key issues need to be paid attention to.
For example, we need accurate, sensitive, low-cost, unified detection methods, and high-throughput processing techniques for soil samples to better understand the impact of microplastics on the turnover and transformation of soil elements. Also, research needs to cover more types of ecosystems.
Current research mostly focuses on farmland ecosystems that may contain the highest microplastics, which is caused by the use of large amounts of sludge, compost, and plastic films. However, we don’t know much about microplastics in other terrestrial ecosystems (such as drylands or forests). The differences in these ecosystems may lead to significant differences in the dynamic migration and transformation processes and ecological effects of microplastics.
The feedback of the earth system to microplastics is predictable. Microplastics themselves represent fossil carbon, which may indirectly affect net primary productivity and soil carbon storage and change the flux of greenhouse gases. The positive and negative effects, scale, and balance of these changes should be the focus of future research.
The outdoor natural ecosystem (mesocosm) based on carefully designed controlled conditions is an important means to solve these problems. Microplastic pollution is a global problem, and it is necessary to carry out international cooperative research to jointly cope with challenges and overcome scientific problems.