Articles

Soils are basically the main source of life of all terrestrial ecosystems and one of the most important natural sources for human society. They provide useful services for the ecosystem such as carbon sequestration from the atmosphere, the removal of pathogens and pollutant agents from the water, the recycling process of organic waste and, obviously, they are the basis for the production of native crops and plants from which both humans and animals depend. An important push factor behind these services is the activity of soil microorganisms, which catalyze many of the bio-geochemical transformations. On its part, these microorganisms’ function is highly influenced by the physical and chemical conditions that these organisms are exposed to. For instance, it’s well known that soils disposal in different functional aggregates, Ad esempio, è noto che la disposizione dei suoli in vari aggregati funzionali, pores’ room configuration and hydrological properties influence microorganisms’ metabolic rates and the decomposition of organic matter. In other words, the fundamental properties that influence the biophysical environment of the soils are also important for soil processes and functions.

Our mechanistic comprehension of the effects of ecosystems stress factors, such as the presence of microplastics on functional soil changes is delayed compared to the studies about water ecosystems and soil biophysical factors can be the focus for the comprehension of these effects.
Soils could be even more prone to pollution caused by microplastics compared to the oceans, since there is a more consistent plastic release in terrestrial ecosystems.

Once microplastics are abandoned in the upper soils, their fragmentation and aging process take place due to the exposure to UV-lights, as well as to the increased availability of oxygen and temperature. These fragments can vertically migrate across the soil profile and horizontally along the soil surface, leading to the spread of contamination. This contamination can reach up to deep soil layers, groundwater and aquatic ecosystems. Indeed, intense anthropogenic activities can stimulate the dispersion of microplastic pollutants, which can influence the bio-geochemical properties of the soil and biodiversity.

Microplastics can also affect soil physical characteristics, including the bulk density and water dynamics, interestingly reducing the overall bulk density of the soil but at the same time increasing the density of the rhizosphere (the portion of soil surrounding plants’ roots). This ‘ecosystem engineering’ may be partially responsible for the effects of microplastics on soil microbial communities and plants. Plant roots can absorb very small plastic particles (nanoplastics) and their roots can suffer physical damage due to absorption of the same particles. Plastic can also serve as a substrate for the growth of soil microorganisms or even as a matrix for their extensive colonization.

Researchers suggest that the way microplastics affect the systems plants-soil causes a cascade of events that alter the biophysical environment of the soil. For instance, the changes in the soil structure and composition can affect pores’ room and their connection, also influencing their  capacity to retain water and their permeability. An increased water evaporation causes the reduction of the available amount of water in the soil (also affects biological processes such as roots growth and the microbial activity), influencing in this way the frequency and the intensity of the dry/wet  cycles between the soil. These cycles regulate the expansion and the contraction of soil particles, and therefore the soil structure. Plants, on their part, adapt their traits to the new biophysical soil condition, changing both in their shape and in their function.

Once released into the environment, microplastics interfere with normal life cycles, so much so that we can speak of a real “plastic cycle” that generates many retroaction effects on the ecosystem, including – in addition to the modification of the soil structure and carbon sequestration – an alteration of the phosphorus and nitrogen cycles, essential components for the balance of the entire ecosystem.

However, a direct relationship between microplastics and desertification has not been found yet. But, if we think about how they change the structure and ecosystem of the soil, it is inevitable to think of desertification as a possible consequence. As with all the environmental problems that we live in the present: do we have to wait to see the consequences of our behavior or do we begin to do prevention to avoid them?

SOURCES:

Maxwell S. Helmberger, Lisa K. Tiemann, Matthew J. Grieshop. Towards an ecology of soil microplastics- Review. Functional Ecology. 2020;34:550–560. 

Microplastic effects on plants. Matthias C. Rillig, Anika Lehmann, A. Abel de Souza Machado and Gaowen Yang. New Phytologist (2019) 223: 1066–1070 

de Souza Machado, A. A. Lau, C. W.; Kloas, W.; Bergmann, J.; Bachelier, J. B.; Faltin, E.; Becker, R., Goerlich, A. S., and Rillig, M. C. (2019). Microplastics can change soil properties and affect plant performance. Environmental Science and Technology. 2019, 53, pp6044-6052. 

Anderson Abel de Souza Machado, Chung Wai Lau, Jennifer Till, Werner Kloas, Anika Lehmann, Roland Becker, and Matthias C. (2018). Rillig. Impacts of Microplastics on the Soil Biophysical Environment. Environmental Science and Technology. 2018, 52, pp 9656-9665.

https://ilbolive.unipd.it/it/news/inquinamento-bianco-pericoli-microplastiche-nei

Young-Nam Kim, Jung-Hwan Yoon, Kye-Hoon Kim (2020). Microplastic contamination in soil environment – a review. Soil Science Annual  2020, 71(4), 300–308