What do we know and don’t know about soil fauna communities’ response to soil pollution?
Dr. Léa Beaumelle
Postdoctoral Fellow, INRAE, France
Figure 1. Soils can be visibly affected by human and industrial activities (A): dumping site in Ukraine, (B): industrial soil pollution in China), but pollution and contamination are often invisible, (C): site historically polluted with metals but hosting metal-tolerant, plant species and (D): site formerly affected by iron industry with a diversified vegetation.
Photo credits: (A) Andy Shustykevych, Creative Commons Attribution-Share Alike 3.0 Unported license; (B) JungleNews, Creative Commons Attribution Share Alike 4.0 International License; (C) Guillaume Lemoine; (D) Apolline Auclerc (Beaumelle et al., 2021).
Soil pollution or contamination represent invisible threats for soil biodiversity (Fig 1). Contamination means that substances such as metals, pesticides, or pharmaceuticals are present in the soil at higher than normal concentrations. We talk about pollution when those concentrations are associated with adverse effects on organisms and ecosystems. When we think about pollution we often imagine something very visible, such as plastic accumulation, or industrial wastes (Fig 1A, B). But often times, contamination and pollution are invisible to the naked eye (Fig 1C, D), yet they represent a considerable threat for soil health (FAO, 2018).
The adverse effects of contaminants are most often studied in simplified experimental conditions, at the organism and population levels. Such studies are highly valuable to derive standard toxicity values and understand the mechanisms by which contaminants affect organisms. However, in real-world scenarios, species are not alone in their test tubes, they interact with each other and with their abiotic environment. At this ecological scale, the adverse effects of contaminants can emerge through indirect effects mediated by species interactions. For example, in a Japanese lake, a recent study found that since farmers started using neonicotinoids, fisheries yield decreased due to strong declines in the biomass of zooplankton, an important food source for fish (Yamamuro et al., 2019).
Such ecological, cascading consequences of contaminants have received a lot of attention in recent studies in aquatic ecosystems. As a soil ecologist, I started wondering how much we do and do not know about the ecological consequences of contaminants for natural soil fauna communities (all interacting species in a given area): What are the main knowledge gaps and how to overcome them in future research?
To answer these questions, I surrounded myself with great scientists from my former team in Germany, all having different perspectives to soil ecology (global scale macroecology, trophic interactions and foodweb, plant-soil interactions, and biodiversity-ecosystem functioning research). Together, we compiled the most comprehensive database to date, gathering all the studies that have been published about soil contaminants effects on natural soil fauna communities. We read the papers and collected information such as the geographic location, the taxa groups and contaminants considered. Based on the information available in the literature, and our collective expertise, we then proposed future research avenues to better forecast soil fauna responses to soil pollution (Beaumelle et al., 2021).
Figure 2. Geographical coverage of the 274 studies, by country where the studies were conducted showing limited knowledge in the Global South (Beaumelle et al., 2021).
We ended up with 274 studies spanning a wide range of contaminants (metals, pesticides, pharmaceuticals, antibiotics, hydrocarbons etc.) and an even wider range of soil fauna groups (earthworms, nematodes, mites, springtails, etc.). They used all sorts of approaches, comparing soil communities in contaminated or polluted areas versus reference areas, or using experiments in the lab or field. The results revealed that most research is located in the Northern Hemisphere (Fig. 2). A better global coverage of soil fauna responses to pollution is necessary, because many countries in the Global South are threatened by soil pollution (FAO 2018), and because species sensitivities could be different in warmer and drier climates compared to temperate areas.
We also found that few studies are yet available to anticipate the combined effects of contaminants and climate change on soil fauna communities. An important question for future studies is to better understand how soil communities in contaminated ecosystems will be able to cope with climate change. Will they be more sensitive to climate change, or less sensitive compared to un-polluted ecosystems? Those are crucial questions that require complex experiments and highly collaborative research.
Our study revealed many other knowledge gaps, as well as many exciting opportunities for future research. For example, we found a tremendous amount of studies on nematodes response to metals in soil. Compiling data from those papers could yield important insights to anticipate nematode responses to these contaminants. For us, the next step will be to summarize the results from our database of 274 studies, using an approach called meta-analysis (Phillips et al., 2019). Meta-analysis will allow us to compile together the results from all those different studies, and ask questions such as: which soil fauna groups are the most sensitive across all studies? Which soil contaminants have the strongest negative impacts on soil fauna communities?
In the end, I hope you remember one thing from this blog post, is that science is not always about experimenting in the lab to test new hypotheses. Sometimes compiling existing research is a fantastic source of inspiration to create new research ideas, based on all that we already know, and all that we don’t.
References
Beaumelle, L., Thouvenot, L., Hines, J., Jochum, M., Eisenhauer, N., & Phillips, H. R. P. (2021). Soil fauna diversity and chemical stressors: A review of knowledge gaps and roadmap for future research. Ecography, 44(6), 845-859. https://doi.org/10.1111/ecog.05627
Rodríguez-Eugenio, N., McLaughlin, M. and Pennock, D. 2018. Soil Pollution: a hidden reality. Rome, FAO. 142 pp. ISBN: 978-92-5-130505-8
Phillips, H. R. P., Beaumelle, L., Tyndall, K., Burton, V., Cameron, E., Eisenhauer, N., & Ferlian, O. (2019). The effects of global change on soil faunal communities: A meta-analytic approach. Research Ideas and Outcomes, 5, e36427. https://doi.org/10.3897/rio.5.e36427
Yamamuro, M., Komuro, T., Kamiya, H., Kato, T., Hasegawa, H., & Kameda, Y. (2019). Neonicotinoids disrupt aquatic food webs and decrease fishery yields. Science, 366(6465), 620–623. https://doi.org/10.1126/science.aax3442