Intensive industrial-scale agriculture has led to the rise in the use of pesticides and fertilizers, to maintain high yields to feed a growing global population and appetite. Increases in land conversion and intensive agriculture have contributed to a decline of approximately 60% of soil’s ecosystem services (Gunstone et al., 2021), which adversely affects human and ecosystem wellbeing.
Pesticides are one of the most common soil contaminants and include chemical or biological agents intended for repelling, weakening and killing pests, such as weeds and insects. Based on the target pests, pesticides can be divided into several groups including insecticides, herbicides, rodenticides, fungicides, and larvicides. Pesticide use per unit area of cropland has been increasing globally across all countries, and will likely continue, especially in Asia and America (Fig 1).
Figure 1. Pesticide use per area of cropland. Source: Raffa & Chiampo (2021), data from FAO.
How do pesticides cause soil pollution?
Pesticides are often applied to the soil directly as seed coatings, granules, or drenches. After being sprayed on foliage, pesticides can wash off into soil through overhead irrigation or rainfall. Accidental leaks of pesticides such as from spills, damaged containers, or improper disposal also can occur. The mobility and persistence of pesticides in soil depend on their half-life, chemical characteristics, soil properties, and climatic conditions. Generally, pesticides that have high bioavailability & solubility and low persistence degrade quickly.
Source: Mary-Elika Foundation
However, those that are persistent in the environment can interfere with soil biogeochemical cycles, soil enzyme activity and may lead to acute/chronic effects on soil microorganisms. Organochlorine pesticides are among the most dangerous chemical pesticides because of their long degradation time and semi-volatility which enables them to volatise from the soil to pollute the air or water (Wolejko et al., 2020).
Unsung soil heroes
Efforts to mitigate pesticide contamination are largely focused on reducing water and air pollution. The impacts of pesticides on soil invertebrates and therefore overall soil health has been rather overlooked. In many industrialized countries like the US, the only terrestrial invertebrate pesticides are tested on before commercialization is commercially important honey bees (Decourtye et al., 2013). However, soil invertebrates, from beetles to termites are key to maintaining soil ecosystem services. For example, an earthworm can decrease soil erosion by up to 50% by the construction of channels (Wolejko et al., 2020). Therefore, soil species, including microorganisms, should be the focus when testing pesticide safety and when addressing pesticide-contaminated soils.
A review of 394 studies by Gunstone et al (2021) covering 275 soil invertebrate species and 284 pesticides found that pesticide exposure negatively impacted soil invertebrates in 70.5% of the 2,842 tested parameters. Tested parameters refer to the measurement of a specific endpoint following exposure of a specific pesticide to a specific organism, such as mortality or DNA damage. This study highlights how vital soil organisms are harmed by soil pollution caused by pesticides.
Extensive use and misuse of pesticides need to be curbed to save our soils. In China, for example, year-round planting of crops requires large-scale pesticide use which leads to soil pollution, resulting in more pesticide application, thus causing a vicious cycle (Meng, 2012).
References
Arias-Estévez, M., López-Periago, E., Martínez-Carballo, E., Simal-Gándara, J., Mejuto, J.-C., & García-Río, L. (2008). The mobility and degradation of pesticides in soils and the pollution of groundwater resources. Agriculture, Ecosystems & Environment, 123(4), 247–260. https://doi.org/10.1016/j.agee.2007.07.011
Decourtye, A., Henry, M., & Desneux, N. (2013). Overhaul pesticide testing on bees. Nature, 497(7448), 188–188. https://doi.org/10.1038/497188a
FAO and UNEP. (2021). Global assessment of soil pollution: Report.
Gunstone, T., Cornelisse, T., Klein, K., Dubey, A., & Donley, N. (2021). Pesticides and Soil Invertebrates: A Hazard Assessment. Frontiers in Environmental Science, 9. https://www.frontiersin.org/article/10.3389/fenvs.2021.643847
Meng, Y. (2012, September 7). The damaging truth about Chinese fertiliser and pesticide use. China Dialogue. https://chinadialogue.net/en/pollution/5153-the-damaging-truth-about-chinese-fertiliser-and-pesticide-use/
Raffa, C. M., & Chiampo, F. (2021). Bioremediation of Agricultural Soils Polluted with Pesticides: A Review. Bioengineering, 8(7), 92. https://doi.org/10.3390/bioengineering8070092
Wołejko, E., Jabłońska-Trypuć, A., Wydro, U., Butarewicz, A., & Łozowicka, B. (2020). Soil biological activity as an indicator of soil pollution with pesticides – A review. Applied Soil Ecology, 147, 103356. https://doi.org/10.1016/j.apsoil.2019.09.006