Good evidence about the major sources of microplastics in agriculture and suitable mitigation options is still scarce. This new study from the world’s oldest and most famous long-term experiment, Broadbalk at Rothamsted Research, is therefore very interesting. Unfortunately, the conclusions drawn in this paper are somewhat flawed or can easily be misunderstood, particularly with regard to the role of inorganic fertilizers for increasing levels of microplastics found in the soil.
The graph showed the microplastic concentrations in agricultural soil samples collected from the 0–23 cm depth of the Broadbalk winter wheat experiment between 1846 − 2022. They recorded microplastic concentrations across three treatments with FYM as farmyard manure at 35 t/ha/yr; Nil as no soil amendments; and N3(P)KMg at 144 kg N/ha, 35 kg P/ha (until 2000), 90 kg K/ha, 12 kg Mg/ha (between 1974 −2000).
We can derive the following from the paper and its key figure:
- No microplastics were found until the 1920s because synthetic plastics were only invented in the early 20th century.
- A gradual increase in microplastics occurred during the 1920s to 1990s in all treatments, even in the control plot that had never received fertilizer or manure inputs since 1843. This mainly reflects on the increasing natural abundance of microplastics in the environment and thus its deposition in the soil.
- A steep increase since the 1990s in all treatments, but particularly in the mineral fertilizer and manure treatments, with no difference between the latter two. This illustrates the rapid acceleration of plastic use in the food chain, although the specific sources are not known. For example, polymer-coated fertilizers have never been used in long-term experiments as potential and direct sources of microplastic addition. There is clearly an increase in microplastics, but no specific evidence that fertilisers have contributed to this shift. When interpreting the results, it is also important to keep in mind that there is no widely accepted standard method for extracting and measuring microplastics in soils.
This paper provides the unique and first evidence of accumulating particles to appear as microplastics. Yet, a lot more research is required to confirm these results under different conditions and to identify the main sources of microplastics detected in agricultural systems.
