More and more plastic residues are accumulating not only in the groundwater, but also in the arable land, among other things from years of fertilization with sewage sludge. Studies now show that this plastic is still preserved almost undiminished in the ground 30 years later. The large amounts of plastic in farmland can affect fertility, but it can also reach us humans through the food chain.
Constant exposure: Every time we take a shower or do our laundry, microplastic particles are released from the shampoo bottle or from our clothes and pollute the groundwater. The plastic particles find their way into every ecosystem, no matter how remote, and have already been detected in the depths of the Mariana Trench and at the top of Mount Everest. However, not only bodies of water, but also agricultural areas are affected by microplastic pollution. One of the sources of this plastic pollution in agriculture is soil fertilization.
What happens to plastic in the ground?
Farmers often use the waste from sewage treatment plants for soil improvement and fertilization. But in addition to valuable nutrients and organic substances, this method also introduced the plastic into the soil. Because sewage sludge contains an average of almost 1,500 plastic particles per kilogram, as we know today. Due to the sometimes high level of heavy metals and other contaminants in the sewage sludge, its application is now strictly regulated. Sewage sludge is therefore hardly ever used for fertilization today.
But what happened to the plastic particles that were spread out on the fields back then? Collin Weber and his colleagues Peter Chifflard and Alexander Santowski from Phillips University in Marburg have now investigated this. “What happens to the particles after they get into the agricultural landscape – whether the plastic is broken down or distributed spatially – has so far remained unclear,” says Chifflard. To find out, the scientists took a close look at agricultural areas at the University of Giessen in central Hesse, for which detailed records of use are available. “Sewage sludge has not been used on the examined areas since the mid-1980s,” reports Santowski.
High plastic pollution to date
The results show that 30 years in the life cycle of a plastic is not a long time: The surface of the farmland that used to be fertilized with sewage sludge still has a high density of macroplastics – i.e. plastic particles from a size of five centimeters. If you dig up to ninety centimeters deep in the ground, you will find up to 56 plastic particles per kilogram of dry matter, as the researchers report. The contamination with microplastics was also significantly higher on the fields previously fertilized with sewage sludge than on the untreated fields.
Weber reports that there are also certain patterns in the distribution of the plastic on the arable land: “The areas with direct sewage sludge input contain the most plastic – there is significantly less all around”. The plastic particles therefore remain in the same place over a longer period of time and are only distributed to a limited extent. According to the researchers, the fact that they are spreading at all is probably mainly due to plowing the surface and less to natural causes such as wind or erosion.
Regulation comes too late
The large and small plastic particles therefore remain in the earth for a very long time. So what does this mean for agriculture? According to Weber, it cannot be ruled out that the pollution also affects the function of the arable land. “Previous studies have already shown that the presence of plastic and microplastics in the soil negatively affects soil properties, soil organisms and plant growth,” report Weber and his colleagues. Additionally, when plants ingest the plastic particles, they could enter the food chain and cause health problems in humans.
Weber and his colleagues conclude that new regulations and strategies to avoid plastic in farmland are already too late. The plastic lasts for decades in farmland, which has long since become a reservoir for man-made plastic pollution.
Source: Phillips University of Marburg; Specialist articles: Scientific Reports, doi: 10.1038/s41598-022-10294-w