Plastic mulch can lead to ingestion by humans
By GOLDSCHMIDT CONFERENCE
Despite its benefits, plastic mulch fragments are
difficult to entirely remove and negatively impact soil health over time.
Researchers are seeking solutions with manufacturers to address these
environmental concerns.
Researchers have identified an alarming source of plastic pollution in Californian strawberry farms. Plastic mulch, a common tool in promoting strawberry growth, has been found to shed significant amounts of plastic fragments into the soil.
This phenomenon adversely affects soil
qualities and calls into question the long-term sustainability of plastic mulch
usage. These findings likely have global implications for plastic use in
agricultural production.
Presenting their work at the Goldschmidt geochemistry
conference in Lyon, postdoctoral researcher Dr. Ekta Tiwari (from Sistla group
at California Polytechnic State University) said “What we are seeing a huge
quantity of macroplastic plastic material – particles bigger than 5mm across –
being shed where the mulch is used to enhance strawberry production. These can
remain in the soil for decades or longer.”
Plastics, such as polyethylene, are increasingly used in
agriculture, for example in polytunnels. Plastic mulch films are extensively
used in agriculture, where they provide a range of benefits. They are tucked in
around the base of the plant, which can help control weeds and pathogens,
reduce water evaporation, and prevent soil splashing on fruit (which is
particularly important for strawberries).
Researchers have discovered that plastic mulch, widely used to support the growth of strawberries in California, sheds significant amounts of plastic fragments into the soil. These fragments negatively impact soil quality and challenge the sustainability of plastic use in agriculture. The problem is exacerbated by the fact that even meticulous plastic mulch removal leaves fragments adhered to the soil, resulting in plastic accumulation over time.
The mulch is applied in rows and then removed after the crop’s seasonal production is complete. However, even careful land stewardship by farmers does not ensure all the plastic is removed because fragments get left behind and adhere to the soil during removal.
After decades of
annual plastic mulch application and removal, the researchers observed the accumulation
of plastic fragments within farm soils, even in really well-managed fields. The
researchers looked for macroplastics, which
are plastic pieces more than 5mm across.
Ekta Tiwari continued “We carried out a systematic survey
of strawberry fields after the seasonal removal of these plastic film. We found
that the distribution was fairly uniform. On field surfaces alone, we found up
to 213,500 macroplastic particles per hectare. That doesn’t include subsurface
particles, which we did not survey. In addition, we are currently analyzing the
same soil samples for microplastics, which
are smaller particles, less than 5mm across; these are not yet included in our
findings.”
Note: A hectare is 10,000 square meters. For comparison,
an average professional soccer playing pitch is around 7,100 square meters and
an American football field is 5,350 square meters.
Most of the particles are polyethylene (identified using
Fourier transform infrared spectroscopy). In preliminary findings, the
researchers found that as the levels of macroplastic pollution increased, soil
moisture content, microbial respiration, and plant-available nitrogen declined.
Dr. Tiwari added “The plastic mulch provides benefits,
but at the expense of long-term soil quality. It’s difficult and expensive to
remove these particles from the soil, so once they are there they can stay
there indefinitely.”
We tend to think that strawberries are simply things to
be enjoyed, but this shows that even something as delicious as fresh
strawberries can come with a cost to the environment. We are working with the
manufacturers to see if we can mitigate these costs.”
There are alternatives to using polyethylene mulches,
such as biodegradable plastic mulches, or natural mulches such as straw, but
these choices come with an economic l cost. However, the use of plastics
in agriculture is also increasingly regulated, see for example European Union
information at https://environment.ec.europa.eu/system/files/2021-09/Agricultural%20Plastics%20Final%20Report.pdf
Commenting, Professor Sean Schaeffer (Department of
Biosystems Engineering & Soil Science, University of Tennessee, USA)
said:
“Plastics, and plastic mulches in particular, are vital to maintain agricultural production. They are used for various purposes including soil moisture retention, soil warming/cooling, as well as weed or pest control. Agricultural plastic use is increasing worldwide, with California being the largest user of agricultural plastic in the US.
Research on the fate
and transport of plastics in soil and water systems is relatively recent, so
studies like this are vital to increasing our understanding of the scope the
plastic problem. We currently know relatively little about the distribution,
size, and types of plastic in soils in the largest states, both in land area
and in agricultural production.”
This is an independent comment, Professor Schaeffer was
not involved in this work.
This work is ongoing and has not yet been peer-reviewed. The researchers are currently evaluating the level of microplastic pollution (particles smaller than 5mm) left by plastic mulches.
This study provides
baseline data to understand the extent of plastic pollution in the U.S.
agriculture system and can help improve land management practices by assessing
the biogeochemical consequences of plastic accumulation in agricultural soils.