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Effects of microplastics in soil ecosystems: above and below ground
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Bas Boots*, Connor William Russell, Dannielle Senga Green,
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Applied Ecology Research Group, School of Life Sciences, Anglia Ruskin University,
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Cambridge, CB1 1PT, United Kingdom
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*corresponding author at:
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School of Life Sciences
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Anglia Ruskin University,
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East Road,
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Cambridge,
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CB11PT
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United Kingdom
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Email: boots@anglia.ac.uk
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Keywords: Soil, Plants, Perennial ryegrass, Earthworm, Microplastics, Polylactic acid, High-
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density polyethylene, Synthetic fibres.
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Highlights:
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Microplastics may affect important processes in soil ecosystems.
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HDPE, PLA and fibres affected initial growth of perennial ryegrass.
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Plant primary production may be affected by microplastics in soil.
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Earthworm biomass gain was negatively affected by microplastics.
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Microplastics may alter soil structural stability.
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Abstract
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Environmental contamination by microplastics is now considered an emerging threat to
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biodiversity and ecosystem functioning. Soil ecosystems, particularly agricultural land, have
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been recognised as a major sink of microplastics, but the impacts of microplastics on soil
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ecosystems (e.g. above and below ground) remain largely unknown. In this study, different
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types of microplastics (biodegradable polylactic acid (PLA), conventional high-density
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polyethylene (HDPE) and microplastic clothing fibres were added to soil containing the
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endogeic Aporrectodea rosea (rosy-tipped earthworm) and planted with Lolium perenne
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(perennial ryegrass) to assess biophysical soil response in a mesocosm experiment. When
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exposed to fibres or PLA microplastics, fewer seeds germinated. There was also a reduction in
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shoot height with PLA. The biomass of A. rosea exposed to HDPE was significantly reduced
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compared to control samples. Furthermore, with HDPE present there was a decrease in soil pH.
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The size distribution of water stable soil aggregates was altered when microplastics were
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present, suggesting potential alterations of soil stability. This study provides evidence that
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microplastics manufactured of HDPE and PLA, and synthetic fibres can affect the development
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of L. perenne, health of A. rosea and basic, but crucial soil properties, with potential further
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impacts on soil ecosystem functioning.
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1. Introduction
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Microplastics have been found to contaminate a wide range of aquatic environments around the
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world
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negatively affecting a wide range of organisms and have received much scientific
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attention over the last decade. There are now studies that have reported microplastics present
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in soil environments
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. Soils may represent a large reservoir of microplastics
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, with sources
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such as sewage sludge applied as fertiliser
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, fallout from the air
reference8
, and in precipitation
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therefore microplastics may pose a threat to soil biodiversity and ecosystem functioning
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, but
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there is still a dearth of information
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.
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Soil fauna are critical for maintaining a healthy soil
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. Earthworms are arguably one of the most
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important and are considered key ecosystem engineers
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and bio-indicators of environmental
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quality
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. Through their feeding, burrowing and casting behaviour, earthworms break down
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organic matter, turn over nutrients and aid in the structural development of soil aggregates
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.
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Of particular interest are endogeic species, such as Aporrectodea rosea, which are numerically
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dominant in temperate agroecosystems
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. Intensive farming can result in reduced soil health,
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including less organic matter and can lead to deterioration of soil structure
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.
With potentially
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increasing contamination by microplastics, soil fauna may be exposed to further stress.
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Outside of landfills and industrially intense areas, other terrestrial habitats, such as
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agroecosystems, are likely to be exposed to microplastics
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manufactured of a myriad of
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different polymers. In European agricultural land, microplastic loadings have been estimated at
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between 63,000 to 430,000 tonnes year
-1
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, with studies reporting anywhere between 700-4000
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plastic particles kg
-1
of soil
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and, by dry soil weight, up to 7% microplastic fragments has
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been reported
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Microplastics are thought to accumulate in soils
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with sources of microplastic
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pollution to agroecosystems typically derive from agricultural practices, such as the use of
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plastic mulches
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the spreading of sewage sludge
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and during the irrigation of land
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. Many
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plastic items are manufactured from durable polymers, such as polyethylene (e.g. high-density
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polyethylene), which is not considered biodegradable and can persist in the environment for
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decades
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Increasingly, biodegradable polymers, such as polylactic acid (PLA), are becoming
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a common alternative to conventional agricultural mulches
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, but the degradation of many
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biodegradable polymers under ambient conditions has proven to be lengthy or incomplete
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.
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Agricultural land that has not been exposed to the application of plastic mulches and biosolids,
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may still come into contact with microplastics during the irrigation of crops, with some
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microplastics bypassing the treatment process at waste water treatment works
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.
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Recent research has shown that once in the soil, microplastics can easily be ingested by soil-
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living organisms, potentially affecting their fitness and survival
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. To date, the ingestion of
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microplastics in the soil has been demonstrated in the anecic earthworm Lumbricus
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terrestris
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, with earthworms displaying reduced growth rates after 60 days exposure to
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polyethylene microplastics at concentrations ranging from 0.2 to 1.2% in dry bulk soil
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. Maaß
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et al.
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reported that springtails aid in moving microplastic particles through the soil matrix,
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therefore potentially contributing to the bio-availability of microplastics to the soil food web.
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Despite this, minimal research has explored the effects of microplastics on other important
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aspects of the soil environment, including effects on plant development
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. De Souza Machado
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et al.
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reported that different microplastics can affect several below ground processes
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such
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as soil structure and microbial activity, and physiological components of Alium fistulosum
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(spring onion) when grown in presence of microplastics
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They have proposed a conceptual
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model that links soil biophysical processes to plant performance, in which microplastics alter
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several aspects of the soil environment with cascading effects on soil biota, including plants
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.
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The current knowledge on the impacts of microplastics on soils (physico-chemical
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characteristics and structure) and its associated biota (above and below ground) currently
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remains inadequate to fully address the risks to the terrestrial environment.
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This study, therefore, was set up to assess the above and below ground responses to microplastic
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contamination of soil ecosystems, using the endogeic earthworm Aporrectodea rosea (rosy-
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tipped earthworm) and soil sown with Lolium perenne (perennial ryegrass). The effects of
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synthetic fibres (acrylic and nylon mixture), and microplastics manufactured of conventional
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high-density polyethylene (HDPE) or biodegradable polylactic acid (PLA) were assessed using
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mesocosm systems, providing realistic, but controlled, semi-natural conditions. The experiment
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tested the hypotheses that the addition of synthetic fibres, HDPE and PLA microplastics to soil
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would alter the (i) seedling growth and germination of L. perenne, (ii) shoot and root biomass,
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and root/shoot ratio of L. perenne, (iii) total chlorophyll, chlorophyll-a and -b contents and
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chlorophyll a/b ratio of L. perenne as a potential stress response, (iv) growth of A. rosea and
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(v) pH, organic matter content and stability of soil, with regards to soil aggregate distribution
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and aggregate mean weight diameter.
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2. Materials and Methods
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2.1 Experimental design and set-up
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For this experiment, L. perenne was chosen as a model species on the basis that it is one of the
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most important grass species in temperate regions
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, providing high yields and quality forage
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throughout a wide range of environmental conditions
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. The endogeic earthworm A. rosea was
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chosen as a model species because of its natural abundance in grassland ecosystems
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. The
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experiment was conducted under laboratory settings at Anglia Ruskin University, Cambridge,
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United Kingdom and followed an orthogonal, fully crossed design with two factors, “PLANT”
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and “PLASTIC”. The factor “PLANT” had two levels: Planted or Unplanted; the factor
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“PLASTIC” had four levels: Fibres (synthetic fibres), HDPE (high-density polyethylene), PLA
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(polylactic acid) and a control (Control). All treatments were replicated five times (n = 5, N =
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40).
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The mesocosms were created using clean, opaque polypropylene plant pots with a 1.3 litre
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capacity (height = 13.0 cm, top diameter = 12.5 cm, bottom diameter = 10.2 cm). Each
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mesocosm was filled with top soil sourced from Westland Garden Health (Dungannon,
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Northern Ireland). Top soil was chosen to represent similar soil conditions in which A. rosea is
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commonly found. The topsoil was a sandy clay loam soil composed of 18.6 ± 0.7% (mean ±
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SEM, n =3) organic matter and a pH of 6.9 ± 0.01 (mean ± SEM n = 3). All soil was air dried
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for 24 hours, before being manually sieved through a 2000 µm mesh to remove any stones and
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homogenise the soil.
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Prior to filling mesocosms with soil, virgin HDPE (density of 0.95 g cm
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) or PLA (density of
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1.2 – 1.3 g cm
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) microplastics or synthetic fibres were thoroughly mixed and homogenised by
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