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Particulate plastics in soil: friend or foe?

Particulate plastics in soil: friend or foe?

Plastics can pollute the air, land and water during all stages of their life cycle from manufacture to disposal. Synthetic plastics do not biodegrade and tend to accumulate in the environment. Particulate plastics of varying physical size fractions are becoming major sources of pollutants in terrestrial and aquatic ecosystems.

Particulate plastics are synthetic polymer particles measuring less than 5 mm in diameter. There are two types of particulate plastics that can enter the environment. Primary particulate plastics are manufactured and are a direct result of anthropogenic use of plastic-based materials (eg microbeads in cosmetics). Secondary particulate plastics are plastic fragments derived from the breakdown of larger plastic debris. An example of secondary particulate plastic input to soil is the breakdown and weathering of plastic mulch used mainly in agricultural and horticultural crops (Figure 1).

Photo of rows of lettuce, with plastic mulch covering the lettuce
Figure 1 Breakdown and weathering of plastic mulch is an important source of particulate plastic input to soil 

Both particulate plastics persist in terrestrial and aquatic ecosystems. Because plastics do not breakdown readily, they can be ingested and incorporated into in the tissues of some terrestrial and aquatic organisms.

Sources of particulate plastic contamination

Particulate plastics reach terrestrial ecosystems through their indiscriminate disposal in landfills, and through compost and biosolids application. Although medium and large-sized plastic materials are generally segregated during the composting process, a significant portion of small-sized plastics make it through the sieve and are still composted. Because compost is subsequently milled, most plastics end up as microplastics or nanoplastics. There has been renewed interest in the large-scale application of composts to soil, mainly to increase soil health and to improve carbon sequestration in soil.

Biowastes – including biosolids (treated sewage sludge) and composts – are excellent sources of nutrients and organic matter for agricultural and degraded soils. Although biowastes offer agronomic benefits, they also contain a number of contaminants, including heavy metals, pharmaceuticals, per- and poly-fluoroalkyl substances and particulate plastics. Particulate plastics end up in soils when biowastes are applied to land. It was estimated that more than 280 billion particulate plastics entered the Australian soils through biowaste application in 2017.1,2

The terrestrial ecosystem is a major source of the particulate plastics that reach aquatic ecosystems, through sediment transfer during soil erosion.

Impacts of particulate plastics in soil

Particulate plastics input to terrestrial ecosystem can have both beneficial and detrimental impacts on soil characteristics and organisms. For example, polyacrylamide is used to promote flocculation (particle clumping) and soil aggregation, thereby mitigating soil erosion.3 Particulate plastics in soils can serve as a hidden source of carbon sequestration, thereby contributing to climate change mitigation. However, since particulate plastics are introduced through human activities, it may not be considered a ‘direct action’ approach to mitigate climate change.

Most research on particulate plastics has focused on their detrimental effects in aquatic environments. The results indicate that particulate plastics can act as a vector for pollutants, and transport them long distances to affect aquatic environments.

Our research

Although particulate plastics are recognised as emerging contaminants in soils, their impact in the soil environment remains largely unclear, particularly on microbial functions and contaminant mobility. We quantified the amount of particulate plastics in biosolid samples. We also conducted a laboratory incubation study to examine the impact of particulate plastics on microbial activity and contaminant mobility in soil.

Particulate plastics were extracted from a number of biosolid samples. Two types of particulate plastics, pristine polyethylene (PPP) and surface-modified plastics (BSPPs), were used in the laboratory incubation study. The PPP (size ~100 μm – Microscrub®) was purchased. The BSPPs were prepared by spiking biosolids with PPP, facilitating the adsorption of biosolids-derived dissolved organic carbon onto PPP. Sandy soil samples collected from Grenfell, Sydney, New South Wales, were spiked with copper (500 mg copper/kg soil) and equilibrated for 2 weeks. Both the pristine and copper-contaminated soil samples were treated with PPP and BSPPs. These soil samples were subsequently analysed for bioavailable copper concentration, soil basal respiration, microbial biomass carbon and dehydrogenase activity.

The amounts of particulate plastics in the biosolids samples tested were:

  • 352 particles/kg for the <50 mm size fraction of particulate plastics
  • 146 particles/kg for 50–100 mm
  • 324 particles/kg for 100–250 mm
  • 174 particles/kg for 250–1000 mm.

The incubation study indicated that the bioavailability of copper in contaminated soil decreased with the addition of both PPP and BSPPs. The effect was more pronounced with the BSSP addition. This may be attributed to the adsorption of copper by the dissolved organic matter associated with the particulate plastics.

The results also showed that there was less soil basal respiration in copper-contaminated soil samples, suggesting copper is toxic to soil microbial activity. Adding particulate plastics resulted in an increased soil respiration in both uncontaminated and contaminated soils. The increase in microbial respiration due to the particulate plastic was higher in the BSPP-treated soil than in the PPP-treated soil. This suggests that soil microorganisms used the BSPP – the organic matter that had sorbed to particulate plastics – to increase their microbial activity. BSPPs are likely to interact with and retain the contaminants, thus reducing the toxicity to soil microorganisms. The improved soil aeration (porosity) caused by adding PPP or BSPPs could be another reason for the increase in the microbial activity.4 Similar to the soil respiration observations, copper contaminated and uncontaminated soils treated with particulate plastics appear to increase dehydrogenase activity and microbial biomass carbon.

Conclusions

Particulate plastics in terrestrial ecosystems can have both beneficial and detrimental effects on soil health. Our observations on soil basal respiration, dehydrogenase activity and microbial biomass carbon indicate that adding particulate plastics helps modulate contaminant toxicity on soil microbial activity. However, it is important to examine the long-term effects of particulate plastics on the microbial activity in soil health.

References

  1. Australia & New Zealand Biosolids Partnership (2017). Australian biosolids statistics. ANZBP.
  2. Wijesekara H, Bolan NS, Bradney L, Obadamudalige N, Seshadri B, Kunhikrishnan A, Dharmarajan R, Ok YS, Rinklebe J, Kirkham MB, Vithanage M (2018). Trace element dynamics of biosolids-derived microbeads. Chemosphere 199:331-339.
  3. Lee SS, Gantzer CJ, Thompson AL, Anderson SH (2011). Polyacrylamide efficacy for reducing soil erosion and runoff as influenced by slope. J Soil Water Conserv 66:172–177.
  4. Chen H (2016). Synergistic effects of microplastic and glyphosate on soil microbial activities in Chinese loess soil. Wageningen University, Wageningen, Netherlands.
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