Article: Effects of Microplastics and Drought on Soil Ecosystem Functions and Multifunctionality by Lozano and colleagues (2021)
Background: Research suggests that there is an average of 8.3 million pieces of plastic in every cubic meter of ocean water (Brandon et al., 2019). The smaller pieces of plastic, termed microplastics (<5mm in diameter), come from sources such as car tires, synthetic clothings, cosmetic products, and the breakdown of larger plastics. Microplastics are ingested by filter feeders (e.g., oysters), work their way up the food web, and are eventually consumed by humans. Along the way, microplastics release harmful toxins, which is a process yet to be fully understood. Although many microplastics make their way to the ocean, others settle in soil and inhabit our terrestrial environments. New research by Lozano and colleagues sheds light on how microplastics in soil can interact with drought to threaten ecosystem functioning. Specifically, researchers analyze the effects of microplastics on nutrient cycling, decomposition of organic matter, and the ability of soil particles to clump together (i.e., soil aggregation). With better insights into these terrestrial processes, we will have a better idea of how microplastics will impact future soil-carbon emissions and nutrient availability for plant life.
Methods: Researchers established 20 experimental pots with soil (i.e., microcosm) and added polyester fibers to 10 of them. Seven common grassland plants were chosen to be grown in the pots. Half of the pots were well-watered, while the other half simulated drought conditions. After 3 months, Lozano and colleagues measured several indicators of nutrient cycling, decomposition, and soil aggregation.
Findings: Microplastics were shown to have different effects depending on the amount of water in the soil. In plastic-containing soil, nutrient cycling increased by ~75% in drought conditions. However, in plastic-free soil, nutrient cycling decreased when facing drought by ~39%. Further, plastic increased the rate of organic matter decomposition in well-watered soil, but decreased decomposition under drought conditions. With respect to soil aggregation, the presence of plastic increased soil-particle clumping in both well-watered and drought conditions by 15% and 21.7%, respectively.
Conclusions: Because of the complexity of the results, it may appear that plastic can provide a benefit to some ecosystem functions. For example, plastic fibers promote soil particle aggregation, which allows soil to hold more water in dry conditions and increases nutrient cycling. However, beyond looking at nutrient cycling, organic matter decomposition, and soil aggregation independently, Lozano and colleagues analyzed soil multifunctionality, or the ability of the soil to perform all of these processes at the same time. While having no significant effect on soils in drought conditions, microplastics had a significant negative impact on the multifunctionality of well-watered soils. Moreover, the effect of plastic-derived toxins on terrestrial ecosystem functioning is another factor yet to be considered, which is likely to negatively impact plant life and soil multifunctionality. Overall, unless we seriously curtail our use of plastics, the ability of soil to provide necessary ecosystem services is threatened, in turn affecting our global food and water supply.
Figure: Overall soil function (i.e., multifunctionality) in drought and well-watered soils with/without microplastics. Higher values indicate enhanced soil activity (e.g., nutrient cycling, organic matter decomposition, soil aggregation).
Reference:
Lozano, Y.M., et al. “Effects of microplastics and drought on soil ecosystem functions and multifunctionality.” Journal of Applied Ecology (2021).
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