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Microplastic fibres may be as damaging to soil ecosystems as drought.

Yudi Lozano at the Free University of Berlin and her colleagues suspected that microplastics may affect interactions between soil and water, so they examined the impact on grassland soils.

The researchers collected dry loam soil 鈥� which consists mainly of sand and silt 鈥� and mixed in polyester microfibres that were 1.28 millimetres in length. They planted seven grassland plant species in the soil, keeping some of the plants well-watered and growing others in drought-like conditions.

To simulate a moderately high level of microplastic pollution, the fibres were mixed in at a concentration of 0.4 grams of microfibres for 100 grams of dry soil. Microplastic fibres in strongly polluted areas have previously been recorded at levels up to 7 grams per 100 grams of soil.

The researchers focused mostly on the effects the microplastics had on the soil ecosystem rather than the effects on plant growth. They looked at respiration 鈥� the release of carbon dioxide from the soil 鈥� pH, how well it retained and cycled nutrients, and soil aggregation 鈥� its tendency to form clumps.

They also used a general index known as 鈥渆cosystem multifunctionality鈥�, which takes into account all the various ecosystem roles that soil performs.

The researchers found that microplastic fibres 鈥� which are similar in shape to fine plant roots 鈥� increased soil aggregation by 18 per cent, nutrient retention by 70 per cent and plant litter decomposition by 6 per cent.

However, under well-watered conditions, the presence of microplastic fibres decreased the functionality of soil enzymes and respiration. As a result, overall ecosystem multifunctionality was 34 per cent lower in some cases. The effects of the microfibres weren鈥檛 as significant under drought conditions.

The detrimental impact microplastics have on the ecosystem multifunctionality of well-watered soils is of a magnitude comparable to that caused by drought in non-polluted soils, the team says 鈥� and the problem will probably get worse as pollution levels rise.

鈥淚t is not necessarily the current levels of microplastic contamination that we should be most concerned about, but future levels,鈥� the researchers write.