Coarse microplastic accumulation patterns in agricultural soils during two decades of different urban composts application

Plastic pollution, a global threat to environmental and human health, is now ubiquitous in the environment, including agricultural soils receiving urban compost amendments. Yet, the accumulation pattern of microplastics in soils are still to be disentangled, with regards to their sources and/or their physical properties such as morphotypes. The aim of this study was to identify the accumulation patterns of coarse microplastics (CMP) resulting from the long-term amendment of soil with urban waste composts. To this end, we used a field experiment receiving three different urban composts derived from municipal solid waste, biowaste, and a mixture of sewage sludge and green waste. We isolated 1417 coarse microplastic particles from a 21-year archive of soil and compost samples, using density fractionation followed by oxidation, and used Py-GC/MS for polymer identification. Different compost types led to different coarse microplastics accumulation levels. The accumulation pattern showed increasing CMP contents in soils over time. After 21 years of experiment, the calculated number of CMP was in accordance with the estimated values for all three compost types but it was not the case for the CMP mass. No difference of evolution pattern was found between films and fragments. We proposed that biotic transport or abiotic weathering and fragmentation could explain such differences in CMP evolution pattern.