Microplastic contamination of soil: Are input pathways by compost overridden by littering?

Compost application is a widely recommended agricultural practice to improve soil fertility. As almost all compost is likely polluted with plastic we hypothesize that compost application is a major input pathway for microplastics (MPs) into agricultural soil. To attribute the plastic load of soil to compost application, we investigated MPs in topsoil (0-30 cm) of a controlled, long-term fertilizer trial with application of compost made of municipal biowaste (0, 5, 10, and 20 t ha^-1a^-1), which ended 11 years ago. Microplastics were analyzed via density separation (ZnCl₂) and light microscopy; testing this method recovered 92 ± 10 % of spiked plastic items. The fields of the long-term compost trial showed a MP load of 0-64 items kg^-1, corresponding to MP stocks in the plough layer (0-30 cm) that ranged from 38.2 ± 55.5 million to 171.4 ± 57.5 million items ha^-1. Microplastic stocks and contents increased with increasing amount of compost application. Thus, we confirm compost as a major input pathway for MPs into agricultural soil, with the effect still visible after 11 years. Comparison of calculated plastic input based on MP contents of recent German compost with MP loads found in soil revealed that overall compost application explained <6 % of total MP stocks. We assume that compost applied in earlier days contained higher plastic loads than recent ones, reflecting current awareness and successful efforts in reducing plastic loads during compost production. However, as the plots at the border of the field had up to 18 times higher MP loads than the inner plots of the trial, we suggest that littering also contributed significantly to MP pollution. Thus, even if given compost applications still add plastics to environment, other sources such as littering can already have become the dominating input pathway.