Quantification and Identification of Microplastics in Organic Fertilizers: The Implication for the Manufacture and Safe Application

The application of organic fertilizers has been suspected as an important source of microplastic (MP) pollution in agricultural soils. However, limited data have been reported regarding the quantity and characteristics of MPs in organic fertilizers, giving rise to the difficulties in their risk assessment. This study investigated the occurrence of MPs in 23 commercial organic fertilizers and two farm composts, covering different raw materials and treatment processes. The average MP concentration in the commercial organic fertilizers was 9210.4 ± 1743.1 items/kg, significantly lower than that in farm composts (24344.1 ± 2697.2 items/kg). Among commercial organic fertilizers, the MP loads varied significantly with the source materials and their processing. Organic fertilizers derived solely or proportionally from plant-derived materials through mechanical shredding and sieving had lower MP abundances. The polymer sizes, morphotypes, and colors of MPs in studied samples were mainly < 3 mm, fiber, and black, respectively. The variations in the MP characteristics implied their diverse sources. This study provided a detailed assessment of MP accumulation in organic fertilizers, and confirmed their significance to the MP pollution in terrestrial ecosystem. The results also highlighted the importance of establishing standards to regulate the contents of plastics in raw materials and end products and the treatment processes for the manufacture, thus ensuring the safe application of organic fertilizers.