Occurrence and distribution of microplastics in organic fertilizers in China

The use of organic fertilizers is becoming a routine measure of soil improvement in China, and therefore organic fertilizers are considered an emerging contributor to microplastic accumulation in farmland soils. However, little is known regarding the microplastic contents in organic fertilizers used across China. Here, we investigated the occurrence and distribution of microplastics in 102 organic fertilizer samples across 22 provincial administrative regions of China to serve as a basis for the development of better microplastic management strategies. Organic fertilizers are made of a wide variety of feedstocks, including livestock manure (chicken, cow, goat, and pig manure) and bacterial residues etc., and are mainly produced via aerobic composting. In present study, microplastics within a 0.1-5 mm range in different types of organic fertilizers were separated using a general sequential density flotation method. Microplastic abundances ranged from undetectable to 2550 items/kg, with an average value of 325 ± 511 items/kg and a detection frequency of 80.4 %. The highest microplastic abundances were detected in Beijing City (758 items/kg) and in compound organic fertilizers (average of 386 items/kg). The microplastics in organic fertilizer were primarily white/transparent (75.9 %), 1-3 mm in size (55 %), and film-shaped (39 %). Interestingly, microplastics were not detected in cow dung fertilizer, suggesting that cow dung-based fertilizers might generally contain lower microplastic levels. Moreover, organic fertilizer microplastics tended to be concentrated in provincial areas with intensive agriculture, and the input flux of microplastics from organic fertilizer into fertilized soils reached an average of 5.07 × 10¹² items per year. Collectively, our findings provide key insights into the current state of microplastic levels in organic fertilizers in China and serve as a basis for the creation of novel microplastic mitigation strategies in farmland soils.