From Sea to Land: Setting a Size Definition of Plastics for Soil Ecosystem Studies

Abstract

In soil studies, the current definition of microplastics as particles <5 mm was adopted directly from marine research. To our opinion, a more precise and differentiated size definition is needed to focus studies on specific challenges plastics cause for soil ecosystems. As relevant soil functions such as water, carbon, and nutrient retention and provision are mainly controlled by soil structure, biota, and chemical processes dominantly appearing in the micro- to nanoscale, we suggest adapting size ranges of plastics to the respective process scales in soil ecosystem studies. Even more, we expect that larger particles will not be incorporated into soil until they reach a size threshold compatible to soil structure (<1000 µm, depending on soil properties). Redefining plastic sizes in accordance with soil processes and the International System of Units (SI) should be implemented to focus research. A unified definition of microplastics (1–1000 µm) and nanoplastics (1–1000 nm) will set a standard to further allow relating plastic sizes across research disciplines.