Understanding the risks to soil and plant health following the use of microplastic contaminated digestate in agriculture

In the UK, biodegradable organic waste, such as food waste, can be converted into combustible gas known as 'biogas' through an anaerobic digestion (AD) process. Turning waste products into energy using AD processes prevents waste entering landfills and therefore is a favourable practice. The residue of the AD process, known as digestate, is also a nutrient rich substance and is often land applied to meet nutrient requirements (Barnes, 2020).

Digestate is nitrogen (N) rich and food waste digestates (FWD) typically contain higher quantities of ammoniacal nitrogen (NH4+-N) compared to other types of digestates and it has been widely shown that following application to land digestates can significantly affect N emissions to air and N leaching losses to nearby water bodies. For example, ammonia emissions were greater from FWD applications (~ 40% of total N applied) than from livestock slurry (~30% of total N applied) due to its higher ammonium-N content and elevated pH (Nicholson et al., 2017).

Recent research has also demonstrated that digestates also contain high levels of microplastic pollution (Porterfield et al., 2023). AD plants use mechanical debagging to split plastics from feedstock, operating this practice for both commercial food (manufacture and distribution) and separately collected household biodegradable waste. The potential for imperfect separation of food waste from plastic packaging therefore presents a pathway by which digestates can become contaminated with microplastics.

Research has shown that microplastics affect the biogeochemical cycling in soil by changing several soil physical, chemical, and biological properties (Lehmann et al., 2021; Wang et al., 2022). However, despite claims that digestates are a N rich fertiliser our current understanding is limited with respect to the role that microplastics play in influencing N dynamics in soil-plant systems.

This research gap is also true with respect to the potential impacts of microplastics on wider soil health metrics. For example, Arbuscular Mycorrhizal Fungi (AMF) play a crucial role in soil nutrient cycling, yet little is known about the effect of MPs on AMF diversity in a N rich soil environment (e.g., digestates amended soil).

Despite the potential wider agricultural risks associated with the presence of microplastics in digestates we currently lack evidence which can help better inform regulatory and policy developments on microplastic loads in digestates. This project has been designed in collaboration with the Environment Agency to generate crucial understanding needed to advance scientific developments in this area.

Aim and objectives:

The overall aim of this project is to advance our understanding of the impact of FWD containing microplastics on key soil health metrics. This will be achieved using a combination of an experimental approach in the laboratory and field trials utilising facilities available at the University of Leeds farm. The specific objectives of this study are:

[1] To characterise microplastic fragments (size, shapes, polymer types, and surface area) in FWD; [2] To evaluate the effects of microplastics in FWD on soil N dynamics following land application; [3] To assess the impact of MP contaminated FWD on the diversity and functioning of AMF.