Understanding the risks to soil and plant health following the use of microplastic contaminated digestate in agriculture
Background:
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.
Training:
The student will work under the supervision of Dr Laura Carter and Dr Paul Kay within the River Basin Processes and Management research cluster in the School of Geography, University of Leeds. The successful candidate will develop a range of research skills, including experimental design, field sampling, chemical analysis, statistical analysis and data interpretation, academic writing skills and giving presentations. The successful candidate will benefit from inter-disciplinary training in analytical techniques and chemical fate and effects, as well as wider water management skills. Training will be provided in field/laboratory health and safety procedures and the use of field and analytical equipment.
Supervision will involve regular meetings between all supervisors including the Environment Agency who will contribute to PhD supervision and project design over the course of the PhD. Students present results and receive constructive wider feedback from peers in a Research Support Group and at the University postgraduate research days. The successful applicant will, under guidance of the supervisory team, contribute to all aspects of the research including designing the experiments, laboratory analysis and data collection. The student will be encouraged to write and submit papers for publication during the project and to attend national and international conferences to present results and gain feedback from the wider academic community.