Micro and macroplastics are emerging contaminants in agricultural settings, yet their impact on nitrogen (N) cycling and partitioning in plant-soil-microbial systems is poorly understood. In this mesocosm-scale study, spring barley (Hordeum vulgare L.) was exposed to macro or microplastic produced from low density polyethylene (LDPE) or biodegradable plastic at concentrations equivalent to 1, 10 and 20 years of plastic mulch film use. Partitioning of ¹⁵N-labelled fertiliser into plant biomass, soil and leachate yielded a partial mass balance. Soil N partitioning was probed via compound-specific ¹⁵N-stable isotope analyses of soil microbial protein. Concentration-dependent decreases in plant ¹⁵N uptake occurred with increased leached nitrogen for LDPE microplastic. Assimilation into soil microbial protein was higher for biodegradable plastics, which we associate with early-stage biodegradable plastic degradation. Partitioning of ¹⁵N into inorganic soil N pools was affected by LDPE size, with lower assimilation into the microbial protein pool. While microplastics and macroplastics altered soil N cycling, the limited impacts on plant health indicated the threshold for negative effects was not reached at agriculturally relevant concentrations. This study highlights the difference between conventional and biodegradable plastics, and emphasises that the interplay of micro and macroplastics on soil N cycling must be considered in future studies.

Over the last 50 years, the intense use of agricultural plastic in the form of mulch films has led to an accumulation of plastic in soil, creating a legacy of plastic in agricultural fields. Plastic often contains additives, however it is still largely unknown how these compounds affect soil properties, potentially influencing or masking effects of the plastic itself. Therefore, the aim of this study was to investigate the effects of pure plastics of varying sizes and concentrations, to improve our understanding of plastic-only interactions within soil-plant mesocosms. Maize (Zea mays L.) was grown over eight weeks following the addition of micro and macro low-density polyethylene and polypropylene at increasing concentrations (equivalent to 1, 10, 25, and 50 years mulch film use) and the effects of plastic on key soil and plant properties were measured. We found the effect of both macro and microplastic on soil and plant health is negligible in the short-term (1 to <10 years). However, ≥ 10 years of plastic application for both plastic types and sizes resulted in a clear negative effect on plant growth and microbial biomass. This study provides vital insight into the effect of both macro and microplastics on soil and plant properties.

Plastic mulch films support global food security, however, their composition and the potential release rates of organic, metal and metalloid co-contaminants remains relatively unknown. This study evaluates the low molecular weight organic additives, metal and metalloid content and leaching from low density polyethylene (LDPE) and biodegradable plastic mulch films. We identified 59 organic additives, and non-intentionally added substances in the new LDPE films (39.8 mg m^-2) and 60 in the new biodegradable films (129 mg m^-2). The leaching of organic compounds of high concern for ecosystem and human health (e.g. phthalates, organophosphite antioxidants) was comparable to those of low concern (fatty acids, fatty amides, alkanols). However, the majority of leached compounds have undergone no regulatory scrutiny and their environmental fate and toxicity remain unknown. Leaching of heavy metals (Cu, Zn, Pb) was low relative to inert fillers (Ca, Na). Leaching was higher for both organic and metal/metalloid additives from the biodegradable films (74.6 mg m^-2) than the LDPE films (23.7 mg m^-2). This untargeted approach allowed assessment of the chemical burden posed to individual farms, based on existing use patterns of plastic mulch films, with higher chemical burden coming from biodegradable films, raising the potential for pollution swapping. This research emphasises the need to include the complex mixture of leached additives when assessing the environmental risks presented by plastic mulch films, balancing yield benefits with the protection of our agricultural soils.

Degradation of PHBV in soil, quantified with pyrolysis GC-MS, was nutrient limited at increasing microplastic concentrations. Hydrophobicity increased while plant and soil microbial biomass decreased at higher soil microplastic concentrations.

The adoption of biodegradable plastic mulch films (PMFs) has increased as a sustainable alternative to conventional PMFs. However, evidence suggests that current biodegradability standards may not reflect in-field conditions, leading to potential accumulation of biodegradable PMFs in soil with yet undetermined effects on soil microbial communities. We therefore conducted a field experiment to investigate the above and below ground degradation of one conventional and seven biodegradable PMF blends in a temperate climate (UK) over 12 months. We found that PMFs had no effect on soil properties and a negligible effect on microbial community structure. The main factors driving change in relative abundance of bacteria and fungi were exposure time, followed by the potential of meso- and macrofauna interactions. PMFs buried in soil showed no sign of fragmentation, regardless of presence of meso- and macrofauna, which was likely the cause for no overall difference in microplastic concentrations. The slow degradation of all PMFs combined with the lack of physical fragmentation under natural conditions, indicates that degradation is slower than expected according to current biodegradability standards. This highlights the importance of reframing standards by treating degradation as a system property, influenced by a variety of environmental and intrinsic factors.

Whilst complex interactions of plastic type, size, shape, and concentration have been established, most research to date has focused on new, uniform microplastics that fail to represent the heterogeneous mixture of weathered fragments resulting from natural aging of PMF in agricultural fields. To address this, we investigated the concentration-dependent effects (0.05 %, 0.5 %, 1.5 % w/w) of realistic size mixtures (1:1:1 w/w/w macro-, meso-, microplastic) of both conventional (LDPE) and biodegradable (PLA/PBAT) PMFs in their new and field-weathered forms on radish (Raphanus sativus L.) growth, soil physio-chemical properties, ¹⁵N fertiliser partitioning, N₂O emissions, and microbial activity. We showed that degradation plays a crucial role in how PMFs affect soil and crop functioning, emphasising the importance of utilising size mixtures and field-weathered materials when assessing plastic impacts in agricultural settings. Soil bulk density and moisture reduced with increasing PMF concentration regardless of PMF type or degradation, and aged PMFs increased cumulative ¹⁴CO₂ respiration similarly to organic straw amendment compared to new PMF. Additionally, biodegradable PMF, especially in its aged form, positively affected ¹⁵N plant uptake and plant biomass compared to conventional PMF. This study highlights that using uniform, new microplastics may misrepresent the complex effects on soil and plant properties under realistic conditions.