LDPE and biodegradable plastics differentially affect plant-soil nitrogen partitioning and microbial uptake

Micro and macroplastics, produced from plastic mulch film and polytunnels, are contaminants of growing concern in agricultural settings. However, their impact on nitrogen (N) cycling and partitioning in plant-soil-microbial systems, critical to soil health and food security, is poorly understood. The differing impact of conventional plastics (e.g. low density polyethylene; LDPE) and emerging biodegradable plastics on microbially-mediated N transformations is also unclear, especially with accumulation over long timescales. In this mesocosm-scale study, spring barley (Hordeum vulgare L.) was exposed to macro (1 x 1 cm) or microplastic (< 500 μm) produced from LDPE or biodegradable (polylactic acid/polybutylene adipate terephthalate (PLA/PBAT); 15%/85% w/w) plastic mulch at concentrations equivalent to 1 (0.02%), 10 (0.2%) and 20 (0.4%; LDPE only) years of plastic mulch film use. Mesocosms were fertilised with ammonium nitrate (40 kg N ha−1, 20 atom%¹⁵N), and partitioning of ¹⁵N-labelled fertiliser into plant biomass, soil and leachate yielded a partial mass balance. Soil-N partitioning was probed via diffusion of extractable ammonium and nitrate, and compound-specific ¹⁵N-stable isotope analyses of soil microbial protein. Barley chlorophyll content and growth were used to determined effects on plant health. Plant health parameters were not effected by increasing concentrations of micro or macroplastic, however, there were concentration-dependent decreases in plant ¹⁵N uptake. This was linked to increased leached nitrogen for biodegradable and LDPE micro- and macroplastic, due to changes in physical pore flow pathways. This was also observed for total soil ¹⁵N, while varying patterns in soil ¹⁵N partitioning between plastic type, size and concentrations revealed potential complexities of impacts of N cycling for macro and microplastics. Assimilation into soil microbial protein was higher for biodegradable plastics, which we associate with early-stage degradation. Microbial assimilation in the presence of LDPE was a function of abiotic impacts on leaching, with suppression of inorganic N transformations. While micro- 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 during early-stage barley growth. However, changes in soil N cycling and available N will impact nitrogen use efficiency and soil organic matter dynamics. Thus, the differing impacts of conventional and biodegradable macro and microplastics, and effects of accumulation, must be considered in risk assessments for agricultural plastics.