Addition of inorganic nitrogen significantly increases the real and apparent priming effects of polyethylene and PHBV microplastics in soil

Plastic mulch films (PMFs) enhance crop productivity but degrade into microplastics, raising concerns about their effects on soil health. This study compared the effect of conventional polyethylene (PE) and biodegradable poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) microplastics on soil organic matter (SOM) turnover through real (native SOM) and apparent (microbial biomass) priming effects. Using ¹⁴C-labelled assays, we assessed three microplastic loading rates (10, 100, 1000 kg ha^-1) with addition of nitrogen (N) (80 kg N ha^-1), and without. Comparing responses to inert sand and organic residues (clover, straw). Nitrogen addition significantly enhanced real priming in all treatments. PHBV caused dose-dependent (although non-significant) positive priming, greater than equivalent straw or clover loading. Generally, apparent priming was negative without N but positive with N, suggesting microbial responses depend on substrate stoichiometry and accessibility. Microbial (16S rRNA and ITS2) sequencing showed that N addition mainly drove bacterial community changes, while PHBV uniquely altered fungal communities, increasing Ascomycota dominance in a dose-dependent manner. PE behaved similarly to inert sand, indicating minimal impact on SOM turnover. While biodegradable plastics reduce conventional microplastic accumulation, PHBV may enhance SOM loss via priming (with no direct analogue found in this study), with potential long-term consequences for soil carbon stocks. These findings emphasise the importance of evaluating biodegradable plastics within broader soil nutrient and carbon cycling frameworks.