Nitrogen Availability Governs Priming Effect Induced by Biodegradable Microplastics Through Microbial Life‐Strategies

Microplastics (MPs) have emerged as an increasingly concerning soil contaminant. Although biodegradable plastics are good alternatives to non‐biodegradable plastics in croplands, they can influence soil organic matter (SOM) decomposition through a priming effect. We investigated how the biodegradable MPs‐induced priming effect responds to nitrogen (N) availability in soil. The impact of biodegradable MPs and mineral N on the priming effect was generalized by a meta‐analysis, and the mechanisms were investigated by 13C isotope techniques coupled with 16S rRNA amplicon sequencing. By combining the meta‐analysis of data from 67 publications with an incubation experiment, we tested the MPs‐induced priming effect and their mechanisms depending on four levels of mineral N: 1.50, 0.75, 0.50, 0.30 mg N g−1 soil. The meta‐analysis suggested that the mineral N input decreased the priming effect induced by root exudates (effects size: −1.1) and MPs (effects size: −1.5), but increased the priming effect induced by biochar (effects size: 3.1). The effect size of mineral N input on the priming effect decreased with the increase of carbon‐to‐nitrogen ratio (C/N) between added organic carbon and mineral N. Due to the differences in MPs degradability, the range of the priming effects induced by polyhydroxyalkanoate was from 200% to 250%, while the priming effects induced by polylactic acid were negative (−22% to −5%). Mineral N primarily mitigated the MPs‐induced priming effect by reducing the abundances of microorganisms with K‐strategy (Acidobacteria and Basidiomycota), thereby reducing their N mining from SOM. Priming reduction by N fertilization was minimal when the C/N between added MPs carbon and mineral N was 10 (high N availability), and the abundance of r‐strategists (Proteobacteria and Ascomycota) was large. We conclude that both r‐ and K‐strategists collectively drive the intensity and direction of the MPs‐induced priming effect, which decreases with increasing C/N between added MPs carbon and mineral N.