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.