Microplastics enhance soil nitrogen availability by stimulating nitrogen-acquiring enzyme activity over three decades-long fertilization regimes

Agricultural productivity heavily relies on fertilization to enhance soil nitrogen (N) availability and improve crop yields. Concurrently, agroecosystems are facing increasing contamination from microplastics (MPs), which are known to reduce crop yields and potentially threaten soil health. While the individual effects of fertilization and MPs on soil N dynamics are recognized, the complex interactive impacts of these ubiquitous factors on soil N availability remain poorly understood. Here, we conducted a laboratory incubation experiment on a long-term fertilized cropland amended with different MPs types. Our results revealed that 34-year fertilization decreased soil pH. Notably, the application of mineral fertilizers with crop residue incorporation enhanced soil organic carbon by 8.93 % and total nitrogen by 10.71 %, respectively, compared to the non-fertilized plots. After 60-day incubation, significant effects of fertilization, MPs addition, and their interaction were observed on soil N availability. In fertilized soils, biodegradable MP increased N-acquisition enzyme (NAE) activity by 36.61–41.20 %, and substantially altered the composition (β-diversity) of N-cycling microbial community. In contrast, non-degradable MP significantly decreased the N-cycling microbial α-diversity. We also found that soil N availability was related to soil pH, C:N ratio, and NAE activity. Structural equation modeling further identified the critical role of NAE, which was the most important factor in determining soil N availability. Overall, this study highlights that MPs, by acting as a high-carbon resource, alter the soil C:N ratio, which in turn stimulates microbial N-acquiring enzyme activity to increase N availability. These results underscore the need for greater attention to MPs pollution and its consequences on soil N cycling in global agroecosystems.