Divergent impacts of microplastics and related leachates on sediment carbon and nitrogen transformation by regulating...

Microplastics (MPs) are recognized as potential disruptors of biogeochemical cycles. However, the differential impacts of MPs types and related leachates remain poorly understood, particularly for nitrogen-containing MPs. This study compared the variational responses in sediment carbon and nitrogen transformation to 1% aged MPs (AMPs), leachates (LMPs), washed aged MPs particles (WMPs) derived from nitrogen-containing polyamide (PA) and non-nitrogen-containing polylactic acid (PLA) under natural exposure and freeze-thaw cycles (FTCs). The results showed that the Carbon Pool Management Index (CPMI) in PLA groups was always higher than corresponding PA groups, indicating better sediment quality in the PLA groups. Under natural exposure, LPA and LPLA respectively increased CPMI by 3.4% and 93.6% due to containing biologically available organic matter. FTCs increased CPMI by 34.1% in control groups, whereas only CMPI in AMPs was higher than control groups. Besides, the PLA group reduced the nitrogen mineralization rate by possibly inhibiting the macromolecular organic matter decomposition, and might have reduced the narG/H/I and nirB/D genes, thereby indirectly maintaining the nitrification rate. While the PA group increased the nitrogen mineralization rate by possibly inhibiting the amoA/B/C genes to reduce nitrification rate and enriching ureolytic microorganisms. FTCs mitigated these disparities by intensifying microbial interactions. PLS-SEM suggested that APLA was most strongly associated with carbon and nitrogen transformation within the PLA treatments, whereas the apparent effect of APA was jointly shaped by the opposite associations of WPA and LPA. These findings provide new insights into the complex ecological effects of MPs and their leachates on sediment carbon and nitrogen transformations.