Insights into freshwater lake abiotic and biotic carrier biofilm nitrogen transformation and N₂O emission patterns: A 15N isotope tracing study

Lake biofilms play pivotal roles in nitrogen cycling; however, the influence of carrier type on their nitrogen transformation processes remains inadequately understood. In this study, we employed ¹⁵N isotope tracing, selective inhibition experiments, and microbial community analysis to investigate nitrous oxide (N₂O) emission patterns in biofilms formed on biotic (hydrophyte) and abiotic (rocks and plastics) carriers. Our results revealed that biotic carrier biofilms predominantly generated NO₂^- via denitrification, contributing 99.39 % of nitrogen transformation, whereas abiotic carrier biofilms primarily produced NO₂^- through nitrification. Moreover, nitrogen loss in biotic carrier biofilms was mainly attributed to denitrification, while in abiotic carrier biofilms, anaerobic ammonium oxidation (anammox) accounted for 70.5 % of nitrogen loss. Environmental parameters, particularly temperature and oxidation–reduction potential, significantly modulated nitrogen transformation processes in both biofilm types. Nitrification-related N₂O production showed no significant difference between the two biofilm types, denitrification-related N₂O potentials were substantially higher in biotic carrier biofilms, with greater N₂O production (157.78 vs 84.94 μg N/h/gMLVSS) and reduction rates (150.70 vs 83.36 μg N/h/gMLVSS), resulting in increased net N₂O emissions. Microbial community analysis indicated that abiotic carrier biofilms harbored higher relative abundances of Chloroflexi and Dadabacteria, whereas abiotic carrier biofilms could show enhanced nitrite oxidation and nitrate reduction activities. These findings offer novel insights into carrier-dependent nitrogen cycling mechanisms and bear significant implications for managing N₂O emissions from lake ecosystems.