Elevation Amplifies Carrier-Type Differences in Nitrogen Cycling and N 2 O Dynamics in Lake Biofilms

Abstract Lake biofilms are microbial habitats for nitrogen transformation, but how carrier type and elevation influence N2O dynamics remains unclear. We examined abiotic- and biotic-carrier biofilms from 12 lakes spanning 4–4272 m using anoxic incubations, 15N tracing, selective inhibition, isotopocule analysis, and metagenomics. Net N2O emission potential increased with elevation in both carrier types, with the largest carrier contrast at high elevation, where biotic-carrier biofilms showed the highest net N2O emission potential. 15N partitioning showed a marked shift in emission sources. At low elevation, heterotrophic nitrification dominated abiotic-carrier biofilms (∼91%), whereas denitrification contributed ∼57% in biotic-carrier biofilms. At high elevation, denitrification accounted for 67–97% of net N2O emission across both carriers, and bacterial denitrification dominated the total denitrification rate potential. Metagenomic profiles provided mechanistic support: high-elevation biotic-carrier biofilms showed higher relative representation of genes for dissimilatory nitrate reduction to ammonium and nitrogen fixation, but had few nosZ-carrying genomes, indicating weak terminal N2O reduction capacity. Overall, carrier type and elevation jointly shape nitrogen transformation pathways. High-elevation lake ecosystems, particularly those with abundant biotic substrates, may harbor disproportionately important biofilm N2O hotspots under global change.