This　study　explored　the　impact　of　varying　nitrate　to　sulfide　(N/S)　ratios　on　nitrogen　removal　efficiency　(NRE)　in　the　sulfide-driven　autotrophic　denitrification　and　anammox　(SDAD-anammox)　system.　Optimal　nitrogen　removal　was　observed　at　N/S　ratios　between　1.5　and　2.0.　Isotope　tracing　results　showed　that　the　contribution　of　anammox　to　nitrogen　removal　was　enhanced　with　increasing　N/S　ratios,　reaching　up　to　37　%　at　the　N/S　ratio　of　2.5.　Additionally,　complex　nitrogen　pathways　were　identified,　including　dissimilatory　nitrate　reduction　to　ammonium　(DNRA).　Furthermore,　isotope　tracing　was　innovatively　applied　to　investigate　N2O　emissions,　demonstrating　that　higher　N/S　ratios　significantly　reduced　N2O　emissions,　with　the　lowest　emissions　at　N/S　ratio　of　2.5.　Gene　expression　analysis　indicated　that　nitrogen　and　sulfide　transformation　genes　decreased　with　increasing　N/S　ratios,　while　anammox-related　genes　first　increased　and　then　decreased,　reflecting　the　system＇s　microbial　dynamics.　These　findings　offer　insights　into　nitrogen　transformation　pathways　and　N2O　production　mechanisms　in　the　SDAD-anammox　process.