Salt marsh expansion into estuarine mangrove mudflats reduces nitrogen removal capacity

•Salt marsh invasions decreased the mineralization and denitrification but increased the nitrification in mangrove wetlands.•Carbon quality, functional gene and substrate availabilities controlled the variations in nitrogen transformation rates.•Salt marsh invasions altered the NH4+-N and NO3--N retention or removal in wetlands.•Salt marsh expansion into mangrove mudflats reduces wetland total nitrogen removal capacity.