Quantitatively deciphering the roles of sediment nitrogen removal in environmental and climatic feedbacks in two subtropical estuaries

•Denitrification dominate nitrogen removal in sediments from subtropical estuary.•Particulate organic matter links nitrogen transform between water and sediments.•Estuarine sediments are weak in eliminating riverine nitrogen due to limited area.•Sedimentary denitrification directs a more than half of daily sea-air N2O flux.