Nitrogen removal through denitrification in China's aquatic system

Abstract

Investigating aquatic denitrification is essential for understanding nitrogen (N) removal in ecosystems, especially in China, the largest N fertilizer producer and consumer globally. In this study, we examined benthic denitrification rates (DNR) in China's aquatic ecosystems with 989 data over two decades to overview the long-term trend and spatial and system differences of DNR. Rivers have the highest DNR among the studied aquatic ecosystems (rivers, lakes, estuaries & coasts, and continental shelves) due to high hyporheic exchange, the fast nutrient supply, and more suspended particles. The average DNR in China's aquatic ecosystems is much higher than that of the global average, indicating an effect of higher N inputs and lower N use efficiency. Spatially, DNR increases from west to east in China, and DNR hotspots are on coasts, estuaries, and downstream of rivers. Temporally, DNR shows a slight decline regardless of system differences, owing to national-scale water quality recovery. Human activities indeed impact denitrification, where N fertilization intensity strongly correlates with DNR, and higher population density and human-dominated land may enhance DNR by increasing C and N loadings to the aquatic system. The total N removal via denitrification in China's aquatic systems is roughly estimated to be 12.3 ± 5 Tg N yr−1. Based on the overview of previous studies, we suggest conducting investigations with larger spatial scales and long-term denitrification measurements in the future to better understand the mechanism and hotspots in N removal in the context of climate change.