Assessing dissolved inorganic nitrogen flux in the Yangtze River, China: Sources and scenarios

• The change of Yangtze DIN evolution through time, and its relation to DIN sources
• Nitrogen loads transition to a market-based economy in the last decades.
• Changing diet to protein is becoming the key driver of DIN source.
• DIN concentrations impact on the domestic water demands and scenarios

This study gives a thorough assessment of the occurrences of dissolved inorganic nitrogen (DIN) in the Yangtze River in the past half century. The results have shown that nitrogen fertilizer, a major DIN source, has been replaced by domestic sewage in the last decade, which has dramatically driven up DIN loads in the Yangtze. DIN concentrations showed a rapid increase from < 0.5 mg L− 1 in the 1960s to nearly 1.5 mg L− 1 at the end of the 1990s. Since then DIN has remained steady at ca. 1.6–1.8 mg L− 1. A significant relationship between the historical DIN record at the downstream gauging station (Datong) and nitrogen (N) sources in the Yangtze River basin is established using principal components analysis. This allows us to apportion DIN loads for the year 2007 (the most recent year of measured DIN data available) to various N sources, listed here in order of weight: sewage (0.391 million tons); atmosphere (0.358 million tons); manure (0.318 million tons), N-fertilizer (0.271 million tons). Therefore, we estimated that a DIN load of 1.339 × 106 t was delivered to the lower Yangtze and its estuarine water in that year. We established scenarios to predict DIN concentrations in the lower Yangtze at 10 year intervals to 2050. For a dry year (20,000 m3 s− 1) DIN concentrations would range from 2.2–3.0 mg L− 1 for 2020–2050. This far exceeds the 2.0 mg L− 1 defined on the Chinese National Scale as the worst class for potable source water. The scenario results suggest that upgrading the sewage treatment systems throughout the basin will be an effective way to help reduce DIN concentrations to less than 2.0 mg L− 1 in the lower Yangtze. This would save the Shanghai megacity from the increasing threat of heavily polluted water sources, where > 23 million people are at present dependent on the Yangtze estuary for 70% of their freshwater intake.