Is surface water acidification a serious regional issue in China?

- High pH and ANC were observed for almost all forested headwater streams in both south and northeast China.
- Significant N sinks occur and can greatly buffer the acidifying effect of high N depositions in south China.
- Considerable NO3− leaching to stream waters does not occur in China until N deposition is higher than 2.50 gN·m− 2·yr− 1.

Acidic deposition used to cause severe surface water acidification in Europe and the North America, but no damage of surface water acidification was reported under continuous heavy acid deposition in China. We present the first detailed study on regional surface water acidification in southern and northeastern China based on a survey of 255 forested headwater streams, which were investigated during 2010-2015 to explore the status and mechanism of surface water acidification. South China has a subtropical climate with high acid deposition, whereas northeast China is located in a cold temperature zone with relatively low acid deposition. High pH and acid neutralizing capacity (ANC) were observed for almost all of the streams, indicating that surface water acidification may not be a serious regional issue in China both at present and in the future. In northeast China, where soil types are similar in low weathering rate to those in Northern Europe and North America, the ANC of surface waters was lower than that in South China, indicating a higher acid-sensitivity of the surface water. In comparison, the high pH and ANC of streams in southwest China resulted from high soil weathering and atmospheric calcium (Ca) deposition, despite elevated atmospheric acid inputs due to the high sulfur (S) and nitrogen (N) deposition. Comparison of stream nitrate (NO3−) and sulfate (SO42 −) concentrations with modeled N and S deposition showed significant N and possible S sinks in the catchments in south China, probably due to denitrification of NO3− in groundwater discharge zone and SO42 − adsorption by acid soils respectively, which may also buffer the acidifying effect of S and N depositions. It seems that considerable NO3− leaching to stream waters does not occur in China unless N deposition is higher than 1.8 keq·ha− 1·yr− 1 (2.50 g N·m− 2·yr− 1) (or greater values in certain regions).

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