Monitoring Effect of SO2 Emission Abatement on Recovery of Acidified Soil and Streamwater in Southwest China

Following Europe and North America, East Asia has become a global hotspot of acid deposition, with very high deposition of both sulfur (5) and nitrogen (N) occurring in large areas in southwest and southeast China. Great efforts have been made in reducing national emission of sulfur dioxide (SO2) since 2006 in China. However, the total emission of nitrogen oxides (NOx) continued to increase until 2011. To evaluate the effects of SO2 and NOx emission abatement on acid deposition and acidification of soil and water, we monitored the chemical composition of throughfall, soil water, and streamwater from 2001 to 2013 in a small, forested catchment near Chongqing city in Southwestern China. The deposition of S decreased significantly, whereas N deposition increased in the recent years. This clearly showed the effect of SO2 abatement but not of NOx. Overall the rate of acid deposition was reduced. However, there was delay in the recovery of soil and surface water from acidification, probably due to desorption of previously stored sulfate (SO42-) and increase in nitrate (NO3-) leaching from soil. The average acid input by N transformations has greatly exceeded the H+ input directly by atmospheric deposition. The reversal of acidification with an increase in pH of soil water, requires additional abatement of emissions of both SO2 and NOx.