Critical loads of headwater streams in China using SSWC model modified by comprehensive F-factor

Delayed recovery of surface water chemistry from acidification in subtropical forest region of China

The three largest acid rain regions of current earth are located in northern and western Europe, eastern North America, and East Asia. Sulfur and nitrate concentrations in headwater streams in Europe and North America decreased as atmospheric sulfur and nitrogen deposition decreased, albeit with a considerable delay. However, how water chemistry responds to the declining sulfur and nitrogen deposition in China is unclear. The regional survey of surface water chemistry during 2010 and 2018 within the Sichuan Basin in southwestern China showed that the recovery of the surface water chemistry was delayed for at least 5 years owing to the release of previously deposited sulfur and nitrogen stored in the soil. After sulfur deposition declined from its peak value, the subregions of purplish soil with low sulfate adsorption capacity still exhibited a net sulfur release in 2010, but this release was no longer evident by 2018. The subregions with the red and yellow soils, which have a high sulfate adsorption capacity, operated as sulfur sinks during 2010 and 2018, indicating a continuous immobilization process through sulfate reduction despite a decrease in sulfur deposition. Additionally, this sulfate reduction countered the release of sulfate caused by sulfur desorption. There was a substantial nitrogen sink within the Sichuan Basin. Nitrogen leaching decreased slowly with the declined nitrogen deposition, except in regions where nitrogen deposition exceeded the critical threshold. Compared to temperate forest regions in Europe, the Sichuan Basin and its surrounding areas have experienced higher decline rates in the leaching of sulfur and nitrogen, highlighting that the subtropical forest region undergoes a faster restoration of surface water chemistry.

Assessing critical loads and exceedances for acidification and eutrophication in the forests of East and Southeast Asia: A comparison with EANET monitoring data

Spatial variations in sulfur (S) and nitrogen (N) deposition have changed in East and Southeast Asia in recent decades. Nevertheless, in this region, including the tropics, regional-scale assessments of the long-term risk of acidification and eutrophication (N saturation) for terrestrial ecosystems using a critical load approach have not been updated since 2001. To evaluate future risks, maps of critical loads and exceedances were updated using recently acquired spatial datasets of soil properties, soil minerals, climate, tree plantations, and the annual S and N depositions estimated using the Community Multiscale Air Quality (CMAQ) model. The resulting maps were verified using data on long-term trends in soil pH and nitrate concentration in surface water acquired by the Acid Deposition Monitoring Network in East Asia (EANET). It was found that N deposition exceeded the critical load for eutrophication not only in East Asia but also in some parts of the tropical monsoon and humid regions in Southeast Asia, whereas S deposition partly exceeded the critical load for soil acidification in China and small parts of the tropical monsoon region. The high-risk areas for eutrophication coincided well with the EANET sites, where the increase in nitrate concentration in the surface water was significant over the last 20 years. Hence, the estimated map of the critical load exceedance for eutrophication is more plausible for assessing the risk in East and Southeast Asia than that for acidification, although the critical load exceedance for acidification would be sufficiently significant as an updated risk map based on the latest input values. This update also suggests that increased N deposition around megacities, water discharge, and tree plantations may play an important role in the spatial variability of eutrophication risks in the tropics of Southeast Asia.