Acidity of Soil and Water Decreases in Acid-Sensitive Forests of Tropical China

Acid deposition in China has been declining since the2000s. Whilethis may help mitigate acidification in forest soils and water, littleis known about the recovery of soils and water from previous severeacidification in tropical China. Here, we assessed the chemistry ofmineral soils, water, and acid gases (SO2 and NO (x) ) from three successional forest types in tropicalChina from 2000 to 2022. Our results showed that soil pH increasedsynchronously from 3.9 (2000-2015) to 4.2 (2016-2022)across all three forest types, with exchangeable acid initially decreasingand thereafter stabilizing. Surface and ground water pH also graduallyincreased throughout the monitoring period. Soil pH recovery was strongerin the primary than in the planted forest. However, soil pH recoverylagged behind the increase in rainfall pH by approximately a decade.The recovery of soil pH was likely related to the positive effectsof the dissolution of Al/Fe-hydroxysulfate mineral and subsequentsulfur desorption on soil acid-neutralizing capacity, increased soilorganic matter, and climate warming, but was likely moderated by increasedexchangeable aluminum and potentially proton-producing hydroxysulfatemineral dissolution that caused the lagged soil pH recovery. Surfaceand ground water pH recovery was attributed to increased water acid-neutralizingcapacity. Our study reports the potential for the recovery of acidifiedsoil and water following decreased acid deposition and provides newinsights into the functional recovery of acid-sensitive forests. The recovery of acidified soil and waterin the tropicsis understudied. Decreased acidity of soil and water may benefit environmentalrecovery in acid-sensitive forests of tropical China.