Acidification of soil due to forestation at the global scale

Forestation is a key strategy to mitigate climate change caused by anthropogenic carbon dioxide emissions. However, the impacts of forestation on soil pH remain unclear, despite critical roles of soil pH in regulating key soil biogeochemical processes. Here, we collected a global dataset of soil pH change after forestation, which included 1082 observations from 171 published papers. Results showed that soil pH declined significantly by 0.23 after forestation over the globe. Soil pH consistently declined after forestation, no matter the forest was established naturally or by planting, on croplands or grasslands. The decline of pH after forestation was generally larger in neutral soils (pH 6–7) than in acidic soils (pH < 6) and alkaline soils (pH > 7), and larger in boreal and temperate forests than in tropical forests. Soil pH decreased significantly in humid areas but not in arid regions. Random forest analysis showed that climate was the most important regulatory factor to influence soil pH change after forestation. Mean annual temperature and precipitation probably affected soil pH both directly and indirectly via altering soil physiochemical properties. Given vital roles of soil pH in regulating carbon and nutrient dynamics, our findings have important implications for the long-term impacts of forestation on carbon and nutrient dynamics.