Soil pH neutralization by conversion of cropland to forest in China: A meta-analysis

The “Grain for Green” project is recognized for its promotion of soil nutrient cycling, carbon sequestration, and soil quality. Soil pH is a key regulator of biogeochemical cycling in terrestrial ecosystems and has cascading consequences for the structure and function of these systems. Nevertheless, little is known about how the conversion of cropland to forest impacts soil pH, and available relevant research is inconclusive. Here, we carried out a meta-analysis using 4240 observations collected from 135 studies conducted in China to examine the influence of afforestation on soil pH. We quantified variation in the effects of afforestation-induced changes to soil pH, identified the key factors underlying this variation, and assessed its consequences. We found that soil pH significantly increased by 1.29% in acid soil (tropical/subtropical) and decreased by 1.59% in alkaline soil (temperate) across China. Mean annual precipitation and temperature, initial soil pH, latitude, afforestation duration, and climate zone were the main drivers of variation in soil pH. Moreover, we found that changes in soil nutrient content and organic carbon were strongly linked to alterations in soil pH following afforestation. Our results suggest that forest restoration changes soil pH significantly, raising pH in relatively acidic soils and lowering pH in relatively alkaline soils, thereby potentially promoting ecosystem productivity and improving soil fertility. Considering the critical role of soil pH in regulating carbon and nutrient cycling, these results provide insight for understanding the long-term influence of afforestation on biogeochemical cycling and can be used to inform regionally adapted afforestation efforts.