Crop residue return and nitrogen fertilizer reduction alleviate soil acidification in China's croplands

Accurate knowledge of the soil pH in China's croplands is essential to control soil acidification and improve soil quality and sustainability of agricultural systems. Here, this study analyzed the soil pH in China's croplands and its change based on a comprehensive investigation of 8.9 million soil samples from 2534 counties. We show that China's croplands were mainly composed of weakly acidic soils (34.9%) and alkaline soils (35.3%), and the mean soil pH (0-20 cm) was 6.74-6.83 in 2010 at the national scale. Weakened acidification was observed in China's cropland soils with 0.11 units during 1980-2010. Soil acidification occurred mainly in southern, eastern, and northeastern provinces, especially in Liaoning, Hainan, Jilin, and Heilongjiang. The changes in soil pH in China's croplands during 1980-2010 were driven by initial soil pH, mean annual precipitation, crop biomass removal, and nitrogen fertilizer input, with the latter two explaining 20.9% of the total variation. Given this, four scenarios with different crop residue and nitrogen fertilizer inputs were designed to assess soil pH changes in China's croplands during 2020-2050. The results show that soil acidification will continue under existing agricultural production practices. Increasing crop residue input and reducing nitrogen fertilizer input can effectively alleviate soil acidification and even increase soil pH over the next 30 years. We also proposed targeted fertilization and crop residue returning strategies to further alleviate soil acidification in different provinces by considering current agricultural management practices.