Soil organic carbon content increase in the east and south of China is accompanied by soil acidification

Increased soil organic carbon (OC) in China has been reported in the past two decades, suggesting the sequestration of atmospheric carbon dioxide into soil, mitigating climate change and improving soil health. On the other hand, soil pH decrease had also been reported nationwide. If the two are related, the strategy of increasing soil OC could negatively affect soil quality for food production and the environment. We investigate this thread based on large-scale soil survey data from two provinces with typical soil and cropping patterns in the east and south of China, Jiangsu (102,600 km2) and Guangdong (177,900 km2). The data include >5000 observations from soil surveys conducted over the past four decades, i.e., the 1980s, 2006–2007, and 2010–2011. Using spatiotemporal modelling, we show that across Jiangsu province, the topsoil OC on average has increased from 8.5 g kg−1 to 9.9 g kg−1 from 1980 to 2000 and a further increase to 12.6 g kg−1 in 2010. This increase was accompanied by a decrease in average pH from 7.63 to 6.90. In Guangdong, there was an overall increase in average topsoil OC content from 14.2 g kg−1, 16.5 g kg−1, and 20.2 g kg−1 with a decrease in average pH from 5.58, 4.90, and 4.98. Based on the spatiotemporal modelling results, the structural equation modelling analysis shows that OC and pH changes were significantly correlated and linked by increased soil N content. On croplands, soil N content was mainly attributed to N fertiliser application. The pH decrease was particularly significant in the east of China where the soils were neutral in pH. We recommend that more revolutionary means be taken to sequestrate atmospheric carbon into soil as the current OC increase due to increasing crop productivity via a high rate of nitrogen application may have a potential acidification effect.