Increasing soil carbon stocks in eight permanent forest plots in China

Forest soils represent a major stock of organic carbon (C) in the terrestrial biosphere, but the dynamics of soil organic C (SOC) stock are poorly quantified, largely due to lack of direct field measurements. In this study, we investigated the 20-year changes in SOC stocks in eight permanent forest plots, which represent boreal (1998-2014), temperate (1992-2012), subtropical (1987-2008), and tropical forest biomes (1992-2012) across China. SOC contents increased significantly from the 1990s to the 2010s, mostly in the upper 0-20 cm soil depth, and soil bulk densities do not change significantly during the same period. As a result, the averaged SOC stocks increased significantly from 125.2 +/- 85.2 Mg C ha(-1) in the 1990s to 133.6 +/- 83.1 Mg C ha(-1) in the 2010s across the forest plots, with a mean increase of 127.2-907.5 kg C ha(-1) yr(-1). This SOC accumulation resulted primarily from increasing leaf litter and fallen logs, which accounts 3.6 %-16.3% of above-ground net primary production. Our findings provided direct evidence that China's forest soils have been acting as significant C sinks, although their strength varies in forests with different climates.