National-Scale Meta-Analysis of Soil Carbon and Nitrogen Accumulation Potential in China's Grain for Green Program

Grain for Green Program (GFGP) by establishing revegetation on degraded cropland has been considered effective to capture soil carbon (C) in terrestrial ecosystems. However, the magnitude and direction of soil C sequestration and its relation to soil nitrogen (N) dynamics are still poorly known. This study analyzed the literature to determine the influence of low-yield cropland conversion on soil C and N stocks and summarized the results via meta-analysis from 94 recent publications (534 observations) from 1999 to 2018. Cropland conversion had a positive and significant impact on soil C stocks at depths of 0-20, 20-40 and 0-100 cm, and significantly affected soil N stocks at a depth of 0-20 cm. The changes in soil C significantly correlated with N accumulations, but increased at different rates. Soil C sequestration was highly positively related with initial soil C amounts for forest and grassland (0-40 cm), and shrubland (0-20 cm), but there was no initial soil N limitation for later soil N accumulation at all depths. A redundancy analysis revealed that restoration age was the main parameter affecting both soil C and N accumulations. This study showed significant and positive relationships between soil C and N after cropland conversion, and suggested that soil C sequestration could be sustained with soil N availability in the long term. Together, cropland conversion could effectively gain C stock without N limitation in long-term restoration under GFGP in China.