Afforestation on Cropland Promotes Pedogenic Inorganic Carbon Accumulation in Deep Soil Layers on the Chinese Loess Plateau

Over the past decades, vast croplands have been converted into forestland on the Chinese Loess Plateau (CLP). Although a few studies have investigated the effects of afforestation on soil inorganic carbon (SIC), there has been little information on the changes in the two fractions of SIC, pedogenic inorganic carbon (PIC) and lithogenic inorganic carbon (LIC), along a deep profile following afforestation. We selected forestland (Robinia pseudoacacia; 31 years) and adjacent cropland (control), and investigated the stocks of SIC, PIC and LIC, at 0-300 cm under the two lands on the CLP. The SIC stock significantly decreased by 60.2 Mg ha−1 at 0-80 cm but increased by 57.8 Mg ha−1 at 80-220 cm, and no significant difference in total SIC stock at 0-300 cm was observed between the two lands. Afforestation significantly reduced the LIC stock in each layer; afforestation decreased the PIC stock at 0-80 cm but elevated it at 80-300 cm. In the 80-220 cm layer of forestland, the LIC stock decreased by 41.1 Mg ha−1 while the PIC stock increased by 98.9 Mg ha−1. Afforestation on cropland induces the loss of SIC in upper layers. However, it promotes the accumulation of PIC in deep layers, which causes an increase of SIC in these layers. Because the net increased PIC in deep layers compensates for the loss of SIC in the upper layers, afforestation on cropland does not alter the total SIC storage but redistributes the SIC along the profile on the CLP.