Profile distribution of soil inorganic carbon along a chronosequence of grassland restoration on a 22-year scale in the Chinese Loess Plateau

Understanding the distribution of soil inorganic carbon (SIC) is meaningful and necessary for assessing soil carbon sequestration. However, changes in SIC following vegetation restoration are still not well studied in the Chinese Loess Plateau (CLP), which has undergone extensive changes from farmland to grassland. Here, we chose three study sites, farmland (FL), grassland restored for 12 years (RG-1) and grassland restored for 22 years (RG-2). The changes of SIC storage were determined under the conversion from a farmland ecosystem to extensively used grassland during the past twenty years. Results showed that the SIC content decreased with vegetation restoration, which averaged 18.5, 13.2 and 11.8 g kg− 1 in FL, RG-1 and RG-2, respectively. The SIC content was significantly lower in the top 80 cm in RG-1 and RG-2 than in FL, but showed little variance at depths of 80–200 cm among the study sites. It illustrated that the portion of carbonate was most likely released to the atmosphere as CO2 temporarily due to soil acidification and the increase of soil moisture in restored grassland. The lower δ13C value of SIC in RG-1 and RG-2 than in FL was most likely due to the soil carbonate exchanging with more soil respired CO2, with increasing vegetation coverage in restored grassland. Our results indicate that the SIC storage deceases in the restored grassland in situ probably due to the decrease of soil pH and the increase in soil water content. The SIC plays an important role when assessing the capacity of carbon sequestration with vegetation restoration in the semi-arid CLP.