Changes in soil organic and inorganic carbon stocks in deep profiles following cropland abandonment along a precipitation gradient across the Loess Plateau of China

Determining changes in soil organic carbon (SOC) and inorganic carbon (SIC) stocks following ecological restoration is important for estimating the regional carbon budget and evaluating ecological effects. However, there is limited understanding on the interacting effects of vegetation restoration and climate gradients on SOC and SIC stocks in shallow and deep soil layers over large scales. This study selected seven sites along a climate transect from the east to the west of the Chinese Loess Plateau, and the SOC and SIC stocks were measured to a depth of 300 cm in sites covered by cropland and three vegetation restoration types (grassland, shrubland and woodland). The spatial variations and controlling factors of the SOC and SIC stocks at different depths following vegetation restoration along the precipitation gradient were investigated in detail. The results indicated that the SOC and SIC stocks in the 100–300 cm layers accounted for more than 50% of the total values in the 0–300 cm profile. The total SOC stock in the 0–300 cm profiles significantly increased along precipitation gradient (p < 0.01) in all vegetation types except woodland. The total SIC stock decreased, but the change was not significant, which caused little variation in the total carbon stocks along the precipitation gradient (p > 0.05). Shrubland and woodland plantation following cropland abandonment resulted in soil carbon accumulation, whereas grassland represented carbon loss in sites with mean annual precipitation (MAP) greater than 470 mm. The changes in the SOC stock (ΔSOC) in the surface layer (0–20 cm) and those in the SIC stock (ΔSIC) in the deep layers (100–300) following revegetation significantly decreased along precipitation gradient (p < 0.05). The interactions between ΔSOC and ΔSIC stocks were evident, especially in the upper soil layers. An accumulation of 1 kg SOC was accompanied by 0.73 kg loss of SIC in the 0–40 cm layer and 1.26 kg increase of SIC in the 40–300 cm layer per square meter following revegetation. MAP and mean annual temperature (MAT) mainly affected the spatial patterns of SOC and SIC in the upper layers, while land use and soil texture mainly affected soil carbon in the deep layers. This study indicates that vegetation restoration does not always result in soil carbon sequestration at every depth after cropland abandonment, which depends on climatic conditions and varies among different vegetation types.