Assessment of Soil Organic and Inorganic Carbon Under Afforestation and Natural Vegetation Restoration Using Stable Carbon Isotope (δ 13 C) Measurements

On the Loess Plateau, vast farmlands have suffered from severe land degradation and soil erosion. Afforestation and natural vegetation restoration are two main methods for ecological restoration on the degraded farmland in the last few decades. Previous studies have mainly investigated the effects of the two methods on soil organic carbon (SOC). However, few studies have focused on the influence of the two methods on soil inorganic carbon (SIC), which is the fundamental component of the local soil carbon pool. On the Loess Plateau, we measured SOC and SIC stocks, and the stable carbon isotope values of SOC and SIC (δ 13 C-SOC and δ 13 C-SIC) within 0–100 cm depth in an artificial forestland ( Robinia pseudoacacia , 27 years) and a neighboring abandoned farmland ( Poa annua and Bothriochloa ischaemum , 27 years). The results showed that SOC and SIC stocks at 0–100 cm in forestland were 23.43 Mg ha −1 and 16.89 Mg ha −1 higher than the abandoned farmland, respectively. The δ 13 C-SOC in topsoil was significantly ( p < 0.05) lower than the subsoil in both lands. The δ 13 C-SIC in forestland was significantly ( p < 0.05) lower than in abandoned farmland. The results indicated that afforestation is more suitable to conserve soil carbon than natural vegetation restoration on degraded farmland on the Loess Plateau. The forestland generated more pedogenic inorganic carbon than the abandoned farmland causing the difference in SIC stock between the two lands. Our findings highlight the importance of SIC, and the SIC may be more important than SOC in sequestrating soil carbon following vegetation restoration on the Loess Plateau.