Effects of afforestation on soil carbon and its fractions: a case study from the Loess Plateau, China

Afforestation has been implemented to reduce soil erosion and improve the environment of the Loess Plateau, China. Although it increased soil organic carbon (SOC), the stability of the increase is unknown. Additionally, the variations of soil inorganic carbon (SIC) following afforestation needs to be reconfirmed. After planting Robinia pseudoacacia, Pinus tabuliformis, and Hippophae rhamnoides on bare land on the Loess Plateau, total soil carbon (TSC) was measured and its two components, SIC and SOC, as well as the light and heavy fractions within SOC under bare lands and woodlands at the soil surface (0–20 cm). The results show that TSC on bare land was 24.5 Mg ha−1 and significantly increased to 51.6 Mg ha−1 for R. pseudoacacia, 47.0 Mg ha−1 for P. tabuliformis and 39.9 Mg ha−1 for H. rhamnoides. The accumulated total soil carbon under R. pseudoacacia, P. tabuliformis, and H. rhamnoides, the heavy fraction (HF-SOC) accounted for 65.2, 31.7 and 76.2%, respectively; the light fraction (LF-SOC) accounted for 18.0, 52.0 and 4.0%, respectively; SIC occupied 15.6, 15.3 and 19.7%, respectively. The accumulation rates of TSC under R. pseudoacacia, P. tabuliformis, and H. rhamnoides reached 159.5, 112.4 and 102.5 g m−2 a−1, respectively. The results demonstrate that afforestation on bare land has high potential for soil carbon accumulation on the Loess Plateau. Among the newly sequestrated total soil carbon, the heavy fraction (HF-SOC) with a slow turnover rate accounted for a considerably high percentage, suggesting that significant sequestrated carbon can be stored in soils following afforestation. Furthermore, afforestation induces SIC sequestration. Although its contribution to TSC accumulation was less than SOC, overlooking it may substantially underestimate the capacity of carbon sequestration after afforestation on the Loess Plateau.