Soil erosion, organic carbon and nitrogen dynamics in planted forests: A case study in a hilly catchment of Hunan Province, China

Afforestation has the potential to elevate soil organic carbon (SOC) stocks, but water erosion would negatively impact such SOC increase. To evaluate the dynamics of SOC and total soil nitrogen (TSN) stocks in planted forest catchment in a subtropical region with mean annual precipitation of 1398 mm, ranging from 1200 to 1500 mm, we measured the inventories of 137Cs, SOC and TSN in five planted forest stands with slope gradients of 10–30%. Results showed that the afforestation systems have experienced appreciable net erosion, while different tree species exerted distinct effects on water erosion intensity and the stocks of SOC and TSN. Masson pine plantations were most susceptible to erosion with the highest average soil losses of 4.15 ± 1.25 Mg ha−1 yr−1. Although erosion occurred under afforestation, the soil illustrated large and significant gains of SOC and TSN stocks in the upper 0–10 cm of soil depth (P < 0.05). In contrary, soil in the deposition site had the highest SOC and TSN stocks (P < 0.05) in layers below 10 cm of soil depth. Under Masson pine stands, the lowest accretions of SOC and TSN stocks were observed in the upper 10 cm of soil depth with average gains of 0.3 Mg C ha−1 and 0.6 Mg N ha−1, respectively. In the upper 20 cm of soil, C:N ratio was significantly greater (P < 0.05) in forest stands than that in the deposition site. In conclusion, afforestation had a crucial effect on elevating SOC and TSN stocks in the study watershed, but planting of Masson pine is not an optimal choice in terms of soil loss and carbon sequestration. To maximize the net carbon sequestration and effectively control water erosion, terracing or straw mulching in forest plantations should be implemented.