Effect of Time since Afforestation on Soil Organic Carbon Stock and Turnover Rate

Forests can accumulate large quantities of SOC, but the trend in SOC accumulation with increasing stand age is inconclusive. In this study, we selected five plots in northwestern China: four stands of artificially planted Robinia pseudoacacia of different ages (5, 20, 40, and 56 years old), and a plot of wasteland as the control. The results showed that the SOC contents decreased, whereas δ13C values increased, with soil depth. The soil total nitrogen (TN) content and the carbon/phosphorus (C/P) ratio increased significantly with increasing stand age. The SOC storage in the 0–30 cm soil layer did not differ significantly among stands of different ages. However, SOC storage in the 0–100 and 30–100 cm soil layers was significantly higher in the 40- and 56-year-old stands than in 5-year-old stands. The SOC turnover rate decreased gradually over the 40 years after planting and then rapidly increased between 40 and 56 years after planting. The SOC stocks were most strongly correlated with TN and the C/P ratio, and SOC turnover was most closely related to soil porosity. Afforestation significantly improved soil properties to enhance SOC sequestration, but it took a long time for stored SOC to accumulate in this study area.