Impact of short-rotation Acacia hybrid plantations on soil properties of degraded lands in Central Vietnam

Acacia hybrid (A. mangium x A. auriculiformis) is the main species planted for short-rotation forestry in Vietnam. In this study, the effect of these plantations on some key properties of degraded gravelly soils in Central Vietnam was assessed. Soil samples were collected from second- or third-rotation plantations representative of five age classes (0.5-5 years old), and in adjacent abandoned lands as controls. Compared with abandoned land, stock of total soil carbon (C) was significantly higher at ages 0.5, 1.5, 2.5 and 5 years (18.4-19.5 v. 13.0Mgha^-1), total nitrogen (N) at 0.5 and 1.5 years (1.5-1.7 v. 1.0Mgha^-1), exchangeable calcium at 0.5, 1.5 and 2.5 years (215-294 v. 42Mgha^-1), magnesium at 0.5, 1.5, 2.5 and 3.5 years (39-48 v. 19Mgha^-1), and sodium at all ages (46-59 v. 5Mg ha^-1). Electrical conductivity was significantly higher at all ages (58.5-69.4 v. 32.7 uS cm^-1). Differences in extractable phosphorus and exchangeable potassium were not significantly different between plantations and abandoned land. Bulk density was significantly lower in plantations than abandoned land at all ages (1.36-1.42 v. 1.55Mgha^-1), pH_CaCl2 at 0.5 and 5 years (3.78-3.84 v. 3.98), and pH_H2O at 5 years (4.30 v. 4.52). Because the soils were gravelly, differences in concentration of total C and nutrients between abandoned land and plantations were not the same as those for stocks after correction for gravel content and bulk density. Within a rotation, most soil properties did not change significantly with plantation age, although they appeared to decrease during the first 3 years; total C then recovered to initial levels, but total N and exchangeable cations remained lower. Some soil properties were strongly related to gravel content and elevation, but not to growth rate. We conclude that consecutive plantings of short-rotation Acacia hybrid on degraded and abandoned land can lead to changes in some soil properties.