[Distribution characters and affecting factors of Al(i)/(Ca + Mg) molar ratio in acid forest soils]

By using principle component analysis (PCA) and partial least square (PLS) regression, this paper studied the distribution characters and their affecting factors of Al(i)/(Ca + Mg) molar ratio in acid forest soils in southern and southwestern China. The monitoring data from 2000 to 2002 showed that in most cases, the Al(i)/(Ca + Mg) molar ratio in soil moisture was lower than assumed critical value 1.0, indicating that significant aluminum toxicity to vegetation was not occurred in the study regions. PLS regression suggested that soil aluminum saturation (AlS) was the dominant factor affecting the Al(i)/(Ca + Mg) molar ratio in soil A horizon. Higher AlS led to a higher Al(i)/ (Ca + Mg) molar ratio in soil moisture. Despite of its lower acid deposition, Liuxihe basin had a higher Al(i)/(Ca + Mg) molar ratio in soil A horizon than other catchments, mainly due to its higher AIS. In deeper soil horizons (B1, B2 and BC), the inorganic aluminum (Al(i)) in soil moisture was the main factor affecting Al(i)/(Ca + Mg) molar ratio. Higher inorganic aluminum concentration resulted in higher Al(i)/(Ca + Mg) molar ratio. In each of study catchments, the vertical distribution of Al(i)/(Ca + Mg) molar ratio was basically identical with the distribution of inorganic aluminum (Al(i)) through soil profile. It was concluded that soil aluminum saturation was the main factor controlling the regional variation of Al(i)/(Ca + Mg) molar ratio in soil moisture, and the vertical distribution of soil Al(i)/(Ca + Mg) molar ratio in each site was mainly due to the vertical change of inorganic aluminum in soil moisture.