Influences of Acid Rain on Crown Leaching of Chemical Ions in Different Forest Ecosystem in Guangzhou

The rainfall and throughfall in Pinus massoniana forest and subtropical evergreen broad leaved forest at Baiyunshan and in P.massoniana plantation at Longyandong were measured during the period of April 1998 to March 1999.The collected samples of rainfall and throughfall were used for analyzing pH value and major element compositions (SO42-,NO3-,NH4+,Al3+,Ca2+,Mg2+,K+,Na+).And hence, the crown leaching of main chemical ions in the forests and the relationships between canopy leaching and acid rain were studied.The results indicates that:(1)The frequency of acid rain in Guangzhou is 79.7%, and the amount of acid rain accounts for 95.1% of annual rainfall.(2)The pH value of throughfall in the forest ecosystems is obriously higher than that of rainfall because of the ion exchange in canopy.(3)During the process of some rain, SO42-,NO3-,NH4+,Al3+,Na+ appeared negative leaching in the pine forests and evergreen broad leaved forest, this phenomenon illustrates that the forests can absorb these ions (especially NO3- and Al3+) under the condition of acid rain.In view of annual net crown leaching, the net leaching of NO3- and Al3+ in the broad leaved forest is -0.85 and -0.06 kg·hm2·a-1, respectively, and that in two pine forests are positive leaching, this also illustrates that the evergreen broad leaved forest has great capacity of absorbing NO3- and Al3+ .(4)Increase of acidity of rain can obviously increase the ratio of crown leaching of Ca2+,Mg2+,K+ and Na+.(5)The ratios of crown leaching of NH4+, SO42- and NO3- have close negative interrelation to the concentration of SO42- and NO3- in rainfall.(6) The crown leaching array of major elements from high to low in evergreen broad leaved forest at Baiyunshan is K+Mg2+ SO42- NH4+Ca2+ Na+Al3+ NO3-, in P.massoniana forest at Baiyunshan is K+Mg2+ NH4+SO42- Ca2+ NO3-Na+Al3+, in P.massoniana plantation at Longyandong is K+Mg2+ SO42- NH4+Ca2+ Na+NO3-Al3+ .