Exchange of Ca2+, Mg2+ and K+ and uptake of H+, NH4+ for the subtropical forest canopies influenced by acid rain in Shaoshan forest located in Central South China

A total of ten 30 m × 30 m, researching plots in Shaoshan forest from January to December of 2001 is used for this study in Hunan Province. The atmospheric deposition, the total canopy leaching, the exchange of base cations (Ca2+, Mg2+ and K+), and the uptake of H+ and NH4+ by the canopies are estimated and quantified. The research results show that the maximal element in the total canopy leaching is Ca2+, the minimum is Mg2+ and the mid K+, with the value of 180.98 mequiv. m−2 a−1, 61.48 mequiv. m−2 a−1 and 84.13 mequiv. m−2 a−1, respectively. A conclusion significantly different from the reported results is made that is the largest leaching element in throughfall is not K+ but Ca2+ due to the special two-dimensional canopy structure and the subtropical climate with obvious seasonal characters in a year. The seasonal leaching of base cations varied greatly with the order of winter (333.72 mequiv. m−2), spring (296.80 mequiv. m−2), summer (195.49 mequiv. m−2), and autumn (166.70 mequiv. m−2), but the proportion in the total canopy leaching varied in another order of summer (64.0%), spring (59.0%), winter (57.0%), and autumn (39.2%). The seasonal uptake of NH4+ by the canopies varied from 100 to 150 mequiv. m−2 and the H+ from 50 to 340 mequiv. m−2 in the observed year.