Abstract: Precipitation, throughfall, and stemflow were collected during the period of September, 2007 to August, 2009 from a subtropical natural evergreen and deciduous broadleaf mixed forest in Maolan, a karst area in Southwest China. The concentrations of major ions (Ca²⁺, Mg²⁺, K⁺, Na⁺, NH₄⁺, SO₄^2-, NO₃^-, Cl^-) were determined for collected samples in order to quantify the annual amounts of major nutrient elements and to examine changes in chemical composition of precipitation after passing through the canopy. The results showed that: (1) the concentrations of the major nutrient ions in precipitation, throughfall and stemflow were lower during the summer (from July to September), but higher in the winter (from December to the following February); (2) compared with precipitation, the concentrations of Ca²⁺, Mg²⁺, K⁺, Cl^-, NO₃^-, and SO₄^2- in throughfall and stemflow increased obviously, while there was a decrease in NH₄⁺ concentration and no significant variation in Na⁺ concentration; (3) the annual inputs of nutrient ions via precipitation were in order of Ca²⁺>SO₄^2->NH₄⁺-N>Cl^->K⁺ >Na^+>Mg²⁺>NO₃^--N, and the annual inputs of nutrient elements via throughfall plus stemflow were in order of Ca²⁺>SO₄^2-S>NH₄⁺-N>Cl^->K⁺>Na⁺>Mg²⁺> NO₃^--N. Compared with nonkarst areas, low ion concentrations in local precipitation demonstrated less anthropogenic influences on the studied forests, while the concentrations of K⁺, Ca²⁺ and Mg²⁺ in throughfall and stemflow increased greatly. Our study showed that K⁺, Ca²⁺ and Mg²⁺ mainly derived from carbonate weathering were active in the hydrologic cycling of nutrients in karst forests.

Key words: driving force,  , vegetation landscape, Saihanba., Logistic regression analysis