Abstract
Bulk precipitation, throughfall, stemflow, and stream water were collected and the concentrations of major ions were determined from samples collected from a subtropical natural evergreen and deciduous broadleaf mixed forest in Maolan, a karst area in southwest China, in order to quantify the input-output budgets of major dissolved nutrients and to examine changes in the chemical composition of precipitation after passing through the canopy. Calcium ion and Mg^<2+> derived predominantly from carbonate weathering were the major contributors to the hydrologic system's compartments. Low ion concentrations and a high pH value characterized bulk precipitation quality that was influenced by natural rather than anthropogenic sources in Maolan. Calcium ion and Mg^<2+> had negative annual input-output budgets in contrast to K^+, Na^+, Cl^-, SO_4^<2->-S, and dissolved inorganic nitrogen (DIN) that had annual positive input-output budgets. We used a canopy budget model (Na^+ tracer) to estimate amounts of dry deposition and canopy exchange of ions in throughfall. Calcium ion had the largest annual dry deposition, followed by SO_4^<2-> and NH_4^+. Annual atmospheric deposition of DIN amounted to 12.3kgha^<-1>yr^<-1>. Eighty-six percent of DIN input in the forest was retained in the plant-soil system, indicating effective immobilization by vegetation and/or soil microflora.
