Nutrient Fluxes in Bulk Precipitation and Throughfall in Two Montane Tropical Rain Forests, Colombia

(1) The composition of bulk precipitation (rainfall plus dry deposition as collected by funnels) and throughfall water was studied during one year on a weekly basis in two epiphyte-rich Andean rain forests (Cordillera Central, Colombia, 2550 and 3370 m a.s.l.) (2) Nutrient input to the forest by bulk precipitation was higher at 2550 m because of larger precipitation volumes; rainfall was 2115 mm at 2550 m and 1453 mm at 3370 m. Losses of nutrients from the canopy, both total amounts and amounts per unit of precipitation, were also higher at 2550 m. (3) Net fluxes (throughfall flux minus bulk precipitation flux) and deposition ratios (ratio of throughfall flux to bulk precipitation flux) were generally within the range reported for other montane tropical rain forests. Differences between forests appear to be related to climate (mainly precipitation amounts), geographical situation (e.g. maritime influences) and the availability of nutrients in the soil. The patterns described in the present study are in accordance with other studies that show reduced cycling rates and limited availability of nitrogen and phosphorus in high-altitude rain forests. (4) Volcanic activity in the proximity of the forests (continuous SO2emission and incidental ash-falls) was shown to affect precipitation chemistry. Inputs of SO4-S were high (26.2 and 16.8 kg ha-1year-1at 2550 and 3370 m, respectively) and correlated with rainfall acidity. As a consequence of an ash-fall, concentrations of Ca, Mg and SO4-S in bulk precipitation were far above average on one occasion. Throughfall concentrations fell to normal within two to three weeks in spite of low rainfall.