Internal nutrient fluxes in a Puerto Rican rain forest

Throughfall and soil solution chemistry were studied for 1 y in a tropical montane forest in the Luquillo Mountains of Puerto Rico. Passage of precipitation through the forest canopy resulted in an increase in the concentration and Bur of all solutes except H+ and NO3-. Throughfall chemistry showed no strong seasonal patterns, but concentrations of many solutes declined during weeks of high rainfall. Enrichment in throughfall relative to precipitation was similar to values reported recently for several other tropical sites, with the exception of NH4+, which was particularly high at this site. Based on net throughfall deposition of Cl-, dry deposition of marine aerosols appears to be a relatively minor component (c. 15%) of total deposition. In soil solution, no seasonal patterns in concentration were evident and variability was highest for elements with high biological activity (especially N). Concentrations and fluxes of K+ and dissolved organic carbon (DOC) showed the largest declines as throughfall passed through the soil profile; concentrations of most other elements increased or were relatively constant. Declining DOC flux through the soil profile appears to be due to sorption processes similar to those observed in many temperate forests. Concentrations and fluxes of HCO3- and SiO2, increased substantially in soil solution, but never approached those observed in the stream. This suggests that additional weathering must occur as groundwater moves from this ridgetop site to the stream.